CULTIVATED rice evolved from a semi aquatic perennial ancestor. The wetland ancestry of rice is reflected in a number of morphological and physiological characteristics that are unique among crop species. Lowland rice is extremely sensitive to water shortage and drought effects occur when soil water contents drop below saturation. Rice has a variety of mechanisms by which it reacts to such conditions. 

1. Inhibition of leaf production and decline in leaf area, leading to retarded leaf growth and light interception, and hence to reduced canopy photosynthesis. Drought stress affects both cell division and enlargement, though cell division appears to be less sensitive to water deficit than cell enlargement. Leaf area expansion is reduced as soon as the soil dries below saturation in most cultivars, and when only about 30% of the available soil water has been extracted in cultivars with aerobic adaptation.

2. Closure of stomata, leading to reduced transpiration rate and reduced photosynthesis. Leaf stomata do not close immediately with drought stress, however, and the crop keeps on photosynthesizing for a certain period before stomata close. The assimilates are not used for leaf growth or expansion (see point 1), but are stored in the existing leaves, stems, and roots. When drought stress is relieved, these assimilates may become available and lead to a flush in leaf growth.

3. Leaf rolling, leading to a reduction in effective leaf area for light interception. Leaves unroll again when drought stress is relieved.

4. Enhanced leaf senescence, leading to reduced canopy photosynthesis. 

5. Changes in assimilate partitioning. Roots grow more, at the expense of the shoot, during vegetative development, whereas partitioning of assimilates among various shoot components is not affected. Deeper roots are effective for exploring water stored in deeper soil layers.

6. Reduced plant height (though it is not likely that reduced plant height in itself will result in yield reduction).

7. Delayed flowering. Drought in the vegetative development stage can delay flowering up to 3 to 4 weeks in photoperiod-insensitive varieties. The delay in flowering is largest with drought early in the vegetative stage and is smaller when drought occurs later.

8. Reduced tillering and tiller death. Drought before or during tillering reduces the number of tillers and panicles per hill. If the drought is relieved on time, and the source size (i.e., photosynthesizing leaves and stems) is sufficiently large, the reduced number of tillers/panicles may be compensated for by an increased number of grains per panicle and/or by an increased grain weight.

9. Reduced number of spikelets with drought between panicle initiation and flowering, resulting in decreased number of grains per panicle.

10. Rice is very sensitive to reduced water availability in the period around flowering as this greatly affects spikelet sterility. Increased spikelet sterility with drought at flowering results in decreased percentage of filled spikelets and, therefore, decreased number of grains per panicle. Especially at anthesis, there is a short time span when spikelet fertility is especially sensitive to drought.
11. Decreased grain weight with drought after flowering.
The above processes appear roughly in order of crop development and/or severity of drought, though numbers 2–4 also occur in the reproductive stage. Some effects lead to irreversible processes of yield reduction, such as numbers 4, 9, 10, and 11, whereas others may be restored when drought is relieved, such as numbers 2 and 3, and others may be compensated for by other effects later in the growing season, such as numbers 1, 2, and 8.

Drought may also affect nutrient-use efficiency by the crop since water flow is the essential means of nutrient transport. How yield is finally affected by drought depends on its timing, severity, duration, and frequency of occurrence. The most sensitive stage of rice to drought is around flowering.
BISSASAR CHINTAMANIE 
Research Scientist