تواین تاپیک ترجمه متون مربوط به علوم گیاهی روقرارمیدیم.هرکسی هم که مایل به همکاری باشه میتونه اعلام کنه.
Auxins There is only one naturally occurring auxin: indole-3-acetic acid (IAA) and this is chemically related to the [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ] tryptophan. There are many synthetic auxins - aromatic compounds with carboxylic sidechains often affect plant growth in the same way that IAA does. These are used commercially rather than IAA because they are cheaper and more stable. For example naphthalene acetic acid (NAA) is used to control fruit set and sucker growth on trees after pruning. Indole butyric acid is used to promote rooting in cuttings. Far and away the biggest use of auxin-like compounds is as herbicides (2,4-D and MCPA). Applied at high concentration they promote uncoordinated growth and finally death, particularly in broad-leaved weeds.
There are a number of naturally occuring cytokinins all related to the nucleotide [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ]. They can occur as the free base or as a riboside. Synthetic cytokinins include benzyladenine and kinetin. Cytokinins are used in tissue culture media, and for growth control in fruit. Ethylene Ethylene is the only gaseous hormone in the plant world; it is a simple hydrocarbon gas that is derived from the [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ], methionine, via an unusual cyclic compound which is also an amino acid, ACC (1-aminocyclopropane-1-carboxylic acid). The gas is used commercially for ripening fruit, particularly bananas. There are also synthetic compounds, such as ethephon (chloro-ethanephosphonic acid) that can be sprayed onto plants in solution; once inside the tissues ethephon breaks down to liberate ethylene. Ethephon is used to promote ripening on the tree, leaf abscision in ornamentals, growth control in seedlings and flowering in pineapples.
Abscisic acid (ABA) is one of two related compounds (the other is xanthoxin) that are in the isoprenoid group and related to [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ] ABA is a very expensive material and so far there are no synthetic analogs or practical uses Gibberellins The gibberellins (GAs) are the largest group with over 70 compounds although not all are biologically active. Like ABA they are derived from the [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ] pathway. Gibberellins are used commercially to break dormancy of "difficult" seeds, and to promote set of grapes and other fruits. Many growth retardants used on flowering pot plants, woody plants and turf are "anti- gibberellins". Compounds such as ancymidol and uniconazole block GA synthesis and produce dwarf plants. Genetic dwarfs are often deficient in gibberellin.
At the cell level hormones attach to a protein receptor which sends a signal down a transduction pathway to switch on particular genes. Through [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ] this leads to production of an enzyme protein which actually causes the change in plant growth. A good example from the early stages of plant development is the role of GA in cereal seed germination. As the seed imbibes water the embryo produces GA. This induces synthesis of amylase in the aleurone layer which secretes the enzyme to the endosperm. Amylase breaks down starch to glucose which diffuses to the embryo and is used for the early stages of plant growth.
ایندول-3-استیک اسید تنها اکسین طبیعی است که ازلحاظ شیمیایی مربوط به امینواسیدتریپتوفان می باشد.
تعداد زیادی از ترکیبات اروماتیک مصنوعی اکسینی با زنجیره جانبی کربوکسیلیک وجود دارد که همانند اکسین عمل کرده و اغلب روی رشدگیاهان تاثیر می گزارند، چراکه که مقرون به صرفه تر و پایدارتر از ان می باشند. برای مثال نفتالین استیک اسیدبرای کنترل میوه ها ورشد پاجوش روی درختان بعدازهرس کردن استفاده می شود. ایندول بوتیریک اسید درجلوانداختن ریشه زنی در قلمه زنی استفاده میشود. از ترکیبات شبه اکسینی نظیر 2-متیل -4-کلرو فنوکسی استیک اسید و 2،4- دی کلرو فنوکسی استیک اسیدبه عنوان علف کش استفاده می شود.کاربرد وسیع انها سبب رشد غیر طبیعی وسرانجام مرگ به ویژه در علفهای هرزمی گردد.
تعدادی ازسیتوکینین های طبیعی وجودداردکه همه به نوکلئوتید ادنین مربوط می شوند.انها میتوانند بصورت مستقل و یا ریبوزید وجود داشته باشند.سیتوکینین های طبیعی شامل بنزیل ادنین و کیتین می باشد. سیتوکینین ها درکشت بافت وبرای کنترل رشد درمیوه استفاده می شوند.
تنها هورمون گازی گیاهان اتیلن می باشد، که گازهیدروکربن ساده ای است که ازامینواسید متیونین همراه با یک چرخه ی مرکب غیرمعمول از امینو سیکلو پروپان کربوکسیلیک اسید مشتق شده است. گاز تجاری برای رسیدن میوه ها بویژه موزهامی باشد. همچنین ترکیبات مصنوعی ازقبیل اتفون(کلرو-اتان فسفونیک اسید)وجود دارد که میتواند روی گیاهان بصورت محلول پاشیده شود. اتفون در بافتهای درونی به اتیلن ازاد می شکند. اتفون درجلوانداختن رسیدگی درختان میوه، قطع برگهای زینتی، کنترل رشد دربذرافشانی و گلدهی دراناناس استفاده میشود.
ابسیزیک اسیدیکی از دو ترکیب ایزوپرنوئیدیه مربوط به کارتنوئید است( ترکیب دیگر گزانتوکسین می باشد). ابسیزیک اسیدیک ماده ی خیلی گرانقیمت با مصارف زیاد بوده و جایگزین مصنوعی ندارد.
ژیبرلین ها بزرگترین گروه بابیش از 70 ترکیب هستند که همه ی انها فعالیت زیستی ندارند. مثل ابسیزیک اسید انهاازایزوپرنوئید مشتق شده اند. ژیبرلین ها برای شکستن دوره ی خواب دانه ها ودرجلو انداختن رسیدگی انگورها ودیگرمیوه ها استفاده ی تجاری دارند. بسیاری از عوامل ضد رشد روی گلدهی گیاهان گلدانی، گیاهان چوبی و چمن ترکیبات انتی ژیبریلین هستند. ترکیباتی ازقبیل انسیمیدول و یونیکونازول سنتز ژیبریلین رامسدود وگیاهان کوتاه قد را افرایش می دهند. ژنتیک کوتاه قدها اغلب درژیبرلین ناقص است.
هورمون های سلولی به یک پروتئین گیرنده وصل شده و یک پیام برش مسیر ترجمه برای اتصال روی ژن های نسخه برداری و ترجمه فرستاده و باعث تغییردررشد گیاهان میشوند. یک مثال خوب ازمراحل سریع از توسعه گیاه نقش ژیبرلین درجوانه زدن دانه ی حبوبات است. جنین اب جذب کرده، ژیبرلین تولید میکند که موجب سنتز امیلاز درلایه ی الئورون که انزیم رابه اندوسپرم ترشح میکند میشود. امیلازنشاسته رابه گلوکزتجزیه میکند که به مصرف جنین و مراحل ابتدایی رشد می رسد.
