3Agronomy Department, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran.
Sustainability of rice production under limited water conditions is threatened by increasing irrigation water scarcity. Therefore, physiological and morphological responses of rice to varying water stress management strategies should be determined. The physiological and morphological responses of a semi dwarf rice (Hashemi cultivar) to water stress intensities (mild and severe, i.e., short-duration of stress with early recovery and long-duration stress with late recovery, respectively) and timing (mid-tillering, booting and 50% of flowering) were studied in a pot experiment. The severe water stress at mid-tillering significantly (P<0.05) decreased plant height and the number of panicle per hill and delayed flowering. The severe water stress at different growth stages caused substantial yield losses by large percentage of unfilled grains. Root weight was highest under mild water stress at 50% of flowering followed by severe water stress at mid-tillering. Relative transpiration (RT) was not lowered until the thresholds of the fraction of transpirable soil water (FTSW), plant available water (PAW) and soil water tension (h) reached to 0.46 (-), 0.9 (-) and 78.0 kPa, respectively. These values were approximately close to those obtained for the relative leaf development rate (RL), as 0.44 (-), 0.91 (-) and 74.6 kPa, respectively. These results indicated that transpiration and leaf development rates have the same sensitivity to water deficits. However, in the mid-tillering, RL was more sensitive to water deficit than RT. The results of thresholds for RT indicated that booting stage is more sensitive than other stages. It is concluded that water tension of 1500 kPa as considered for permanent wilting point is not suitable for rice. Therefore, PAW can not be a suitable soil-water criteria for rice plants that are sensitive to water deficit. Furthermore, It is concluded that Hashemi cultivar is very sensitive to mild and severe drought stress during reproductive stage.