Preface It is well known fact that physiology studies are fundamental study and it help in clearing the fundamental issues of other disciplines of agriculture. This book on the physiological issues under abiotic stress has its own importance due to shift in climatic pattern under global warming regime. As the growing world population together with the lack of expansion, or even reduction of available arable lands needed to maintain agricultural sustainability, implies that the relative importance of plant breeding to raise crop yield potential and adaptiveness is now greater than in the past. This is not a minor expectation. Although there is a rising demand for irrigation and chemical fertilizers, the technological progress made in cereal cultivation over recent decades has led to a decline in the cost of cereal production per unit of output. Without this growth in productivity, many developing countries would have been forced to further extend cultivation into marginal lands, thus aggravating the dilemma of how to sustain natural resource bases. In the case of wheat, yield has been genetically enhanced virtually all over the world. The magnitude of the improvement has depended upon the environmental conditions of the region. However, when wheat yields are expressed as a percentage of the mean yield of the trial in which they were assessed, the data appear to converge into a single trend.

The extreme temperatures those are consequences of present-day global climate changes are considered as major abiotic stresses for crop plants. Different studies clearly show that temperatures exceeding the limits of adaptation substantially influence the metabolism, viability, physiology, and yield of many plants. Plants exposed to extreme temperatures often show a common response in the form of oxidative stress. However, the extent of damage caused by extreme temperatures depends greatly on the duration of the adverse temperature, the genotypes of the exposed plants, and their stage of growth. There is ample need to develop temperature tolerance in crop plants by exploring suitable strategies. Numerous research findings support the notion that induction and regulation of antioxidant defenses are necessary for obtaining substantial tolerance against environmental stress. The development of genetically engineered plants, by the introduction and/or overexpression of selected genes, would to be one feasible strategy. However, plant adaptation to either high temperature or low temperature is a multigenic response which is very complex in nature. Thus the task of identifying the traits those correlate with stress tolerance is incredibly difficult for researchers. Plant response to continuous light depends on plant tolerance and can be modified by alteration in temperature, light intensity, CO2 level, and other environmental factors.

Most food legumes have a long history of domestication almost as long as cereals, and during this time they have been subjected to conscious and unconscious selections for better adaptation to environmental condition as well as better seed yield. In general, crops during their ontogeny faces a number of abiotic stress viz., excessive and/or low soil water stress, soil salinity stress, high as well as low temperature stress. It has been well documented that crop yields would be greater in many cropping regions if more water were available.  Availability of water for agriculture is being challenged increasingly because of growing demand for water from other sectors such as industry, urban use, and for social and environmental purposes. Water stress may conceivably arise either from an insufficient or from an excessive water activity in the plant’s environment. Many physiological characteristics are correlated with the water potential of mesophyll tissue but the correlations are species specific. There is a general hierarchy of sensitivities among general physiological activities. Most sensitive are cell expansion, cell wall synthesis, protochlorophyll formation, and nitrate reduction. Generally turgor pressure is still accepted as the best indicator of water stress in plants. The specific mechanism by which turgor regulate physiological function probably relate to cell walls and membranes. Cell membrane structure, and spatial arrangement of enzyme, transport channels, cellulose synthesis rosettes, and receptor proteins may be dependent on turgor pressure.

We do not know when intercropping began nor why early civilizations fostered its use. Whether by design or accident, intercroppinq dominated early agriculture and is still practiced in many areas of the world. With the advent of “modern” agriculture, intercropping began disappearing from many areas. This shift was driven primarily by mechanization and specialization. Despite pressures to abandon intercropping, it has survived and flourished Inereasing interest in sustainability and environrnental concerns have drifted attention back to intercropping as a means of better utilization of resources while preserving the environment. Through intercropping, farmers can achieve the full production of the main crop and also an additional yield (bonus) associated with an increased plant population of the second component. Hence, intercropping can increase incomes obtained by smallholder farmers in areas where labor is not shortage, through reduction of economic risk and market fluctuation resulting from growing a single crop which is more prone to natural hazards and helping the farmers in better utilization of land by having more than one crop produced per unit area. Though all intercrops produced higher productivity, the farmers could better use the appropriate population of component crops in intercropping systems in order to maximize yield of both crops as well as total productivity. It is, therefore, important to support intercropping systems with appropriate agronomic practices such as timely irrigation, pest protection and the likes to sustain the cropping system in the countries where labor is not problem for proper management in general even though sometimes sole cropping may became more productive.

