Activities of enzymes involved in sucrose-to-starch metabolism in rice grains subjected to water stress during filling

Slow grain filling of rice (Oryza sativa L.) is a problem due to the heavy use of nitrogen fertilizer or the high lodging resistance of some cultivars. This study investigated if controlled water deficit during grain filling could enhance sink strength by regulating key enzymes involved and lead to faster grain filling. Two rice cultivars with high lodging resistance and slow grain filling were grown in the field and treated with either normal nitrogen (NN) or high nitrogen (HN) at heading. Well-watered (WW) and water-deficit stressed (WS) treatments were imposed from 9 days after anthesis (DAA) until maturity. Leaf water potentials of both cultivars markedly decreased during the day as a result of WS treatments, but completely recovered by early morning. WS promoted the reallocation of prefixed ¹⁴C from stems to grains, facilitated starch accumulation in grains and increased grain-filling rate although it shortened grain-filling period. In contrast, HN behaved in the opposite way. Sucrose synthase (EC 2.4.1.13) activity was substantially enhanced by water stress, and was positively correlated with starch accumulation rate (SAR) in the grains. Both soluble and insoluble invertase (EC 3.2.1.26) activities were less enhanced by WS and showed no significant correlation with SAR. Starch branching enzyme (BE) (EC 2.4.1.18) and soluble starch synthase (EC 2.4.1.18) activities were also enhanced by the WS, with the former enhanced more than the latter, and were significantly correlated with SAR. Adenine diphosphoglucose pyrophosphorylase (EC 2.7.7.27) activity was little affected by WS. The results suggest that WS-increased remobilization and grain-filling rate were attributed to enhanced sink strength by regulating sucrose synthase and starch BE activities in rice grains when subjected to water stress during the grain-filling period.