(باتشکرازاقای حاجی بهزاد)
Plant secondary metabolites is a generic term used for more than 30,000 different substances which are exclusively produced by plants. The plants form secondary metabolites e.g. for protection against pests, as colouring, scent, or attractants and as the plant's own hormones. It used to be believed that secondary metabolites were irrelevant for the human diet. The importance of these substances has only recently been discovered by scientists. Secondary metabolites carry out a number of protective functions in the human body. Plant secondary metabolites can boost the immune system, protect the body from free radicals, kill pathogenic germs and much more.
In contrast to the primary metabolites (carbohydrates, fats, proteins, vitamins and mineral nutrients) secondary metabolites do not have nutrient characteristics for human beings. They are usually found in very small amounts but have an effect on humans.
Secondary metabolites have a scientifically proven effect on health. However many of these effects are unknown. The exact requirement of the individual substances is likewise unknown. A diet which is rich in plant foods contains a variety of secondary metabolites and contributes to protecting the body against cancer and cardiovascular illnesses. Secondary metabolites and their effects are currently being intensively researched
Carotenoids are organic pigments occurring in plants and are mostly found in red, orange and yellow fruits and vegetables. Other vegetables such as broccoli, spinach or curly kale also contain carotenoids. Carotenoids have antioxidative effects and prevent cancer. In addition to this they boost the immune system and reduce the risk of getting heart attacks.
- Phytosterols are found in plant foods such as sunflower seeds, sesame, nuts and Soya beans. Phytosterols protect against colon cancer and lower cholesterol levels. Phytosterols are chemically similar to cholesterol and therefore they compete against each other for absorption in the body.
Saponins are flavour additives, which are found in legumes and spinach. Saponins boost the immune system, lower the cholesterol levels in the blood and reduce the risk of getting intestinal cancer.
- Glucosinolates are flavour additives, which are found in all types of cabbages, mustard, radish and cress. Glucosinolates prevent infections and inhibit the development of cancer.
Flavonoids are organic pigments occurring in plants which give plants a red, violet or blue colour. Flavonoids have a particularly broad spectrum of efficacy. Flavonoids inhibit the growth of bacteria and viruses, protect the cells against the damages of free radicals, protect against cancers and heart attacks, have a repressive effect against inflammations and they influence blood coagulation.
- Protease-inhibitors are found in plants that are rich in protein such as legumes, potatoes and wheat and they inhibit the decomposition of protein. Protease inhibitors protect the body against cancers and regulate the blood sugar levels.
Terpenes are plant flavours for e.g. the menthol in peppermint oil or the essential oils in herbs and spices. Terpenes decrease the risks of cancer.
Phytoestrogens are natural plant hormones which are similar to the ***ual hormones. Phytoestrogens are mostly found in wheat, legumes and wheat products. Phytoestrogens protect the body against hormonal dependant cancers such as breast, uterine and prostrate cancer.
- Sulphides are compounds containing sulphur which are mostly found in plants that belong to the lily family such as onions, leeks, asparagus and garlic. Sulphides inhibit the growth of bacteria, lower cholesterol levels, protect the body from free radicals and have preventive effects against cancer.
- Phytic acid
Phytic acid is found in wheat, legumes and flaxseeds. Phytic acid was considered undesirable for a long time because it binds trace elements such as iron and zinc and it also affects various digestive enzymes. However new studies have proved that phytic acid has an antioxidant effect in the large intestine
متابولیت های ثانویه گیاهی
متابولیتهای ثانویه گیاهی واژه ای کلی برای بیش از 30 هزار محصول منحصرا گیاهی می باشد. اشکال گیاهی متابولیتهای ثانویه به عنوان مثال عاملی برای حفاظت در مقابل افات، رنگ، عطر، جلب و یا به صورت خود هورمون گیاهی هستند.مصرف کنندگان عقیده دارند که متابولیتهای ثانویه در رژیم غذایی انسان تاثیر گذار نیستند. اهمیت این مواد اخیرا توسط دانشمندان کشف شده است. متابولیتهای ثانویه برخی از اعمال حفاظتی در بدن انسان انجام میدهند. متابولیتهای ثانویه گیاهان میتوانند سیستم ایمنی را تقویت کرده، بدن را از رادیکال های ازاد حفظ کرده و عوامل بیماریزا را نابود کنند.
در مقابل متابولیتهای اولیه (کربوهیدرات ها - چربی ها - پروتئینها و ویتامینها و مواد غذایی معدنی) نظیر متابولیتهای ثانویه ویژگی های غذایی برای انسان ها ندارند.انها در مقادیر ناچیز و بسیار کم اثر برای انسان می باشند.
اگرچه تعدادی از اثرات متابولیتهای ثانویه به صورت اختصاصی ناشناخته هستند اما به طور علمی ثابت شده که در سلامتی تاثیر گذار هستند. یک رژیم غذایی گیاهی که سرشار از انواع متابولیتهای ثانویه می باشد به حفاظت بدن در برابر سرطان و بیماریهای قلبی عروقی کمک می کند. در حال حاضر بررسیهای زیادی در مورد این محصولات در حال انجام می باشد.
رنگدانه های الی گیاهی بوده و عمدتا در میوه ها و سبزیجات قرمز، نارنجی و زرد رنگ یافت می شوند. دیگر سبزیجات از قبیل کلم بروکلی، اسفناج و کلم پیچ نیز حاوی کاوتنوئیدها می باشند. کارونوئیدها اثرات انتی اکسیدانی داشته و از سرطان جلوگیری میکنند.علاوه بر این انها سیستم ایمنی را بالا برده و خطر ابتلا به حمله قلبی را کاهش میدهند.
فیتواسترولها در غذاهای گیاهی از قبیل دانه افتابگردان، کنجد، اجیل و دانه های سویا یافت میشوند. فیتو استرولها بدن را در مقابل کلنی های سرطانی محافظت کرده و سطح کلسترول را کاهش میدهند. از نظر شیمیایی مشابه کلسترول بوده و در بدن در جذب رقابتی قرار دارند.
ساپونین ها چاشنی های افزودنی بوده و در حبوبات و اسفناج یافت می شوند. سیستم ایمنی را بالا برده، سطح کلسترول خون را پایین اورده و خطر ابتلا به سرطان روده را کاهش می دهند.
چاشنی های افزودنی اند که در همه انواع کلم ها، خردل، تربچه و شاهی یافت میشوند. از عفونت جلوگیری کرده و سبب مهار پیشرفت سرطان می شوند.
رنگدانه های الی بوده و به گیاهان رنگ قرمز، بنفش و یا ابی میدهند. فلاونوئید ها طیف اثر گسترده ای دارند. رشد باکتری ها و ویروس ها را مهار کرده و سلولها را در مقابل تخریب رادیکالهای ازاد محافظت میکنند.اثر سرکوب کنندگی علیه التهاب داشته و سبب انعقاد خون می شوند.
در گیاهانی که غنی از پروتئین اند یافت میشوند، نظیر حبوبات، سیب زمینی و گندم و از تجزیه پروتئین جلوگیری میکنند. از بدن در برابر سرطانها محافظت کرده و سطح قند خون رو تنظیم میکنند.
چاشنی های گیاهی نظیر منتول در اسانس نعنا یا اسانسهای گیاهی و ادویه جات که خطر ابتلا به سرطان را کاهش میدهند.
