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Alternate wetting and drying irrigation (AWD) has been reported to save water compared with continuous flooding (CF) in rice cultivation. However, the reported effects on yield varied greatly and detailed agrohydrological characterization is often lacking so that generalizations are difficult to make. Furthermore, it is not known how AWD modifies nutrient use efficiencies and if it requires different N-fertilizer management compared with CF. This study quantified the agro-hydrological conditions of the commonly practiced AWD and compared the impact of AWD and CF irrigations at different N-fertilizer management regimes on rice growth and yield, water productivity, and fertilizer-use efficiencies in five crop seasons in 1999 and 2000 at two typical lowland rice sites in China (Jinhua, Zheijang Province and Tuanlin, Hubei Province), with shallow groundwater tables.Grain yields varied from 3.2 to 4.5 t ha -1 with 0 kg N ha -1 to 5.3-8.9 t ha -1 with farmers' N-rates (150 kg N ha -1 in Jinhua and 180 in Tuanlin). In both sites, no significant water by nitrogen interaction on grain yields, biomass, water productivity, nutrient uptakes and N-use efficiency were observed. Yield and biomass did not significantly differ (P >0.05) between AWD and CF and among N timings. The productivity of irrigation water in AWD was about 5-35% higher than in CF, but differences were significant (P <0.05) only when the rainfall was low and evaporation was high. Increasing the number of splits to 4-6 times increase the total N uptake, but not total P-uptake, and total K-uptake compared with farmers' practices of two splits. Apparent Nitrogen recovery (ANR) increased as the number of splits increased, but there was no significant difference in ANR between AWD and CF. During the drying cycles of AWD irrigation, the perched water table depths seldom went deeper than -20 cm and the soil in the root zone remained moist most of the time. The results suggest that in typical irrigated lowlands in China, AWD can reduce water input without affecting rice yields and does not require N-fertilizer management differently from continuous flooding. The results can be applied to many other irrigated lowland rice areas in Asia which have a shallow groundwater table.

: The effects of the osmotic component of salt stress on rice cultivar IR64 were examined. Treatments were four combinations of two levels of osmotic stress at two developmental stages: medium-and highlevel stress applied at the vegetative and reproductive stages using salt (NaCl) and polyethylene glycol-6000 (PEG) as sources of osmotic stress. Both PEG and NaCl reduced the total above ground biomass and delayed flowering and maturity, with a longer delay observed with the high-level stress. The reduction in number of filled spikelets, 1,000-grain weight, and hence grain yield was significantly greater when they were applied during the reproductive stage than during the vegetative stage. The sodium concentration in plant tissues also increased in plants treated with NaCl, indicating that besides osmotic stress, plants were also subjected to ionic stress. Treatment with NaCl decreased the potassium concentration in plant tissues but did not cause significant differences in phenology, biomass accumulation, yield or N uptake compared with PEG. We concluded that the response of IR64 to NaCl was attributed to the osmotic component. These findings may be specific to IR64, which has a medium tolerance to salinity stress. Further studies are needed with longer stress durations to achieve a higher Na + concentration in plant tissues in several varieties with contrasting tolerance to salt stress to further establish the relative importance of osmotic versus ionic components of salt stress in rice.

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