Grain sink strength may be related to the poor grain filling of indica-japonica rice (Oryza sativa) hybrids

The physiological and biochemical factors contributing to poor grain fillin g of indica-japonica rice (Oryza sativa L.) hybrids were studied by analyzi ng the role of grain sink strength in dry matter accumulation of grains of two types of rice cultivars, Yayou 2 (an indica-japonica hybrid) and Yanjin g 2 (a japonica cultivar). Carbon dioxide enrichment and plant hormone appl ication were imposed at anthesis and the number of endosperm cells, dry mat ter accumulation and the activities of some sugar-metabolizing enzymes of g rains were measured during grain filling. In Yayou 2, strong-potential grai ns (SPGs) accumulated dry weight much earlier than weak-potential grains (W PGs), but this difference was not obvious for Yanjing 2. Carbon dioxide enr ichment imposed after heading significantly stimulated dry matter accumulat ion of WPGs of Yayou 2, but had little influence on WPGs of Yanjing 2 and S PGs of both cultivars. Leaf sheath dry matter decreased steadily in both cu ltivars during early stages of grain filling and accumulated during the lat er stages. Carbon dioxide enrichment increased leaf sheath dry matter. Dry matter accumulated by grains was linearly related to the increases in endos perm cell numbers and the activities of sucrose synthase (SS) and ADP-gluco se pyrophosphorylase (AGPase) in SPGs and WPGs of both cultivars. Applicati on of either 6-benzyladenine or abscisic acid had no significant influences on both endosperm cell number and grain dry matter accumulation. These res ults suggest that grain sink strength, determined by both cell numbers, SS and AGPase activities in the endosperm control the dry matter accumulation of grains.