Phytotoxicology - Introduction

Definition:

The study of plant poisons is known as phytotoxicology. Most of the poisonous higher plants are angiosperms, or flowering plants, but only a small percentage are recognized as poisonous. Several systems have been devised for the classification of poisonous plants, none of which is completely satisfactory. Poisonous plants may be classified according to the chemical nature of their toxic.

Phytotoxicity: Is a term used to describe the toxic effect of a compound on plant growth. Such damage may be caused by a wide variety of compounds, including trace metals, pesticides, or salinity.

Toxic metals: are metals that form poisonous soluble compounds and have no biological role, i.e. are not essential minerals, or are in the wrong form. Such as, if the concentration of mercury in environment is increased from its normal level then it will cause cancer and it is called as toxic element. Excess boron leads to rotting of roots, chlorosis, and widespread formation of galls.

Source and occurrence of trace elements in soil:

The total amount of the trace elements in soil is derived in the first place from weathering of rocks minerals, but may be increased substantially by man’s industrial and agricultural activities. Igneous and metamorphic rocks both account for 95% of the earth’s crust with sedimentary rocks making up remaining 5%.

Anthropogenic sources of elemental pollution are important in many places and regions.

Agricultural sources, inorganic insecticides and fungicides, sewage sludge and various metals; Cd of greatest concern. Automobile emissions, mining, municipal incinerators are also a source of trace and toxic elements in environment.

Many chemical processes are involved in the transformation of trace elements in soils, but precipitation–dissolution, adsorption–desorption, and complexation are the most important processes controlling bioavailability and mobility of trace elements in soils. Both deficiency and toxicity of trace elements occur in agro ecosystems. Application of trace elements in fertilizers is effective in correcting micronutrient deficiencies for crop production, whereas remediation of soils contaminated with metals is still costly and difficult although phytoremediation appears promising as a cost-effective approach. Soil microorganisms are the first living organisms subjected to the impacts of metal contamination. Being responsive and sensitive, changes in microbial biomass, activity, and community structure as a result of increased metal concentration in soil may be used as indicators of soil contamination or soil environmental quality. Future research needs to focus on the balance of trace elements in an agro ecosystem, elaboration of soil chemical and biochemical parameters that can be used to diagnose soil contamination with or deficiency in trace elements, and quantification of trace metal transport from an agro ecosystem to the environment.

Effect trace elements in soil:

Whenever any toxic and trace elements is added to soil and its concentration is increased from its threshold level it will cause adverse effect on soil composition. It will change the soil quality and soil texture.  Microbial life and activity in soil will be disturbed and the bacteria which are responsible for proper functioning of soil will be altered.  Again if artificial fertilizers are used in large quantity the threshold level of phosphorus, nitrogen and other elements such as sulpher is increased it will causes disorders and diseases in plants such as chlorsis, and plant will die.

 an accumulation of toxic components of waste in the soil, e.g. trace metals; a risk of (soil) biodiversity decline; unhealthy plant growth caused by unbalanced nutrient supply; the potential for water eutrophication through nutrient transfer from soil.

Toxic effects of various heavy metals in plants:

Contamination of agricultural soil by heavy metals has become a critical environmental concern due to their potential adverse ecological effects. Such toxic elements are considered as soil pollutants due to their widespread occurrence, and their acute and chronic toxic effect on plants grown of such soils.

a)      Effect of cadmium on plant:

The regulatory limit of cadmium (Cd) in agricultural soil is 100 mg/kg soil. But this threshold is continuously exceeding because of several human activities. Plants exposed to high levels of Cd causes reduction in photosynthesis, water uptake, and nutrient uptake. Plants grown in soil containing high levels of Cd show visible symptoms of injury reflected in terms of chlorosis, growth inhibition, browning of root tips, and finally death.

b)      Effect of zinc on plant:

Soil is also contaminated with zinc (Zn) in addition to Cd by the sewage sludge or urban composts, fertilizers, emissions from municipal waste incinerators, residues from metalliferous mining, the metal smelting industry, and other human activities. Zn is an essential nutrient for living organisms, while Cd is nonessential nonessential and potentially toxic for higher plants, animals and humans. Concentrations of Zn found in contaminated soils frequently exceed to those required as nutrients and may cause phytotoxicity. High levels of Zn in soil inhibit many plant metabolic functions; result in retarded growth and cause senescence. Zinc toxicity in plants limited the growth of both root and shoot. Zinc toxicity also causes chlorosis in the younger leaves, which can extend to older leaves after prolonged exposure to high soil Zn levels. The chlorosis may arise partly from an induced iron (Fe) deficiency as hydrated Zn+2 and Fe+2 ions have similar radii. Excess Zn can also give rise to manganese (Mn) and copper (Cu) deficiencies in plant shoots. Such deficiencies have been ascribed to a hindered transfer of these micronutrients from root to shoot. This hindrance is based on the fact that the Fe and Mn concentrations in plants grown in Zn-rich media are greater in the root than the shoot. Another typical effect of Zn toxicity is the appearance of a purplish-red color in leaves, which is ascribed to phosphorus (P) deficiency.

c)      Effect of copper on plant:

 Cu is also an essential component of various proteins like plastocyanin of photosynthetic system and cytochrome oxidase of respiratory electron transport chain. But enhanced industrial and mining activities have contributed to the increasing occurrence of Cu in ecosystems. Cu is also added to soils from different human activities including mining and smelting of Cu-containing ores. Mining activities generate a large amount of waste rocks and tailings, which get deposited at the surface. Excess of Cu in soil plays a cyto- toxic role, induces stress and causes injury to plants. This leads to plant growth retardation and leaf chlorosis. Exposure of plants to excess Cu generates oxidative stress and Oxidative stress causes disturbance of metabolic pathways and damage to macromolecules.

d)     Effect of mercury on plants:

The large input of mercury (Hg) into the arable lands has resulted in the widespread occurrence of mercury-contamination in the entire food chain Hg is a unique metal due to its existence in different forms e.g. HgS, Hg2+, Hg° and methyl-Hg. However in agricultural soil, ionic form (Hg2+) is predominant. Hg released to the soil mainly remains in solid phase through adsorption onto sulfides, clay particles and organic matters. Increasing evidence has shown that Hg2+ can readily accumulate in higher and aquatic plants. High level of Hg2+ is strongly phytotoxic to plant cells. Toxic level of Hg2+ can induce visible injuries and physiological disorders in plants. For example, Hg2+ can bind to water channel proteins, thus inducing leaf stomata to close and physical obstruction of water flow in plants. High level of Hg2+ interfere the mitochondrial activity and induces oxidative stress by triggering the generation of ROS. This leads to the disruption of biomembrane lipids and cellular metabolism in plants.

  

e)      Effect of chromium on plants:

Chromium is now a day’s added to our environment by tanning factories through waste water released from factories; this water may be used or mixes with irrigation water and cause dangerous effects on plant and its growth Toxicity of Cr has been studied in many plants. Excess of Cr causes inhibition of plant growth, chlorosis in young leaves, nutrient imbalance, wilting of tops, and root injury.

  

f)       Effect of Lead on plants:

Lead is one of the world over distributed most abundant toxic elements in the soil. The toxic level of Pb in soil results from disposal of municipal sewage sludge, mining and smelting activities, Pb containing paints, paper and pulp, gasoline and explosives. It exerts adverse effect on morphology, growth and photosynthetic processes of plants. High level of Pb also causes inhibition of enzyme activities, water imbalance, alterations in membrane permeability and disturbs mineral nutrition. Pb inhibits the activity of enzymes at cellular level by reacting with their sulfhydril groups.