Simultaneously improving grain yield and water and nutrient use efficiencies by enhancing the harvest index in rice

Global agriculture is facing the major challenge of providing enough food for a growing population while increasing resource use efficiency. The harvest index (HI) is an independent variable in the function not only for crop yield (biomass x HI) but also for water productivity (HI x biomass / water transpired) and nutrient productivity (HI x biomass / nutrient absorbed by plants) in crops. Therefore, enhancement in HI could simultaneously improve grain yield and water and nutrient use efficiencies in rice. An increase in the grain-leaf ratio (the ratio of the total number of spikelets to the leaf area at heading time) to coordinate source-sink relationships, an increase in the sugar-spikelet ratio [the ratio of the amount of nonstructural carbohydrates (NSC) in the stems to the total number of spikelets at heading time] to enhance sink strength and NSC remobilization in stems during grain filling, and an increase in the percentage of productive tillers to improve canopy structure are the three important agronomic and physiological approaches to increase HI. The adoption of an alternate wetting and moderate drying irrigation regime and the use of the "three-based on" application technology of nitrogen fertilizer (based on the soil fertility, leaf color, and variety in rice) are two major crop management techniques to increase HI. Further studies are needed to elucidate the role of HI in reducing environmental risk, the association of an enhanced HI by more remobilization of prestored NSC in stems during grain filling with lodging resistance, and the relationship between HI and grain quality in rice.