هورمون های گیاهی طبیعی اند که شبیه هورمون های جنسی عمل می کنند. اکثرا در گندم، حبوبات و محصولات گندمیان یافت میشوند. بدن را علیه سرطان های وابسته به هورمون از قبیل سرطان پستان، رحم و پروستات محافظت میکنند.
ترکیباتی اند حاوی سولفور که اکثرا در گیاهان متعلق به خانواده زنبق از قبیل پیاز ها؛ تره فرنگی، مارچوبه و سیر یافت میشوند. سولفید ها مانع رشد باکتری ها شده و سطح کلسترول خون را پایین می اورند. از بدن در برابر رادیکالهای ازاد محافظت کرده و بازدارنده پیشروی سرطان می باشند.
در گندم و حبوبات و تخم کتان یافت میشوند. به دلیل ایجاد باندهای اتصالی با عناصر کمیاب از قبیل اهن و روی برای مدتی طولانی نامطلوب در نظر گرفته می شدند و همچنین انزیمهای گوارشی مختلف را تحت تاثیر قرار می دهند. به هر حال مطالعات جدید نشان می دهند که فیتیک اسیدها اثر انتی اکسیدانی در روده بزرگ دارند.
مترجم:[مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ]
باتشکرازاقای حاجی بهزاد
INTRODUCTION TO TAXONOMY
What is Taxonomy and Where Did it Originate?
Taxonomy is the method by which scientists, conservationists, and naturalists classify and organize the vast diversity of living things on this planet in an effort to understand the evolutionary relationships between them. Modern taxonomy originated in the mid-1700s when Swedish-born Carolus Linnaeus (also known as Carl Linnaeus or Carl von Linné) published his multi-volume Systema naturae, outlining his new and revolutionary method for classifying and, especially, naming living organisms. Prior to Linnaeus, all described species were given long, complex names that provided much more information than was needed and were clumsy to use. Linnaeus took a different approach: he reduced every single described species to a two-part, Latinized name known as the “binomial” name. Thus, through the Linnaean system a species such as the dog rose changed from long, unwieldy names such as Rosa sylvestris inodora seu canina and Rosa sylvestra alba cum rubore, folio glabro to the shorter, easier to use Rosa canina. This facilitated the naming of species that, with the massive influx of new specimens from newly explored regions of Africa, Asia, and the Americas, was in need of a more efficient and usable system.
Although trained in the field of medicine, botany and classification were the true passions of Linnaeus and he actively explored northern Europe and described and named hundreds of new plant species during his lifetime. As well, Linnaeus spent a great deal of time describing and naming new plant specimens that were sent to him from around the world by other botanists, including from the newly explored regions of the New World. Linnaeus classified this multitude of new plant species based upon their reproductive structures, a method which is still largely in use today. In fact, the majority of the species described by Linnaeus are still recognized today, indicating how far ahead of his time he truly was. Although somewhat rudimentary by today’s standards, Linnaeus’ methods of describing species in such a way as to represent the relationships between them changed the face of taxonomy and allowed biologists to better understand the complex natural world around us.
How Do We Classify Plants?
Plants, and indeed all organisms, are classified in a hierarchical system that attempts to illustrate the evolutionary relationships between the various groupings within the hierarchy. This concept of relatedness forms the backbone of modern classification schemes. Scientists who attempt to classify organisms and place them within an evolutionary framework are called Taxonomists, the most famous of which would be Linnaeus himself.At the broadest level, all organisms on the planet are classified into 5 Kingdoms: Animalia (animals), Plantae (plants, some multicellular algae),Fungi (fungi), Monera (prokaryotic bacteria), and Protista (eukaryotic bacteria, most algae, etc.), representing the most ancient branches of the evolutionary “tree of life.” Organisms in any given Kingdom may be separated from organisms in any other Kingdom by many hundreds of millions, if not billions, of years of evolution. Historically, all organisms known were grouped into only two Kingdoms: organisms that had finite growth, moved, and ate were grouped into the Kingdom Animalia, while organisms that had indefinite growth, didn’t move, and didn’t eat were grouped into the Kingdom Plantae. Of course, as science progressed, it became increasingly evident that such a simplistic approach to taxonomy was ineffective and many species were found that did not fit either grouping particularly well. The proposal to move to an eight-Kingdom system suggests that our current classification system, with its five Kingdoms, may yet change again as our understanding of the diversity of organisms around us continues to grow.
Within each Kingdom, the organisms are grouped into several Phyla (sing. Phylum), also known as Divisions, which represent smaller groupings of more recognizable forms. Although the Kingdom Animalia contains a large number of Phyla (such as chordates [including vertebrates], echinoderms, annelids, arthropods, etc.), Kingdom Plantae contains only ten. The Phylum Bryophyta (mosses, liverworts, hornworts), the most primitive of all true plants, differs from other plant Phyla in that it is non-vascular, meaning that it lacks water-conducting tissues which bring water from the roots of the plant up into the crown, and that the gametophyte (vegetative) generation predominates over the sporophyte (reproductive) generation. The Phyla Psilophyta (whisk ferns), Lycopodiophyta (club-mosses, spike-mosses, quillworts), Equisetophyta(horsetails), and Polypodiophyta (true ferns), including all vascular plants that reproduce using spores, also form an ancient, though largely artificial, grouping and are often referred to as Pteridophytes. The Phyla Cycadophyta (cycads),Ginkgophyta (ginkgo), Gnetophyta (vessel-bearing gymnosperms), and Coniferophyta (conifers) form a second primitive grouping of vascular plants, known as Gymnosperms, which are characterized by the presence of ***** seeds (the literal translation of “gymno-sperm”). The final Phylum, Magnoliophyta, contains all of the vascular, flowering plants that are considered to be the most advanced and recently-evolved plants occurring on the planet today.
Within each Phylum, the organisms involved are grouped into progressively smaller, more refined groupings of similar individuals. Below Phylum, organisms are grouped into Classes, Orders, and Families, the latter being the largest-order taxonomic grouping that is commonly used by amateur botanists. As an example, the Phylum Magnoliophyta is split into 2 well-known Classes: Magnoliopsida (Dicotyledons) and Liliopsida (Monocotyledons) based on a variety of features from leaf venation and flower structure to growth form, root structure, and seed structure, each class with its subsequent Orders and Families. Each family is further divided into Genera (sing. Genus) representing organisms with similar morphology, structure, reproductive organs, and, perhaps most importantly, evolutionary history. These genera represent groupings that many of us are most familiar with, such as Rhododendron, Rosa, Chrysanthemum, etc. and are designed to illustrate that the individual organisms grouped within the same genus are very closely related to each other. In fact, the genus is the taxonomic grouping that represents the closest relationship between organisms which, at the smallest taxonomic level, are called Species. Each individual species is given a specific name that, when combined with the generic name, produces the two-term “binomial” naming system that Linnaeus pioneered. For example, within the genus Rosa are a variety of species such as acicularis, nutkatensis, and woodsii. Through the binomial naming system, these species becomeRosa acicularis, R. nutkatensis and R.woodsii (the generic name is shortened to the first initial when listing several species in the same genus).
