EFFECTS OF WATER-DEFICIT ON PHOTOSYNTHETIC RATE AND OSMOTIC ADJUSTMENT IN TETRAPLOID WHEATS

Osmotic adjustment, accumulation of soluble saccharides, and photosynt hetic gas exchange were studied in five durum wheat (Triticum turgidum L. var. durum) and one wild emmer wheat (Triticum turgidum L. var. di coccoides) cultivars of contrasting drought tolerance and yield stabil ity. Soil water contents (SWC) were 100, 31, 20, and 12 % of maximum c apillary capacity. Under mild water stress (SWC 31 to 20 %), osmotic a djustment 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 landrac es 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 se vere water stress, the decrease in P-N was mainly due to stomatal and non-stomatal factors, respectively.