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Contents Summary 223 Introduction 224 Problems in grain filling: unfavorably delayed whole‐plant senescence 224 Controlled soil drying improves carbon remobilization and grain filling as a result of enhanced whole‐plant senescence 225 Hormonal regulation of whole‐plant senescence and grain filling 229 Activities of key enzymes involved in carbon remobilization and grain filling 230 Conclusions 232 Acknowledgements 232 References 232 Summary Monocarpic plants require the initiation of whole‐plant senescence to remobilize and transfer assimilates pre‐stored in vegetative tissues to grains. Delayed whole‐plant senescence caused by either heavy use of nitrogen fertilizer or adoption of lodging‐resistant cultivars/hybrids that remain green when the grains are due to ripen results in a low harvest index with much nonstructural carbohydrate (NSC) left in the straw. Usually, water stress during the grain‐filling period induces early senescence, reduces photosynthesis, and shortens the grain‐filling period; however, it increases the remobilization of NSC from the vegetative tissues to the grain. If mild soil drying is properly controlled during the later grain‐filling period in rice (Oryza sativa) and wheat (Triticum aestivum), it can enhance whole‐plant senescence, lead to faster and better remobilization of carbon from vegetative tissues to grains, and accelerate the grain‐filling rate. In cases where plant senescence is unfavorably delayed, such as by heavy use of nitrogen and the introduction of hybrids with strong heterosis, the gain from the enhanced remobilization and accelerated grain‐filling rate can outweigh the loss of reduced photosynthesis and the shortened grain‐filling period, leading to an increased grain yield, better harvest index and higher water‐use efficiency.

A major challenge in rice (Oryza sativa L.) production is to enhance water use efficiency (WUE) and maintain or even increase grain yield. WUE, if defined as the biomass accumulation over water consumed, may be fairly constant for a given species in given climate. WUE can be enhanced by less irrigation. However, such enhancement is largely a trade-off against lower biomass production. If WUE is defined as the grain production per unit amount of water irrigated, it would be possible to increase WUE without compromising grain yield through the manipulation of harvest index. Harvest index has been shown to be a variable factor in crop production, and in many situations, it is closely associated with WUE and grain yield in cereals. Taking rice as an example, this paper discussed crop management techniques that can enhance harvest index. Several practices such as post-anthesis controlled soil drying, alternate wetting and moderate soil drying regimes during the whole growing season, and non-flooded straw mulching cultivation, could substantially enhance WUE and maintain or even increase grain yield of rice, mainly via improved canopy structure, source activity, sink strength, and enhanced remobilization of pre-stored carbon reserves from vegetative tissues to grains. All the work has proved that a proper crop management holds great promise to enhance harvest index and, consequently, achieve the dual goal of increasing grain production and saving water.

The possible regulation of senescence-initiated remobilization of carbon reserves in rice (Oryza sativa L.) by abscisic acid (ABA) and cytokinins was studied using two rice cultivars with high lodging resistance and slow remobilization. The plants were grown in pots and either well-watered (WW, soil water potential = 0 MPa) or water-stressed (WS, soil water potential = -0.05 MPa) from 9 days after anthesis until they reached maturity. Leaf water potentials of both cultivars markedly decreased at midday as a result of water stress but completely recovered by early morning. Chlorophyll (Chl) and photosynthetic rate (Pr) of the flag leaves declined faster in WS plants than in WW plants, indicating that the water deficit enhanced senescence. Water stress accelerated starch remobilization in the stems, promoted the re-allocation of pre-fixed (14)C from the stems to grains, shortened the grain-filling period and increased the grain-filling rate. Sucrose phosphate synthase (SPS, EC 2.4.1.14) activity was enhanced by water stress and positively correlated with sucrose accumulation in both the stem and leaves. Water stress substantially increased ABA but reduced zeatin (Z) + zeatin riboside (ZR) concentrations in the root exudates and leaves. ABA significantly and negatively, while Z+ZR positively, correlated with Pr and Chl of the flag leaves. ABA, not Z+ZR, was positively and significantly correlated with SPS activity and remobilization of pre-stored carbon. Spraying ABA reduced Chl in the flag leaves, and enhanced SPS activity and remobilization of carbon reserves. Spraying kinetin had the opposite effect. The results suggest that both ABA and cytokinins are involved in controlling plant senescence, and an enhanced carbon remobilization is attributed to an elevated ABA level in rice plants subjected to water stress.

To understand the effect of water stress on the remobilization of prestored carbon reserves, the changes in the activities of starch hydrolytic enzymes and sucrose-phosphate synthase (SPS) in the stems of rice (Oryza sativa L.) during grain filling were investigated. Two rice cultivars, showing high lodging-resistance and slow remobilization, were grown in the field and subjected to well-watered (WW, psi(soil)=0) and water-stressed (WS, psi(soil)=-0.05 MPa) treatments 9 d after anthesis (DAA) till maturity. Leaf water potentials of both cultivars markedly decreased during the day as a result of WS treatment, but completely recovered by early morning. WS treatment accelerated the reduction of starch in the stems, promoted the reallocation of prefixed (14)C from the stems to grains, shortened the grain filling period, and increased the grain filling rate. More soluble sugars including sucrose were accumulated in the stems under WS than under WW treatments. Both alpha- and beta-amylase activities were enhanced by the WS, with the former enhanced more than the latter, and were significantly correlated with the concentrations of soluble sugars in the stems. The other two possible starch-breaking enzymes, alpha-glucosidase and starch phosphorylase, showed no significant differences in the activities between the WW and WS treatments. Water stress also increased the SPS activity that is responsible for sucrose production. Both V(limit) and V(max), the activities of the enzyme at limiting and saturating substrate concentrations, were enhanced and the activation state (V(limit)/V(max)) was also increased as a result of the more significant enhancement of V(limit). The enhanced SPS activity was closely correlated with an increase of sucrose accumulation in the stems. The results suggest that the fast hydrolysis of starch and increased carbon remobilization were attributed to the enhanced alpha-amylase activity and the high activation state of SPS when the rice was subjected to water stress.

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Genotypes of Oryza sativa, O. glaberrima and Triticum aestivum were investigated for leaf gas-exchange parameters in relation to anatomical characteristics. Species-dependent relationships provide leads for growing rice like dryland cereals.