Of course, as with many scientific theories or strategies, there are problems with this system in the way it is currently applied and as a result it is in a continual state of flux, especially at the lower levels of the hierarchy. Even at the highest level (Kingdom), several groups are still cause for debate among taxonomists as to their placement. For example, how do we classify lichens? Lichens were originally placed within the Kingdom Plantae until further research showed that what we call “lichens” are actually a symbiotic relationship between certain species of fungi and certain species of algae. The two species, which can often survive independent of each other, combine to form a third plant-like “species” of organism called “lichen” that differs greatly from either of its two parent species yet functions as its own reproductive, evolutionary organism (thus meeting the criteria for a “species”). Currently lichens are included within the Kingdom Fungi since the fungal partner is the driving force behind the union (essentially “cultivating” its algal partner in order to produce its own nourishment) but this treatment still does not really fit with traditional taxonomy.Another example of how nature continually confounds attempts to classify it is the vast array of plant-like organisms grouped under the term “algae.” The confusion results from the fact that most algae are unicellular or, if multicellular, composed of a single or very few cell types amassed together to function as a larger individual. So, do we classify multicellular algae based on the characteristics of the single cell (Protista) or as an independent multicellular organism (Plantae)? Most algae are currently placed within the Kingdom Protista despite their often plant-like appearance, with only a few of the multi-cellular forms remaining within the Kingdom Plantae. This treatment is not followed by all authors, however, as some retain all of the algae as a subkingdom within the Kingdom Plantae. Regardless of the treatment, it is obvious that the great diversity within the group “algae,” as well as its unusual morphological and cellular characteristics, is a hindrance to botanists who attempt to classify them within our current taxonomic systems.
What is a “Species”?
At the lowest level of the classification hierarchy is the “species”, a human-derived concept that, to this day, is still not completely understood by scientists. The general consensus in past decades has been that a “species” is a group of similar individuals which can reproduce successfully with each other while at the same time being reproductively isolated from other similar species (known as the “Biological Species Concept”). This interpretation worked reasonably well when it was first proposed, but the more we learn about ecological systems the more apparent it becomes that nature is by no means so simple. The evolutionary process is a continuum whereby a portion of the population of one entity gradually becomes more and more distinctive and discrete, eventually reaching a state in which it is reproductively isolated from its parent “species.” The infinite range of variation between the two ends of this evolutionary process means that many populations are difficult to assign to either a parent species or a new, independent species.A newer species concept, known as the “Phylogenetic Species Concept”, attempts to give specific status to any identifiable populations that have a unique evolutionary history and differ collectively in some characteristics from other populations. This system, which places more weight on the evolutionary process and genetic differences between populations, naturally results in a far greater number of recognizable species than the more conservative Biological Species Concept. In truth, however, neither of these widely accepted concepts appears to fully represent the extraordinary complexities of the natural world, and perhaps the most effective current method of species classification is a combination of both systems.
<Ranunculus californicus and Plectritis congesta
Another method used by taxonomists to deal with the variation within species is the use of “infraspecific” or “subspecific” taxonomy. Many species are not uniform in appearance throughout their distribution, and by assigning subspecies and varietal names to the identifiable populations scientists are able to catalogue and name this variation.
Populations that are approaching species status are typically categorized as subspecies (often written as “ssp.” or “subsp.”), especially when these forms have discrete geographic distributions. For example, in the species Salix reticulata(net-leaved willow) individuals occurring throughout the mountain ranges of the interior of the province with hairy capsules and a strong net-like pattern of venation on the leaves are named S. reticulata ssp.reticulata, while the populations on the Queen Charlotte Islands that have hairless capsules and a weaker net-like venation pattern on the leaves are known as S. reticulata ssp.glabellicarpa. These two subspecies have different geographic ranges and represent evolutionary lines that are fairly well defined, but are similar enough to be classed within the same species.
Heavy Metal Toxicity in Plants
Abstract Plants are sessile organisms that must cope with the surrounding soil
composition in order to survive and reproduce. Soils often contain excessive levels
of essential and non-essential elements, which may be toxic at high concentrations
depending on the plant species and the soil characteristics. Many metals share
common toxicity mechanisms, and plants deal with these metals using similar
scavenging pathways. The impact of metal toxicity is made more complex by
competition, since high levels of one metal may imbalance the uptake and
transport of others, therefore contributing to the toxicity symptoms. Here, the
toxicity symptoms and mechanisms of the most common essential and nonessential
heavy metals will be considered.
Keywords Heavy metal
1.1 Heavy Metals: Nutrients or Toxic Elements?
Plants acquire mineral elements from soil primarily in the form of inorganic ions.
The extended root apparatus and its ability to absorb ionic compounds even at low
concentrations makes mineral absorption highly ef*cient.
Mineral elements can be divided into two groups: essential nutrients and toxic
non-nutrient elements. The essential minerals include the macronutrients nitrogen
(N), potassium (K), calcium (Ca), magnesium (Mg), phosphorous (P), sulfur (S) and
G. DalCorso (&)
Department of Biotechnology, University of Verona, Ca Vignal 1,
Strada Le Grazie 15, 37134 Verona, Italy
e-mail: [مشاهده ی لینک ها فقط برای اعضا امکان پذیر است. ]
A. Furini (ed.), Plants and Heavy Metals, SpringerBriefs in Biometals,
DOI: 10.1007/978-94-007-4441-7_1, DalCorso 2012
2 G. DalCorso
silicon (Si), and the micronutrients chlorine (Cl), iron (Fe), boron (B), manganese
(Mn), sodium(Na), zinc (Zn), copper (Cu), nickel (Ni), and molybdenum(Mo). These
are essential components of plant metabolism and structure, and their absence or
de*ciency reduces *tness and inhibits growth and reproduction. Micronutrients are
required in only small quantities and their excessive abundance in the soil (especially
Cu, Ni and Zn), due to natural occurrence or introduction by anthropogenic activities, is
also detrimental to the majority of plant species. Other minerals such as cadmium(Cd),
mercury (Hg), lead (Pb), chromium (Cr), arsenic (As), silver (Ag), and antimony (Sb)
are toxic to plants even at low concentration. These metals, collectively de*ned as
heavy metals since their density is higher than 5.0 g cm
, are not considered to be
nutrients because they have no known function in plant metabolism and appear to be
more or less toxic to both eukaryotic and prokaryotic organisms (Sanità di Toppi and
Gabbrielli 1999). Only recently, a carbonic anhydrase has been shown to bind Cd as a
cofactor in the marine diatom Thalassiosira weiss*ogii (Lane and Morel 2000).
