Grain filling of cereals under soil drying

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. Contents Summary 223 I. Introduction 224 II. Problems in grain filling: unfavorably delayed whole‐plant senescence 224 III. Controlled soil drying improves carbon remobilization and grain filling as a result of enhanced whole‐plant senescence 225 IV. Hormonal regulation of whole‐plant senescence and grain filling 229 V. Activities of key enzymes involved in carbon remobilization and grain filling 230 VI. Conclusions 232 Acknowledgements 232 References 232