ABSTRACT
Stress conditions emanating from unfavorable environmental conditions created by, for example, intensive radiation, extreme temperatures, lack of oxygen, excessive heat, pollutants, and water scarcity influence the plants in various ways. Irradiance far above the light saturation point of photosynthesis causes light stress leading to photoinhibition and reduction in photosynthetic rates. Photosynthesis is more sensitive to temperature fluctuations than respiration. High temperatures strike an imbalance between photosynthesis and respiration and also result in an increase in membrane-lipid fluidity coupled with a decrease in hydrogen-bonding strength and damage of the three-dimensional structures of proteins, including enzymes. Shearing of cell organelles and cell membranes, intercellular crystallization of the water, external crystallization followed by cellular dehydration, decrease of water potential in the apoplast and increase in the symplast, and severe dehydration are the main consequences of the plants’ exposure to low temperatures. Stress caused by hypoxia and anoxia damage plant roots and seeds and cause lower water diffusion rates, accumulation of toxic chemicals, damage of roots, etc. Stress due to salinity causes adverse effects on photosynthesis, plant growth, and osmotic potential. Heat stress leads to water deficit and inhibition of photosynthesis and respiration and faster depletion of carbohydrate reserves. A variety of pollutants imposes physical, chemical, and biological constraints not only on the species productivity but also on the structure and functions of an ecosystem. Stress caused due to heavy metals commonly leads to the generation of the reactive oxygen species, such as O−2, H2O2, and OH, that bear strong oxidizing activities that can attack all types of biomolecules. Prolonged water stress leads to morphological, anatomical, and cytological changes, reduced efficiency of photosynthesis, adverse effects on protein and lipid synthesis, poor mineral nutrition, abscisic acid accumulation, proline accumulation, and oxidative stress.
