**3. Drought stress**

Drought is complex and destructive in plant biology to such an extent that it is compared with cancer in mammalian biology [27]. The effect of drought varies with the timing and intensity of stress on a plant's growth and development [28].

Maize is a drought-sensitive crop, mainly in a critical stage of growth such as the seedling stage and is grown in a wide range of climatic conditions from semi-arid to temperate regions, including drought-prone areas of Africa, North and South America, Asia and Europe [2]. Drought stress during vegetative growth, especially during V1 to V5, reduces plant growth, increases the vegetative growth period and reduces the growth period of the reproductive stage [29]. The relative water content and water potential are reduced under stressed conditions.

Plants undergo morphological and physiological changes under drought stress conditions. This process can be covered under three major categories. They are drought escape, drought avoidance and drought tolerance. The combined impact of these strategies is drought resistance [30]. According to Osmolovskaya et al. [30] drought resistance is the ability of plants to maintain favorable water balance and turgidity under water stress conditions. Drought escape is a strategy in which plant complete their life cycle before the onset of drought. They show seasonal responses [30, 31]. The drought avoidance strategy integrates increased water uptake and decreased water loss by plants. Plants develop strategies such as osmotic adjustment, an extension of antioxidant capacity, and desiccation tolerance to develop drought tolerance.

Further research has been conducted to identify drought-tolerant varieties. Along with advancements in technology, the research focus has changed from morphological characterization to identifying genes responsible for drought tolerance. Photosynthesis, a major metabolic pathway in plants, is sensitive to drought stress and is involved in plant response [32]. The photosynthetic pigments are damaged by drought, which decreases the light absorption efficiency of plants [2]. Though stomata closure is a way forward to ameliorate the adverse effects of drought, a decrease in stomata opening reduces the amount of CO2 entering in leaves, which reduces carbon assimilation reaction and transpiration decreases root absorbance.