When studying heavy metal toxicity in plants, researchers must take into
account the nature of the pollution phenomenon. First, the stress caused by contaminated
soils is permanent, and therefore long-term rather than short-term
molecular responses must be considered. Most studies have been carried out in
hydroponic or in vitro culture, and have involved the application of extremely high
metal concentrations in the growth media. This seldom resembles actual environment
and represents the consequences of acute stress caused by a single metal
species. Second, the toxicity of a heavy metal depends on its oxidation state, e.g.,
Cr(VI) is considered the most toxic form of Cr, and usually occurs associated with
oxygen as chromate (CrO
) or dichromate (Cr
) oxyanions. Cr(III) is less
mobile, less toxic, and predominantly bound to organic matter in soil and aquatic
environments (Shanker et al. 2005). Third, the ability of heavy metals to persist in
the soil in the form that is bioavailable to roots (i.e., soluble and ready for
absorption) is inﬂuenced by their adsorption, desorption, and complexation in the
soil matrix, processes that are strongly inﬂuenced by soil pH, composition, and
structure. Heavy metals tend to be more mobile in acidic soils. Finally, heavy
metal toxicity is species dependent. For instance, metal-tolerant plants and certain
plants known as hyperaccumulators [able to accumulate at least 100 mg g
(0.01% dry weight) Cd, As, and some other trace metals, 1,000 mg g
weight) Co, Cu, Cr, Ni, and Pb and 10,000 mg g
(1% dry weight) Mn and Ni
(Reeves and Baker 2000; Watanabe 1997) have defense mechanisms that avoid
damage caused by heavy metal-induced stress, although the duration and magnitude
of exposure and other environmental conditions, contribute to heavy metal
sensitivity (Sanità di Toppi and Gabbrielli 1999).
Metal toxicity is also greatly inﬂuenced by the coexistence of many metals in
the soil, which could have both synergic and antagonistic effects depending on the
relative concentrations and other soil properties (i.e., presence of nutrient
elements). For example, Ca
strongly inhibits the uptake of Ni in Arabidopsis
bertolonii, whereas the opposite effect is seen in Berkheya coddii (Gabbrielli and
Pandol*ni 1984). Ni can induce Fe deﬁciency either by retarding its uptake or
trapping Fe in the roots (Mysliwa-Kurdziel et al. 2004), and these effects can be
1 Heavy Metal Toxicity in Plants 3
partially overcome by supplementation with Mg (or Fe) ions suggesting a competitive
interaction (Le Bot et al. 1990). In Pisum sativum, Mn toxicity can be
reduced by applying indole acetic acid (IAA). This also promotes seedling growth,
and it has been suggested that IAA protection is mediated by regulation of both the
ammonium content and the activities of enzymes involved in ammonium assimilation
(Gangwar et al. 2011). Mn toxicity is also reduced in the presence of Si.
The biochemical and physiological basis of this phenomenon is poorly understood
and may involve the modiﬁcation of metabolic stress responses (Führs et al. 2009)
and a change in the apoplastic Mn-binding properties that lead to a reduction in the
concentration of Mn in the apoplast (Horst et al. 1999). As already stated, different
species respond to combinations of ions in different ways. As an example, pea
plants are protected from Cd toxicity by Ca, which limits Cd accumulation,
whereas in Brassica juncea, Ca promotes the accumulation of As but reduces its
toxicity (Rai et al. 2011a).
Fertilization is also known to inﬂuence heavy metal toxicity. The addition of
phosphate reduces As toxicity in *eld-grown Medicago truncatula and Hordeum
vulgare without modifying the speciﬁc uptake of As(V), and this may be due to the
higher phosphate concentration into cells that outcompetes As in metabolic
reactions (Christophersen et al. 2009). The accumulation and translocation of As in
rice plants is inhibited when sulfur is abundant but enhanced when its availability
is limited. This may reﬂect the prominent role of sulfur, which is a component of
PCs and GS (both of which form complexes with heavy metals) and the impact of
its availability on the synthesis of thiolic compounds, elements that ultimately
affect As accumulation and metabolism (Zhang et al. 2011).
1.2 Toxicity Mechanisms of Heavy Metals
Heavy metal toxicity in plants occurs through four major mechanisms:
(1) Induction of oxidative stress and changes in the cell membrane permeability
and integrity. Many heavy metals induce the formation of ROS such as H
, which may be a direct process (via the Fenton and Haber–
Weiss reactions, as shown in Fig. 1.1) or as secondary effect due to their
toxicity into the cell. ROS have a negative impact on plant cells, for instance
by inhibiting water channel and transporter proteins and enhancing lipid
peroxidation. The latter alters membrane *uidity, stability, and structure,
inhibiting membrane-dependent processes such as electron *ow in chloroplasts
and mitochondria. ROS are counteracted by the activation of antioxidant
enzymes such as SOD, APX, GPX, CAT, and GSR, whose reaction mechanisms
are shown in Fig. 1.2.
(2) Reaction with sulfhydryl groups (–SH). Heavy metals have a strong af*nity for
–SH groups [Cd, for example, shows a threefold higher af*nity for –SH groups
than Cu (Schützendübel and Polle 2002)] and therefore bind to structural
4 G. DalCorso
Fig. 1.1 Generation of ROS
by heavy metals, including
examples of reactions
catalyzed by Fe (Halliwell
and Gutteridge 1984)
proteins and enzymes containing them. This can prevent correct folding,
interfere with catalytic activity, and perturb enzyme-mediated redox regulation
(3) Similarity to biochemical functional groups. As(V) in arsenate (AsO
example, is an analog of the micronutrient phosphate (PO
) and competes
with it in many cellular functions. AsO
displaces phosphate in ATP,
leading to the formation of the unstable complex ADP-As that interferes with
the energy *ows in the cell (Meharg and Hartley-Whitaker 2002).
(4) Displacement of essential (cat)ionic cofactors in enzymes and signaling
components. Metal ions in the active sites of enzymes can be displaced by
heavy metal ions resulting in the loss of activity, e.g., the displacement of Cu,
Zn, Fe, and Mn cofactors from superoxide dismutase by Cd. The displacement
of the ionic cofactors from signaling proteins (e.g., calmodulin and transcription
factors) results in aberrant proteins that may perturb gene expression.
This process can also interfere with homeostatic pathways for essential metal
ions (Roth et al. 2006). For example, the displacement of Cu and Fe from
proteins releases free ions that may cause oxidative damage, e.g., via Fe/Cucatalyzed
Fenton reactions (DalCorso et al. 2008).
The large number of targets for heavy metal toxicity means that negative effects
tend to be *rstly observed in those cells that are exposed *rst, i.e., cells responsible
for the metal uptake. Heavy metals interfere with ionic homeostasis and enzyme
activity, and these effects are apparent in physiological processes involving single
organs (such as nutrient uptake by the roots) followed by more general processes
such as germination, growth, photosynthesis, plant water balance, primary
metabolism, and reproduction. Indeed, visible symptoms of heavy metal toxicity
1 Heavy Metal Toxicity in Plants 5
Fig. 1.2 Antioxidant enzymes responsible for the detoxiﬁcation of H
include chlorosis, leaf rolling and necrosis, senescence, wilting and stunted
growth, low biomass production, limited numbers of seeds, and eventually death.
