PHYSIOLOGICAL-RESPONSES OF SPRING DURUM-WHEAT CULTIVARS TO EARLY-SEASON DROUGHT IN A MEDITERRANEAN ENVIRONMENT

The Mediterranean climate of North Africa is characterized by uncertai n rainfall immediately after seedling emergence, leading to drought ea rly in the growing season which depresses durum wheat production. Howe ver, there is limited understanding of the physiological basis of resi stance of spring durum wheat to drought in rainfed Mediterranean regio ns. The objectives of this study were to examine differences in some p hysiological characters among spring durum wheat cultivars in response to duration of early-season drought, and to determine the relationshi p 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 reg imes: well irrigated and three different water deficits from emergence until the onset of tillering, mid-tillering or at the end of tillerin g. Cultivars differed in their response. Decreases in photosynthesis s oon after drought stress was imposed resulted mainly from reduced stom atal conductance. Continued water deficits also reduced mesophyll phot osynthetic activity. Possible factors determining the drought-resistan ce of a cultivar are lower sensitivity of CO2 exchange rate, net CO2 u ptake to water loss ratio, stomatal resistance, relative water content and greater osmotic adjustment under stress. Furthermore, there is su fficient intraspecific variation in these physiological attributes to suggest their use as selection tools. (C) 1998 Annals of Botany Compan y.