Traits to Study Under Drought Stress in Wheat
Authors: R.S. Tomar and S. Tiwari
Drought is a major constraint and environmental factor in limiting wheat productivity in many parts of the world particularly in rainfed agriculture. In the Indian sub-continent too, the wheat production is often limited by a sub optimal moisture conditions largely due to fluctuations in rainfall from year to year and among sites within the year. Cultivars that produce high yield or suffer minimum loss under limited water during drought seasons are preferred by the farmers. Year to year fluctuations in the amount, frequency and duration of rain, higher or lower precipitation and other such factors such as high temperature at grain filling stage influences crop growth and leads to substantial productivity decline.
Direct indices like that of breeding for abiotic stresses are not available with breeders for developing genotypes under drought situations. Slow development of cultivars for drought conditions has been tempered by the lack of knowledge of mechanism of tolerance, poor understanding of mode of inheritance of tolerance to drought, low heritability and lack of efficient techniques for screening the germplasm and the segregating generations. Developing new varieties with greater resistance/tolerance to abiotic stresses is considered a sustainable and economically viable solution to this problem. However, the multifaceted nature of drought coupled with limited knowledge on genetics and physiological bases of productivity under water deficit have hindered breeding to improve crop adaptation to such conditions. Under the circumstances of climatic change, new crop varieties and the technology to cultivate them is a necessity of time. The task of breeders will be complicated if they are not provided with the genetic resources. As of today, the movement of germplasm is impeded by international binding. To meet these challenges, the germplasm collections, their assessment and characterization is necessary. A proper documentation system that serves the interest of breeders and researchers has to be developed.
Up till now in India wheat production has increased as a result of increased yield potential under high input, proper water management and maneuvering biotic stresses. Productivity has enhanced markedly under high input environments, whereas it remained static or marginally improved under abiotic stresses. Serious concerns are expressed over yield plateau but unforeseen factors like increased magnitude of water stress coupled with high temperature due to climate change can further suppress the productivity, more particularly under stress environment. Cropped area under wheat cultivation is unlikely to increase because of pressure from urbanization and industrialization, therefore, the increased productivity under any given environment can come from economically sustained methods of crop improvement and judicious use of available moisture. One of the best and still not greatly exploited means is to breed wheat resistant to variable abiotic stresses.
Symptoms in abiotic stresses
The expression of drought induced symptoms appears extremely unpredictable because they are confounded with effect from several other factors like temperature. The impact of drought is further influenced by soil type and plant density etc. and complicated by rainfall pattern and distribution, wind velocity, evapo-transpiration, the type of genotypes grown and the growth stages. .Symptoms under various types of stresses vary with the severity of the stress. Abiotic stresses are able to adversely affects plant growth, development and reproduction through different mechanism operating and there are no universal symptoms of stress as with certain mineral deficiencies create confusion, Therefore, many a times breeders are not able to discern the cause, which are some times visible injuries of plant parts and many a times invisible but measurable injuries. The traits which have been suggested by various workers for drought tolerance can be mainly grouped as follows:
Germination and seedling emergence, seedling establishment in low moisture content, tillering, biomass accumulation up to anthesis, shape and size of the leaf, leaf area, leaf glaucousness, root depth, root thickness, root shoot ratio, duration of greenness, number of grains per spikelet and thousand kernel weight.
Stomatal conductance, leaf rolling, osmotic regulation and adjustment, excised leaf water retention capacity, relative water content in leaf, cell membrane stability, chlorophyll content, chlorophyll fluorescence, grain growth period, evapotranspiration, carbon assimilation and its partitioning and translocation etc.
Biochemical or metabolic traits
Chlorophyll stability, net photosynthetic rate, photorespiration, abscicic acid (ABA) level, proline content and accumulation, betaine accumulation, occurrence of specific fatty acids and production of heat shock proteins etc.
About Author / Additional Info:
Research Associate at NRCPB