Increasing rice yields through exploitation of heterosis

Commercial exploitation of heterosis to increase rice yields has been successfully demonstrated in China, where 15 million ha of a total 32.5 million ha of riceland were planted with F1 hybrids in 1990. The hybrids yield about 20% higher than inbred rice, and helped China produce 33 million to more rice per year over the past decade. Research in countries outside China indicates that heterosis in rice can increase yields by 15-20% over the best available semidwarf inbreds and commercial rice hybrids are being developed. Rice hybrids are likely to show higher yield potential not only in irrigated conditions but also under some rainfed conditions. Cytoplasmic male sterility (CMS) is the most effective genetic tool for developing hybrid rice. Photoperiod-sensitive (PGMS) and thermosensitive male sterility (TGMS) systems have recently shown promise. In China, the chemically induced male sterility system used is not efficient for large-scale use. Well-developed hybrid seed production practices give average yields of 2 t seed/ha. Outside China, experimental yields of 1-2 t seed/ha have been obtained. Application of gibberellic acid is the most effective component of hybrid seed production technology. Hybrid seeds produced at some locations in the tropics have a high proportion of discolored seeds. Apomixis would fix heterosis in rice and permit farmers to use the F1 harvest as seeds for the next crop.