We now consider the effects of the most relevant heavy metals individually
Non-Essential Heavy Metals: Cadmium, Mercury, Lead,
Chromium and Arsenic
ascorbate peroxidase, AsA ascorbate, CAT catalase, DHA dehydroascorbate, DHAR dehydroascorbate
reductase, GSR glutathione reductase, GSH reduced glutathione, GSSG oxidized
glutathione, GPX glutathione peroxidase, MDHAR monodehydroascorbate reductase, NADP
nicotinamide adenine dinucleotide phosphate, reduced (NADPH) and oxidized (NADP
Cadmium (Cd) is one of the most phytotoxic heavy metals because it is highly
soluble in water and promptly taken up by plants. This also represents its main
entry into the food chain, making it a threat to human health. Even at low concentrations,
the uptake by roots and the transport of Cd to vegetative and reproductive
organs have a negative effect on mineral nutrition, homeostasis, growth
and development (DalCorso et al. 2010).
In root cells, Cd imbalances water and nutrient uptake, interfering with the
absorption of Ca, Mg, K, and P. It inhibits root enzymes involved in nutrient
metabolism, such as Fe(III) reductase, nitrate reductase, nitrite reductase, glutamine
synthetase, and glutamate synthetase, leading to Fe(II) deﬁciency, and reduced
nitrogen assimilation and metabolism (glutamine and glutamate synthetases are
responsible for the incorporation of ammonium into the carbon skeleton, DalCorso
et al. 2008). Nitrogen *xation and primary ammonia assimilation is also inhibited in
the nodules of soybean plants in the presence of Cd (Balestrasse et al. 2003).
Cd inhibits root growth and lateral root formation, with the concomitant
6 G. DalCorso
differentiation of numerous root hairs, for instance, in Arabidopsis and tobacco
(Farinati et al. 2010). Tomato roots exposed to Cd are thicker and stronger (Chaffei
et al. 2004). In shoot tissues, the most evident symptoms of Cd toxicity are leaf roll,
chlorosis, water uptake imbalance, and stomatal closure (Clemens 2006). Chlorosis
may reﬂect changes in the Fe:Zn ratio that negatively affect chlorophyll metabolism
(Chaffei et al. 2004). Cd causes stomata to close independent of water status probably
because its similarity to Ca allows Cd to enter guard cells through voltage-dependent
channels and to mimic Ca
activity in the cytosol (Perfus-Barbeoch et al.
2002). Indeed, stomatal closure can be actively driven by Ca
guard cell cytosol. The increase in cytosolic free Ca
accumulating in the
causes plasma membrane
anion and K
channels to open, resulting in the loss of water and turgor that drives
stomatal pore closure (MacRobbie and Kurup 2007).
Both cellular and organellar metabolism are compromised by Cd. In chloroplasts,
Cd damages the photosynthetic apparatus, targeting the light-harvesting
complex II and the two photosystems (PSI and PSII) which are particularly
sensitive. This reduces the chlorophyll and carotenoid content, increases
non-photochemical quenching and limits both photosynthetic ef*ciency and
effective quantum yield (Sanità di Toppi and Gabbrielli 1999). Moreover, by
inhibiting enzymes involved in CO
*xation, Cd reduces carbon assimilation
(Perfus-Barbeoch et al. 2002). Cd also affects sulfur metabolism in the chloroplasts
by inducing the accumulation of thiolic compounds with a concomitant reduction
in leaf ATP-sulfurylase and O-acetylserine sulfurylase activity, i.e., the *rst and
the last enzymes in the sulfate assimilation pathway (Astolﬁ et al. 2004).
Cd is toxic at the cellular level by interfering with mitosis and inhibiting cell
division, due to chromosomal aberrations and inhibition of mitotic processes
(Benavides et al. 2005). In Arabidopsis, Cd induces mutations, leading to *oral
anomalies, embryonic malformations, and poor seed production (DalCorso et al.
Although Cd does not take part in the Fenton and Haber–Weiss reactions,
without Cd ions altering their oxidation state (Clemens 2006), exposure can still
induce oxidative injuries such as protein carbonylation and lipid peroxidation,
disrupting cell homeostasis and interfering with membrane functions (RomeroPuertas
et al. 2002; Schützendübel et al. 2001). This appears to reﬂect a
Cd-induced imbalance in the activity of antioxidative enzymes, CAD and SOD
in primis, leading to the accumulation of ROS, which may be a general effect of
redox imbalance or a speciﬁc response to heavy metal stress (Romero-Puertas et
al. 2004). Other plants induce GDH in response to Cd (Boussama et al. 1999).
GDH activity is correlated with the onset of senescence and associated changes
in nitrogen metabolism (Masclaux et al. 2000). Similar changes in nitrogen
metabolism are observed in plants exposed to Cd so it is possible that the toxic
effects of Cd reﬂect the induction of senescence. In peroxisomes, Cd induces
glyoxylate cycle enzymes (malate synthase and isocitrate lyase) as well as
peroxisomal peptidases, the latter being well known as leaf senescence-associated
factors (Chaffei et al. 2004).
1 Heavy Metal Toxicity in Plants 7
A secondary effect of ROS accumulation is the perturbation of signaling
pathways mediated by H
and oxygen radicals. Indeed, H
plays a role as
signal molecule in triggering, for instance, defence mechanisms against both
abiotic stresses (Dat et al. 2000; Sharma et al. 1996) and pathogen attack
(Bestwick et al. 1998; Thordal-Christensen et al. 1997). Interfering with H
accumulation, Cd meddles with the signal transduction pathways in which ROS
are involved. Cd
can also displace the chemically similar Zn
from zinc *nger
transcription factors, thus interfering with gene expression (Sanità di Toppi and
Gabbrielli 1999). Similarly, Cd
can displace Ca
from calmodulin proteins, thus
perturbing intracellular calcium level and altering calcium-dependent signaling,
e.g., the regulation of stomatal closure discussed above (DalCorso et al. 2008).
Mercury (Hg) is generally found only in trace concentrations in soil, and it is tightly
bound to organic matter and clay particles or as a sulﬁde precipitates (Schuster 1991).
The predominant source of Hg in the soil is from mining and industrial waste (Zhou et
al. 2007). The toxicity of Hg depends on its chemical state (e.g., HgS, Hg
methyl-Hg). The predominant form in agricultural soils is Hg
, which is not particularly
phytotoxic at normal concentrations, but it is soluble, highly reactive, and
readily taken up by plants (Han et al. 2006). Alternatively, the uncharged and volatile
can enter leaves via the stomata and diffuse to the mesophyll cells where it
is oxidized to Hg(II) (Zhou et al. 2007). The *rst visible symptoms of Hg toxicity are
the profound inhibition of root and shoot growth (Cho and Park 2000). The molecular
basis of Hg phytotoxicity remains uncertain but probably reﬂects: (i) the af*nity of
Hg for –SH groups; and (ii) the direct generation of ROS via the Fenton reaction,
which in turn induces oxidative stress (Fig. 1.1).
