Water management in irrigated rice - Rice Knowledge Bank ...

tive to water deficit than cell enlargement.Leaf area expansion is reduced as soon asthe soil dries below saturation (tensionshigher than 1 kPa) in most cultivars, andwhen only about 30% of the available soilwater has been extracted in cultivars withaerobic adaptation (Lilley and Fukai 1994,Wopereis et al 1996).2. Closure of stomata, leading to reduced transpirationrate and reduced photosynthesis.Leaf stomata do not close immediately withdrought stress, however, and the crop keepson photosynthesizing for a certain periodbefore stomata close. The assimilates arenot used for leaf growth or expansion (seepoint 1), but are stored in the existing leaves,stems, and roots. When drought stress isrelieved, these assimilates may becomeavailable and lead to a flush in leaf growth.In the modern high-yielding variety IR72,stomatal closure starts at soil water tensionsof 75 kPa (Wopereis et al 1996).3. Leaf rolling, leading to a reduction in effectiveleaf area for light interception. Leafrolling in IR72 starts at soil water tensionsof 75 kPa (Wopereis et al 1996). Leaves unrollagain when drought stress is relieved.4. Enhanced leaf senescence, leading to reducedcanopy photosynthesis. Enhancedsenescence in IR72 starts at soil water tensionsof 630 kPa (Wopereis et al 1996).5. Changes in assimilate partitioning. Rootsgrow more, at the expense of the shoot,during vegetative development, whereaspartitioning of assimilates among variousshoot components is not affected. Deeperroots are effective for exploring water storedin deeper soil layers.6. Reduced plant height (though it is not likelythat reduced plant height in itself will resultin yield reduction).7. Delayed flowering. Drought in the vegetativedevelopment stage can delay floweringup to 3 to 4 weeks in photoperiod-insensitivevarieties. The delay in flowering islargest with drought early in the vegetativestage and is smaller when drought occurslater.8. Reduced tillering and tiller death. Droughtbefore or during tillering reduces the numberof tillers and panicles per hill. If the droughtis relieved on time, and the source size(i.e., photosynthesizing leaves and stems)is sufficiently large, the reduced number oftillers/panicles may be compensated for byan increased number of grains per panicleand/or by an increased grain weight.9. Reduced number of spikelets with droughtbetween panicle initiation and flowering,resulting in decreased number of grains perpanicle.10. Rice is very sensitive to reduced wateravailability in the period around floweringas this greatly affects spikelet sterility (Cruzand O’Toole 1984, Ekanayake et al 1989).Increased spikelet sterility with drought atflowering results in decreased percentageof filled spikelets and, therefore, decreasednumber of grains per panicle. Especially atanthesis, there is a short time span whenspikelet fertility is especially sensitive todrought.11. Decreased grain weight with drought afterflowering.The above processes appear roughly in orderof crop development and/or severity of drought,though numbers 2–4 also occur in the reproductivestage. Some effects lead to irreversible processesof yield reduction, such as numbers 4, 9, 10, and11, whereas others may be restored when droughtis relieved, such as numbers 2 and 3, and othersmay be compensated for by other effects later inthe growing season, such as numbers 1, 2, and 8.Drought may also affect nutrient-use efficiency bythe crop since water flow is the essential means ofnutrient transport. How yield is finally affected bydrought depends on its timing, severity, duration,and frequency of occurrence. The most sensitivestage of rice to drought is around flowering.15