Physiology of Spikelet Development on the Rice Panicle: Is Manipulation of Apical Dominance Crucial for Grain Yield Improvement?

Cultivated rice, Oryza sativa L., originated in the flood plains of Asia with their variable environmental conditions. Heterogeneous architecture, leading to intergrain apical dominance in spikelet development, was an important strategy for plant survival under such uncertain conditions. When inclement weather conditions coincide with the sensitive stage of spikelet development, the plant sacrifices some spikelets while preserving the rest to enable some grains to fill to provide seed for the next generation. The nature of the interaction between a genotype and its environment determines the ultimate spikelet number. This heterogeneous architecture was considered a liability for the increased demand for food. Innovative breeding efforts have changed the plant type in favor of extra-heavy panicles bearing a large number of spikelets, mostly in a homogeneous distribution. However, the reduced intergrain apical dominance decreased the strength of the sink for assimilate partitioning to the spikelets, much to the detriment of increased grain production. The metabolic control of assimilate partitioning and the genes controlling starch synthesis are reviewed and the importance of hormonal control of grain filling is emphasized. A model has been constructed to project the role of metabolites, hormones, and other factors in intergrain apical dominance. The panicle architecture of rice is amenable to modification by both intrinsic and extrinsic factors, and there is extensive variation of panicle structure among genotypes. Therefore, we suggest that an architecture with controlled intergrain apical dominance, amounting to production of a greater number of responsive sinks, which is a compromise between heterogeneous and homogeneous architecture of the inflorescence, should benefit spikelet filling in rice with large panicles.