Plant Breeding for Drought Resistance
There are several environmental factors that have adverse effects on normal growth ad development of crop plants. Such factors include deficiency or toxicity of minerals, moisture deficit and low temperature, soil salinity and alkalinity. Soil acidity and environmental pollutants. These factors are known as abiotic or no biotic factors. The stress or adverse condition caused by such factors for growth and development of crop plants are referred to as abiotic stress or environmental stress. Crop plants often suffer from abiotic stresses resulting I significant reduction in both yield and quality. In some areas, the soil and weather conditions are so much unfavourable that cultivation of certain crops becomes very difficult and non profitable. Under such situation, it is essential either to develop stress resistant cultivars or to modify the environment is the most practical way of solving such problems. This chapter deals with breeding for drought and salt tolerance.
In India, problems of drought, salinity and alkalinity are of major concern among environmental stresses. Drought refers to the condition of soil moisture deficiency or water scarcity. Soil drought is more common in the arid and semi arid tropics and in the areas of steep slope. Thus desert areas are more prone to drought conditions. The ability of crop plants to grow, develop and reproduce normally under moisture deficit conditions is referred to as drought resistance. In other words, drought resistance refers to survival of plants under water deficit or scarcity conditions without injury. Improvement in the drought tolerance ability of a plant is known as drought hardening. Main features of drought are given below:
1. Drought is characterized with soil moisture deficit or low soil moisture.
2. Arid and semi-arid areas are more prone to drought than humid zones. About 36% of the land area constitute arid and semi arid zones.
3. Drought leads to reduction in both yield and quality of economic product in crop plants. It has adverse effects on plant growth and development.
4. Drought damages chloroplasts and lowers output of the photosynthetic apparatus.
5. There is an increase in proline level in the leaves of plants which are subjected to drought, Proline level can be used as an indicator of water stress, but not as a measure of drought resistance.
6. The occurrence of drought depends on the amount and distribution pattern of rainfall. If the rainfall is adequate and well distributed over the crop season, there are less chance of drought. On the other hand , when rainfall is erratic there are more chances of drought. Soil type and topography also affect drought.
7. Drought resistance is a genetically controlled physiological property of plant species. Resistance to drought is associated with various morphological and physiological features of the plant.
8. Xerophytic plants are more resistant to drought than mesophytes.
9. There is an increase in abscisic acid content in leaves of barley, and in ethylene level in cotton and wheat under drought conditions.
Mechanisms of Drought Resistance:
There are four different mechanisms which help in survival of plants under moisture deficit conditions. There are: 1) Drought escape, 2) Drought avoidance, 3) Drought tolerance, and 4) Drought resistance. The last one refers to true drought resistance. These are briefly described below:
1. Drought Escape:
The simplest way of survival under drought conditions is to escape drought. Generally, drought occurs either in the mid or late-crop season. Drought escape is most common in case of plants grown in desert regions. They complete their life cycles in 4 to 6 weeks. Drought escape also plays an important role in some crop plants. For Example, yields of early varieties of wheat, sorghum, maize, and rice are less affected by severe drought than late maturing ones. All these crops have determinate growth habit. In spring wheat, late maturing varieties give higher yield than early types especially when drought occurs early in the season and is over before anthesis.
2. Drought Avoidance:
Drought avoidance refers to ability of the plant to maintain a favourable internal water balance under moisture stress. In other words, plants which avoid drought retain high water contents in their tissues. Drought avoidance can permit a longer growth period in the crop through reduced water use or increased water uptake. However, drought avoidance leads to reduction in photosynthesis and thereby reduction in the growth of aerial parts. It leads to increase in root development and therefore, is more important than drought tolerance. In cereals, drought avoidance operates during vegetative phase, while tolerance operates during reproductive phase. Drought avoidance mechanisms are of two types. First those which reduce water loss through transpiration. Such features include stomatal characteristics and shape, size and orientation of leaves. The second, those which maintain water uptake during drought period.
3. Drought Tolerance:
The ability of crop plants to withstand low tissue water content is referred to as drought tolerance. Drought tolerance is more desirable because the crop can produce more yield at lower water potential. In cereals, drought tolerance generally operates during reproductive phase. Tolerant cultivars exhibit better germination , seedling growth and photosynthesis. In Sorghum, a drought resistant line exhibited higher photosynthetic rate at leaf water potential than a less drought resistant line. Drought tolerance differs from drought avoidance in several aspects.
4. Drought Resistance:
Drought resistance is the sum of drought avoidance and drought tolerance. In other words, drought resistance refers to the ability of crop plants to give good yield under moisture deficit conditions. Drought resistance is measured in terms of various mechanisms associated with drought tolerance and yield under soil moisture deficit. In winter wheat, both avoidance and tolerance features are important for drought resistance.