Morphological Characters of drought resistant plants
Drought resistance is associated with various morphological and physiological features or actors of crop plants. Morphological characters which are associated with drought resistance included earliness, shape, size and structure of stomata, size, number and orientation of eaves, presence of cuticle, Waxiness on leaf lamina and stem, rooting pattern, growth habit etc. They are 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 stomata 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 aperture 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 vapor out of the leaf.
Now infrared thermometers are used to measure leaf water status and stomatal activity. Leaves with closed stomata will exhibit higher temperature than those with open stomata. Leaves with open stomata 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 and 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.
7. High tiller survival
Comparison of old and new varieties have shown that under drought older varieties over produce tillers many of which fall to set grain while modern drought tolerant lines produce fewer tillers most of which survive.
8. Stay green
The trait may indicate the presence of drought avoidance mechanism but probably does not contribute to yield if there is no water left in the soil profile by the end of the cycle to support leaf gas exchange. It may be detrimental, if it indicates lack of ability to remobilize stem reserves. However, research in sorghum has indicate that stay green is associated with higher leaf chlorophyll content at all stages of development and both were associated with improved yield and transpiration efficiency under drought.
9. Large seed size
Help emergence, early ground cover and initial biomass.
10. Long coleoptiles
For emergence from deep sowing.
11. Early-ground cover
Thinner, wider leaves and a more prostrate growth habit help to increase ground cover thus conserving soil moisture and potentially increasing radiation use efficiency.
12. Good capacity for stem reserves and remobilization.
13. High spike photosynthesis capacity.
14. High pre-Anthesis biomass.