Roots show the *rst signs of Hg toxicity because these are the *rst tissues to be
exposed to the metal. The suppression of root growth by Hg has been observed in
tomato seedlings, in Brassica spp. and in Spinacia oleracea (Cho and Park 2000;
Ling et al. 2010). At high concentrations, Hg can bind to water channels in the
plasma membrane, interfering with water *ow and stomatal functions. When
wheat root cells were exposed to HgCl
, the hydraulic conductivity of the membranes
was reduced, suggesting that membrane depolarization may inhibit water
transport (Zhang and Tyerman 1999). Hg also strongly inhibits photosynthesis by
interacting with metal ions in the PSII proteins D1 and D2 and with the Mn-cluster
of the OEC (Patra et al. 2004). Oxygen evolution and thylakoid electron transport
are also inhibited because Hg depletes the 33-kDa manganese stabilizing protein
on the luminal side of PSII (Bernier and Carpentier 1995). PSI is also compro-
mised by Hg, which oxidizes the P
chlorophyll a when present as HgCl
et al. 1998). In addition to Hg-induced chlorophyll depletion, these negative effects
eventually result in a dramatic reduction in the photosynthetic quantum yield (Cho
and Park 2000).
8 G. DalCorso
Laboratory experiments with various explants have shown that high concentrations
of Hg are genotoxic, causing chromosomal damages, interfering with
mitosis and meiosis, and inducing polyploidy (Patra et al. 2004).
Hg has a global impact on the redox state of the cell because it catalyzes the
formation of ROS. In tomato seedlings, exposure to Hg induces the formation of
(Cho and Park 2000), whereas alfalfa leaves exposed to Hg
levels of both H
(Zhou et al. 2008). This increase in ROS affects many
other cellular functions by damaging nucleic acids and proteins, and by inducing
lipid peroxidation thus modifying membrane integrity and permeability (Patra et
al. 2004). In tomato, the production of ROS correlates with an increased activity of
CAT, SOD, and PRX enzymes, in both roots and shoots (Cho and Park 2000).
Lead (Pb) is one of the most abundant heavy metals in both terrestrial and aquatic
environments, predominantly arising from human activities such as mining,
smelting, the use of fuels and explosives, and the disposal of Pb-enriched municipal
sewage sludge. Together with Cd, Pb is also considered one of the most
serious hazards to human health, since it is readily taken up by plants and therefore
can easily enter the food chain. Pb toxicity causes similar symptoms to other heavy
metals, namely growth inhibition, chlorosis, and (in the most severe cases) death.
Roots that absorb Pb respond by reducing their growth rate and changing their
branching pattern. In Picea abies, the emergence and growth of secondary roots
are particularly sensitive to Pb toxicity (Godbold and Kettner 1991). In maize, Pb
perturbs the organization of the microtubule network of the root meristem,
resulting in a shorter branching zone with more compact lateral roots emerging
nearer to the root tips (Eun et al. 2000). The inhibition of root growth by Pb also
affects nutrient uptake and nitrogen assimilation. For example, the enzymes nitrate
reductase and glutamine synthetase are inhibited by Pb in Cucumis sativus and
Glycine max, respectively (Sharma and Dubey 2005). Pb also nonspeciﬁcally
blocks the uptake of other cations such as K, Ca, Mg, Mn, Zn, Cu, and Fe,
probably by modifying the activity and permeability of membranes or binding
them to ion carriers, making them unavailable for uptake and transport into the
plant (Patra et al. 2004).
High concentrations of Pb cause a water deﬁcit, reducing the transpiration rate,
altering the osmotic pressure of the cell sap and the water potential of the xylem.
These effects contribute to an overall negative change in the plant water status
(Parys et al. 1998).
Pb interacts with –SH groups like many other heavy metals, but it can also
interact with –COOH groups, inhibiting enzymes and altering protein conformation
(Sharma and Dubey 2005). Pb can also displace metal cofactors from
metalloenzymes, which includes Mn in the OEC and Mg in the chlorophyll
porphyrin ring, thus interfering with photosynthesis and electron transport
1 Heavy Metal Toxicity in Plants 9
by reducing oxygen evolution and chlorophyll levels and altering the thylakoid
membrane structure (Patra et al. 2004). Key chlorophyll biosynthesis enzymes are
also strongly inhibited by Pb, as well as many enzymes in the Calvin cycle (e.g.,
RuBisCO, phosphoenol pyruvate carboxylase, and ribulose 5-phosphate kinase)
thus reducing the rate and ef*ciency of CO
*xation (Sharma and Dubey 2005).
One unique effect of Pb is the disruption of the cell cycle by interfering with the
alignment of microtubules on the mitotic spindle. This effect cannot be replicated
with, e.g., Al and Cu, even at concentrations suf*cient to inhibit root growth (Eun
et al. 2000).
Pb is not a redox metal so cannot generate ROS directly, but oxidative stress is
caused indirectly as shown by the increased lipid peroxidation in rice and pea
plants exposed to the metal (Malecka et al. 2001). This is countered by the activation
of antioxidant enzymes such as SOD and PRX, but whereas CAT activity
increases in pea plants, it declines in rice, perhaps explaining in part why there is
an increase in lipid peroxidation (Malecka et al. 2001, Verma and Dubey 2003).
This complexity of antioxidant enzyme activity in plants under metal stress may
reﬂect the presence of diverse isoforms which have different spatiotemporal
expression proﬁles, different intracellular locations, and different environmental
triggers for activation and inactivation (Scandalios 1990).
Chromium (Cr) has received comparatively little attention from plant scientists
perhaps because it is ubiquitous in the environment and, due to its complex
electron chemistry, it exists in many oxidation states upon which its toxicity
depends. Cr pollution results from human activities such as leather processing and
*nishing, the production of refractory steel, electroplating, wood preservation, and
the manufacture of specialty chemicals and cleaning agents such as chromic acid.
There is no evidence that Cr has a speciﬁc biological in plants, and its absorption
involves the use of Fe, S, and P transporters and carriers; Cr thus competes with
these essential nutrients for binding sites. Cr ions with different oxidation state
appear to be absorbed by different mechanisms (Shanker et al. 2005). Cr stress
inhibits germination in Phaseolus vulgaris, possibly by promoting the activity of
proteases while suppressing the activity of amylases and perturbing the subsequent
transport of sugars to the embryo axes (Zeid 2001). In adult plants, Cr toxicity
inhibits shoot growth, reduces the number of leaves as well as the leaf area and
biomass, reduces the productivity of crops, causes burns on the leaf margins and
tips, and induces chlorosis and necrosis (Sharma and Sharma 1993; Singh 2001;
Jain et al. 2000). Eventually, the global plant *tness is compromised, giving
reduced plant biomass production and productivity, relevant aspects for crops and
A well-documented effect of Cr toxicity is the inhibition of primary root growth
(observed as reduced root length) and the suppression of new lateral root primordia
10 G. DalCorso
(Prasad et al. 2001). The application of Cr inhibited root elongation in Caesalpinia
pulcherrima, wheat, and Vigna radiata (Shanker et al. 2005) possibly by
disrupting cell division through chromosomal damage (Panda and Choudhury
2005). Cr stress also induces changes in root morphology, increasing the number
of root hairs and the relative proportion of pith and cortical tissue layers (Suseela
et al. 2002). The negative effects of Cr on root growth and development combined
with the tendency of Cr to compete with essential nutrients for uptake and
transport means that Cr has a signiﬁcant impact on nutrient acquisition. Although
there is some variation depending on the plant species and tissue, Cr(VI) seems to
have the most potent effect on the uptake of nutrients such as K, Mg, P, Fe, N, Zn,
Cu, Mo, and Mn (Shanker et al. 2005). As well as reducing root growth and
competing with these essential nutrients for uptake, Cr may also inhibit the activity
ATPases in the plasma membrane, which is required for proton export
from the roots and hence acidiﬁcation of the rhizosphere and the subsequent
mobilization of metal ions. Inhibition would therefore result in a general reduction
in nutrient bioavailability in the soil (Shanker et al. 2005).