Marginal land areas will need to be used to meet the increasing requirement of future generations, especially in developing countries. These marginal areas commonly impose abiotic stresses on crops due to factors such as salinity, drought, flooding, low nutrients and aluminium or heavy metal toxicity. As a consequence, the growth and yield of crops from such areas is typically low and their quality in also poor. Endogenous plant growth regulators play an important role in regulating plant responses to abiotic stress by sensitizing growth and developmental processes. While the physiological and molecular mechanisms linked to the role of ABA and cytokinins in stress tolerance are well explained, there is growing interest to elucidate the associations of auxins, ethylene, gibberellins, brassinosteroids, and polyamines in stress tolerance mechanism and also on possible cross talk mechanism among different growth regulators during stress tolerance acquisition. Identification and characterization of the gene regulating synthesis of different endogenous growth regulators and recent progresses on hormonal signaling, mutant research, and physiological actions have provided scope for manipulating their biosynthetic pathways for developing transgenic crop plants with enhanced abiotic stress tolerance. Researches have also provided some leads in exploiting the potential of growth regulators in enhancing the resistance to abiotic stresses of crops. Plant growth-regulators (PGRs), are biochemical and chemical compounds stimulates plant growth and productivity when applied, even in small quantities at appropriate plant growth stages. These are being extensively used in agriculture to enhance the productivity in agricultural crops. Their central role in plant growth and development is through nutrient allocation and source-sink transitions while most of the plant bioregulators (PBRs) stimulate redox signaling under abiotic stress conditions.

Plant–water relations concern how plants control the hydration of their cells, including the collection of water from the soil, its transport within the plant and its loss by evaporation from the leaves. Flow of water through plant and soil over macroscopic distances is driven by gradients in hydrostatic pressure. Over microscopic distances it is driven by gradients in water potential. Evaporation of water from leaves is primarily controlled by stomata, and if not made good by the flow of water from soil through the plant to the leaves, results in the plants wilting. Plant water deficit is initiated as the crop demand for water exceeds the supply. The capacity of plants to meet the demand and thus avoid water deficit depends on their “hydraulic machinery.” This machinery involves firstly the reduction of net radiation by canopy albedo, thus reflecting part of the energy load on the plant. Secondly, it determines the ability to transport sufficient amount of water from the soil to the atmosphere via the stomata in order to provide for transpiration, transpirational cooling and carbon assimilation. Water is transported by way the soil-plant-atmosphere continuum and it is largely controlled by the resistances in the continuum as determined by root, stem, leaf, stomata and cuticular hydraulic resistances. The gross effects of deficient and of excessive soil moisture on plant growth are well known, but controversy has existed for many years around the question whether the so-called “‘available moisture” is equally available for plant growth or available only with such increasing difficulty that plant growth functions are retarded before the wilting point is reached. Various measurable aspects of plant growth do not respond in the same manner to increasing moisture stress. Crops will respond to irrigations, although measured soil moisture stress is quite low. Problems of relating plant growth to soil moisture stress conditions varying with both time and soil depth are considered. Soil factors affecting density or depth are reviewed and several ways weather factors influence soil, -moisture-plant-growth relations summarized.

Partitioning of assimilated carbon among sink organs is a critical factor that controls the rate and pattern of plant growth. Environmental stress often restricts resource availability while successful acclimation sets in motion processes that restore the supply. A key mechanism contributing to regulation of carbon partitioning is an expression of genes that control activity of the enzymes which initiate sucrose metabolism at specific sites and stages of ontogeny. Drought stress not only limits the size of the source and sink tissues but the phloem loading, assimilate translocation and dry matter portioning are also impaired. However, the extent of effects varies with the plant species, stage, duration and severity of drought. Crop production faces many challenges, due to changing environmental conditions and evolving needs for new plant-derived materials.  The extreme temperatures those are consequences of present-day global climate changes are considered as major abiotic stresses for crop plants. Temperatures exceeding the limits of adaptation substantially influence the metabolism, viability, physiology, and yield of many plants. Plants exposed to extreme temperatures often show a common response in the form of oxidative stress. However, the extent of damage caused by extreme temperatures depends greatly on the duration of the adverse temperature, the genotypes of the exposed plants, and their stage of growth. Numerous research findings support the notion that induction and regulation of antioxidant defenses are necessary for obtaining substantial tolerance against environmental stress. Plant adaptation to either high temperature or low temperature is a multigenic response which is very complex in nature. Plant response to continuous light depends on plant tolerance and can be modified by alteration in temperature, light intensity, CO2 level, and other environmental factors.

Though the pesticide application represents viable solution to pest control, however indiscriminate use poses threat to target as well as non-target crops. The side effects of pesticides therefore have to be considered based on their use and on the agricultural system to which particular pesticide is used. Studies should be implied on effects and persistence of pesticides in crops and is consequent effects on soil microbial flora and associated nitrogen metabolism. Safe alternate methods like development of relatively cheaper biopesticide should be encouraged. More efficient methods have to be developed and validated for dissipation of pesticide residues in food grains. Human health and environment has gained great importance in the modern world. The widespread use of insecticides has resulted in problems caused by its interaction with the natural biological systems. Study showed that higher amounts of acetamiprid have some negative effects on corn plant physiology. Thus, the use of pesticide should be controlled following the standards of developed countries to protect health and the environment.