Basis of Drought Resistance
Drought resistance is associated with various morphological and physiological features or factors of crop plants. Morphological characters which are associated with drought resistance included earliness, shape, size and structure of stomato, size, number and orientation of leaves; presence of cuticle; Waxiness on leaf lamina and stem, rooting pattern, growth habit etc. Various physiological characters which are related to drought resistance are photosynthetic rate, transpiration rate, osmotic concentration etc. These factors are briefly discussed below:
Earliness is a desirable character which leads to drought escape in many crops. For example, in wheat, sorghum, maize, and rice yield of early maturing varieties is less affected by severe drought than late maturing varieties.
2. Stomatal Features:
Sunken, small size and less number of stomato are associated with drought resistance. Control of stomatal aperture is important in drought resistance. The rapid closing of stomata during development of drought helps in maintaining higher water potential in the tissues by reducing transpiration rate and thus resulting in drought avoidance.
The stomatal aperature is measured with the help of porometers. Drought resistant genotypes have rapid closing habit of stomata. Porometers are of two types, viz. 1) Viscous flow porometers which measure out flow rate of air through the leaf, and 2) diffusive flow porometers which measures the rate of diffusion of water vapour out of the leaf. Now infrared thermometers are used to measure leaf water status and stomatal activity. Leaves with closed stomato will exhibit higher temperature than those with open stomata. Leaves with open stomato have cooling effect due to water loss through transpiration.
3. Leaf Characters:
Cuticular thickness and Waxiness of leaf surface help in reducing transpiration. These characters are genetically controlled. Leaf rolling is an indicator of stress. It can also serve as drought avoidance mechanisms. Leaf rolling reduced transpiration from 46 to 63% in some grasses of Mediterranean region. In cotton, small and thick leaves are associated with drought resistance. Leaf hairiness lowers the leaf temperature and thus reduces transpiration. In barley, light green and golden leaves reflect more light than dark green leaves and thus remain cooler. The genotypes which reflect more light have more cooling effect resulting in reduction of transpiration.
4. Rooting Patterns:
Increase in depth, width and branching of root systems leads to decrease in plant water stress. Generally, deep rooted plants exhibit greater drought avoidance than shallow rooted ones. Wheat cultivars that produce greatest root mass under drought conditions are important in breeding for drought resistance if the depth is more than 60 cm. Breeding for root patterns associated with drought resistance has been successful in several crops such as soft and hard wheat, barley, corn and sunflower. Two drought resistant cultivars of durum wheat have been developed in Canada through the use of extensive root system. The new varieties combine deep root system, good grain quality and high yield. seedling root growth is an indication of root growth at maturity.
5. Growth Habit:
In upland cotton , interminate genotypes yielded more than determinate genotypes in a semiarid environment. Interminate plants produce flowers throughout the growing seasons whenever sufficient moisture is availability. This is not possible in case of determinate genotype.
In wheat ad barley, presence of awns appears to be associated with high yield under drought conditions. The increase in yield from awns results due to increase in seed size. Awns play important role in growth and development of seeds through increase in photosynthetic surface of spike.
Resistant genotypes maintain high photosynthesis under moisture stress conditions by restricting transpiration water loss. Hence photosynthesis is a reliable parameter to select drought resistant genotype in segregating materials in breeding programmes. In maize, photosynthesis was reduced by stress and translocation within leaves was also reduced. In spring wheat, dehydration hardiness is associated with small cell size, high concentration of cell sap, increased protoplasmic permeability and high viscosity of cytoplasm.