The impact of Cr on plant water status in unknown, although Cr does induce the
typical symptoms of water deﬁcit and reduced transpiration, such as turgor loss,
plasmolysis, and diminished tracheary vessel diameter (Shanker et al. 2005).
Both photosystems are inhibited by Cr(VI) although the mechanisms are
still under investigation. Exposure to Cr(III) and Cr(VI) reduces the chlorophyll
content of bean seedlings and wheat plants by displacing Mg from the chlorophyll
molecule (Samantaray et al. 2001; Sharma and Sharma 1996). Cr stress also
disrupts the ultrastructure of the chloroplast, particularly the arrangement of
thykaloid membranes, probably reducing the size of the antenna complexes (Panda
and Choudhury 2005; Shanker et al. 2005).
Cr can also inhibit certain enzymes in a species-dependent manner, e.g. nitrate
reductase (Panda and Patra 2000) and root Fe(III) reductase (Barton et al. 2000),
the latter affecting Fe nutrition in the plant. In mitochondria, Cr may hamper the
electron transport interfering with the Cu and Fe ions contained in many electroncarrier
proteins. The severe inhibition of mitochondrial cytochrome oxidation, for
instance, could be due to the extreme susceptibility of complex III and IV to
Cr(VI) (Dixit et al. 2002).
Finally, Cr shares the ability of other heavy metals to induce the formation of
ROS in plant cells. Cr is not considered a redox metal, but studies have shown that
it can participate in Fenton reactions (Panda and Choudhury 2005). Sorghum
plants treated with either Cr(VI) or Cr(III) increased H
content in roots and
leaves, correlated with an increase in lipid peroxidation (Panda and Choudhury
2005; Shanker and Pathmanabhan 2004). Antioxidant enzyme activities are also
modulated by Cr, apparently in a dose-dependent manner. For example, low levels
of Cr induce SOD activity in pea plants, whereas higher concentrations inhibit
both CAT and SOD (Dixit et al. 2002; Jain et al. 2000).
1 Heavy Metal Toxicity in Plants 11
Arsenic (As) is a profoundly toxic heavy metalloid that originates from both
geogenic sources and anthropogenic activities such as mining, the combustion of
fossil fuels, and use of As-based pesticides and wood preservatives (Tu and Ma
2005). It is widely distributed in the environment and recognized as a signiﬁcant
threat to human health. The chemistry of As in the soil is complex because it can
be present in both organic and inorganic forms, but most As is present as the
oxidized mineral arsenate, AsO
As(V), and its reduced form arsenite, AsO
As(III). The bioavailability of As depends on the soil characteristics, including its
redox potential, pH, and composition, the presence of other minerals (particularly
Fe and Al oxides and hydroxides), and the abundance of microbes that can reduce
As(V) to As(III) (Smith et al. 2010). Arsenate is chemically similar to phosphate
and it is probably taken up into many plants via phosphate transporters (Pigna et al.
2009). In contrast, arsenite is more abundant and mobile in soils with a low redox
potential, and is thought to be acquired via aquaporin transporters in the plasma
membrane of root cells (Vromman et al. 2011).
As interferes with cell metabolism by reacting with –SH groups on proteins and
replacing phosphate, and inhibits plant growth (Tu and Ma 2005). The symptoms
of As toxicity include poor seed germination and profound growth inhibition
(Smith et al. 2010). In wheat seeds, for example, germination is considerably
affected by both arsenite and arsenate, probably reﬂecting the inhibition of both
a- and b-amylase (Liu et al. 2005). Maize plants treated with toxic concentration
of As(V) and As(III) produced stunted roots that were thicker and stiffer than
normal, and that had a signiﬁcantly lower mitotic index; micronuclei and
chromosome aberrations were also observed in the root meristems (Duquesnoy et
al. 2010). In some species, the effect of As on root growth depends on its
concentration. For example, root growth in Artemisia annua is stimulated at low
As concentrations but inhibited at higher concentrations (Rai et al. 2011b).
The reduction in root growth combined with changes in the selectivity and
permeability of cell membranes prevent the uptake of water and nutrients resulting
in water imbalance and nutrient deﬁciency, the severity depending on the species
(Paivoke and Simola 2001). For example, As signiﬁcantly increases the accumulation
of N, P, K, Ca, and Mg in the shoots of hydroponically grown Phaseulus
vulgaris plants (Carbonell-Barrachina et al. 1997), but reduces the uptake of
macronutrients such as K, Ca, and Mg, and micronutrients such as B, Cu, Mn, and
Zn, in tomato plants (Carbonell-Barrachina et al. 1994). Similarly, arsenite reduces
the uptake of Si, Mn, Zn, Cu, P, and K in rice plants and the translocation of these
minerals to the shoot, possibly by interacting with the –SH groups of transporters
(Hoffmann and Schenk 2011). Interestingly, in some hyperaccumulator species,
such as Pteris vittata, low levels of arsenate stimulate phosphate accumulation in the
fronds and signiﬁcantly enhance growth (Tu and Ma 2005). The water content, water
potential, and stomatal conductance of Atriplex atacamensis (Phil) leaves and roots
were signiﬁcantly reduced after prolonged exposure to As (Vromman et al. 2011).
12 G. DalCorso
Following absorption, As is thought to interfere with essential phosphate
metabolism because the corresponding enzymes can also reduce As(V) to As(III)
(Smith et al. 2010). Moreover, As(V) can be reduced nonenzymatically by
glutathione (at least in vitro; Meharg and Hartley-Whitaker 2002) which is
abundant in plants. Although As is not redox-active, it can stimulate the production
of ROS through the conversion of arsenate to arsenite (Meharg and HartleyWhitaker
2002), and can thus induce lipid peroxidation and cellular damages
(Gunes et al. 2009). Maize leaves and roots exposed to As(V) produce antioxidant
enzymes such as APX in response to the oxidative stress, whereas SOD activity
declines. Conversely, higher levels of CAT activity were measured in maize shoots
and roots exposed to high concentrations of As(III) (Duquesnoy et al. 2010).
In Bacopa monnieri plants exposed to moderate levels of As, the activities of GSR,
SOD, GPX, APX, and CAT were stimulated in a differential but coordinated
manner in the leaves and roots, presumably representing a global response to As
toxicity (Mishra et al. 2011). Artemisia annua plants treated with As showed a
dose-dependent increase in the activities of SOD, APX, GSR, and GPX followed
by a gradual decline at higher concentrations, again suggesting a coordinated
response to the oxidative stress caused by As toxicity (Rai et al. 2011b).
1.4 Essential Metal Ions: Nickel, Copper, Iron, Manganese,
Zinc, and Selenium