Effects of water deficit on photosynthetic rate and osmotic adjustment in tetraploid wheats

Osmotic adjustment, accumulation of soluble saccharides, and photosynthetic gas exchange were studied in five durum wheat (Triticum turgidum L. var. durum) and one wild emmer wheat (Triticum turgidum L. var. dicoccoides) cultivars of contrasting drought tolerance and yield stability. Soil water contents (SWC) were 100, 31, 20, and 12% of maximum capillary capacity. Under mild water stress (SWC 31 to 20%), osmotic adjustment capacity and high accumulation of saccharides were found in cv. Cham1, a high yielding and drought tolerant cultivar, and in var. dicoccoides, while lowest values were noted in the durum wheat landraces Oued-Zenati and Jennah-Khotifa. Under more severe water stress (SWC 12%), the cv. Cham1 maintained higher net photosynthetic rate (P(N)) than other genotypes. The observed changes in the ratio intercellular/ambient CO2 concentration (c(i)/c(a)) indicated that under mild and severe water stress, the decrease in P(N) was mainly due to stomatal and non-stomatal factors, respectively.