**1. Introduction**

Global wheat production in the key production areas is being threatened by recurrent drought situation which is predicted to increase with climate change. Droughttolerant wheat varieties are the ultimate solution of safeguarding the crop against adverse effects of drought [1]. Plants are frequently exposed to environmental stresses both due to some natural cause and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

rough agricultural practices. Various types of both biotic and abiotic stresses may result in limited plant productivity. Plant stresses like oxidative, chemical toxicity, drought and salinity, extreme temperatures along with the attack of insects, pests, and plant pathogens result in significant crop losses which are a serious threat to agriculture [2].

**2. Plant stresses**

**3. Water-use efficiency**

Most plants take up CO<sup>2</sup>

the leaf. The concentration gradient of CO<sup>2</sup>

Environmental stresses are the main cause of limited crop production in the world. The land is affected by mineral stress about 20%, by drought stress about 26%, and 15% by freezing stress [13]. Environmental stresses are of two types: biotic stresses and abiotic stresses. Biotic stresses include infection and competition by other organisms. Abiotic stresses include light, temperature, water (drought), excess (flooding), radiation, and salinity stress. The capacity of plants to cope with unfavorable environments is known as stress resistance. Plant adaptations to tolerate stress depend upon genetically modified resistance genes that improve resistance as a result of prior exposure of a plant to stress. The mechanisms of drought resistance may fluctuate with climate change and soil conditions. Leaf expansion is restricted by water stress as one of the earliest responses occurring when decreases in turgor resulting from water deficit reduce or eliminate the driving force for cell and leaf expansion. Leaf abscission mechanisms start due to water stress and root extension into deeper, wetter soil, and stomatal closure as a response of water deficiency. Water deficit leads to the gene expression involved in acclimation and adaptation to the stress. The sensing and activation of signal transduction cascades mediating these changes in gene expression involve both an ABAdependent pathway and ABA-independent pathways [9]. Anjum et al. [14] have studied the droughtinduced changes in growth, osmolytes accumulation, and antioxidant metabolisms of maize hybrids. Drought stress in crop production system is much more precarious than any abiotic stress due to climatic changes. According to them, physiobiochemical regulation of plants under drought stress can be used as markers for drought tolerance in selection and breeding purposes. The maize growth and yield responses were highly related to ROS production, osmolytes accu-

Role of Osmolytes and Antioxidant Enzymes for Drought Tolerance in Wheat

http://dx.doi.org/10.5772/intechopen.75926

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mulation, and activation of antioxidative defense system under drought situation.

The water-limited productivity of plants depends on the total amount of water available and on the water-use efficiency of the plant. Any plant capable of acquiring more water or that has a higher water-use efficiency will resist drought better. When water shortage develops slowly, it is sufficient to permit changes in developmental process as water stress has more than a few adverse effects on plant growth. In this situation, compatible solutes like proline, glycine-betaine, and trehalose are produced to counter the unfavorable cellular conditions. Osmotic adjustment (OA) is a net increase in these solute contents per cell, and it develops slowly in response to tissue dehydration and maintains turgor and osmotic pressure of effected plant species. Osmotic potential fluctuation by the soil solution creates the stress in plants by water ultimately leading to plant death as a result of growth arrest and molecular damage. Osmotic adjustment

in plant cells helps to maintain plant water balance to carry on regular life processes [9].

exposed plant surfaces as an effective barrier to water loss that protects the plant from desiccation. These plants cannot prevent outward diffusion of water without excluding CO<sup>2</sup>

gradient that drives water loss. If water reservoir is higher than usual, this triggers regulation

from atmosphere while limiting water loss. The cuticle covers

uptake is much smaller than the concentration

from

Drought is significantly damaging the plant that further limits the crop productivity, and most countries are facing this big disaster. Plant productivity is greatly inclined due to stressful conditions that affect almost every aspect of plant growth. All plants develop a unique pattern of biochemical and molecular mechanisms to manage odd situations linked with stress tolerance. Environmental stresses like drought, high salinity, and low temperature initiate gene expression that raise osmolytes and antioxidant enzyme levels in plant cells to tolerate stress responses.

Due to everincreasing population around the globe, food crop productivity is highly enviable, and it is the need of the hour to expand measures for maximum produce. Wheat occupies an important place as staple food and the yield improvement of the wheat germplasm under different stresses and agro-climatic situations [3] have an essential task for researchers to deal within the present scenario. Human food comprises many valuable produce like rice, pulses, and meats but wheat is among the most important for human consumption worldwide. A part of the total wheat crop production is also used as a feed for livestock. *Triticum aestivum* is common bread wheat, but the other two species of wheat are of commercial importance as *T. durum* is pasta product wheat and *T. compactum* is pastry flour wheat. About 35% of the human population consumes wheat as food, covering 29% of caloric intake. Wheat shares the largest cereal market due to its global production at more than 651. 4 million metric tons per annum [4]. The high nutritive value (>10% protein, 2.4% lipids, and 79% carbohydrates) of wheat is based largely on its ingredients and the versatility of its use in the production of a wide range of food products [5]. The global climate change is facing adverse effects, while some other areas that have adopted the effects of climate change have shown benefits to crop system. Wheat is grown in the regions where rainfall ranges 30–113 cm [6]. Plant productivity is hampered by environmental stresses [7], and water shortage definitely limits plant growth and productivity even more than any other environmental factor [8]. Elevated levels of stress hormone viz. ABA has been associated with water stress tolerance in crop plants. Heat regulates stomatal conductance and water loss under desiccation that results in the accumulation of osmolytes like proline, mannitol, glycine-betaine, and soluble sugars like trehalose which lower the osmotic potential of the cell sap and thus prevent the movement of water out of the cell [9].

Pakistan has faced recently a big problem of wheat shortage due to drought situation and had to import wheat to fulfill the need of the country. Our globe is affected by drought: about 45% of the land area mostly of Africa (Ethiopia) and its surroundings, most of the Mediterranean, Mexico, Australia, and some parts of Middle East, India and Sindh province area of Pakistan. Irrigated land is only 15% of total cultivated land which yields twice as much as rain-fed land and producing one-third of the world's food [10]. Plant stress also plays a major role in determining the distribution of plant species due to soil texture and climate limitations. To maintain a gradient of water flow into the plant, the soil water potential is much important but its reduction may lead to increased soil solutes that make it increasingly difficult to establish osmotic pressure. The resulting osmotic stress leads to stomatal closure in some plant species [11] and a reduced rate of photosynthesis [12].
