Physiological responses of spring durum wheat cultivars to early-season drought in a Mediterranean environment

The Mediterranean climate of North Africa is characterized by uncertain rainfall immediately after seedling emergence, leading to drought early in the growing season which depresses durum wheat production. However, there is limited understanding of the physiological basis of resistance of spring durum wheat to drought in rainfed Mediterranean regions. The objectives of this study were to examine differences in some physiological characters among spring durum wheat cultivars in response to duration of early-season drought, and to determine the relationship of these characters to drought resistance. In two held experiments (1995 and 1996 growing seasons) and a glasshouse experiment (1996), six spring sown durum wheat cultivars were evaluated under four water regimes: well irrigated and three different water deficits from emergence until the onset of tillering, mid-tillering or at the end of tillering. Cultivars differed in their response. Decreases in photosynthesis soon after drought stress was imposed resulted mainly from reduced stomatal conductance. Continued water deficits also reduced mesophyll photosynthetic activity. Possible factors determining the drought-resistance of a cultivar are lower sensitivity of CO2 exchange rate, net CO2 uptake to water loss ratio, stomatal resistance, relative water content and greater osmotic adjustment under stress. Furthermore, there is sufficient intraspecific variation in these physiological attributes to suggest their use as selection tools. (C) 1998 Annals of Botany Company.