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Abstract
Although it is generally accepted that drought tolerance is a critical agronomic trait, efficient and predictable improvement in drought tolerance in bread wheat (Triticum aestivum L.), in varying drought stress conditions, has not been fully achieved. This study aimed at assessing the responses of bread wheat to drought, using physiological traits associated with drought tolerance. Two experiments were undertaken in 2001 and 2002. In Experiment I, 17 wheat genotypes were planted in the field in Katumani, Kenya, using a randomised complete block design. Experiment II was conducted under rain shelter (by simulating early season drought at seedling stage) at National Plant Breeding Research Centre, Njoro, Kenya. Treatments were imposed in a split-plot design with water regime (low, medium and high) as main plots and 12 wheat genotypes as sub-plots. Evapo-transpiration (ET) was determined by monitoring soil moisture content at 7 d intervals using a neutron probe, stomatal conductance (g) and instantaneous transpiration (T) rates were measured on the uppermost fully expanded leaves at booting stage using a steady-state porometer, net leaf CO2 exchange rates (CER) was measured on selected leaves using a portable Infrared Gas Analyser, fitted with Parkinson Leaf chamber, and crop biomass, grain yield and harvest index (HI) determined at harvest maturity. Water use efficiency (q), HI, stomatal conductance, and CER were identified as key control points in determining the drought resistance of tolerant genotypes. Therefore it is important to determine the heritability of these traits in order to ascertain their potential usefulness in a wheat breeding program.