Source-sink Characteristics And The Translocation Of Assimilates In New Plant Type And Japonica/Indica Hybrid Rice

Poor grain filling is a major constraint in the increase in grain yield of new plant type (NPT) and japonica/indica hybrid (J/IH) rice (Oryza stativa L.). The objective of this study was to investigate potential causes of poor grain filling of NPT and J/IH rice by examining source-sink relations and translocation of assimilates in typical NPT lines and J/IH combinations. The main results are as follows:1 . Compared with indica hybrid Shanyou 63 (CK), NPT and J/IH had greater sink size ((total number of spikelets per square meter) and less dry matter accumulation before heading with a smaller ratio of dry matter weight to spikelet number (mg/spikelet). However, both NPT and J/IH had greater productivity and more accumulation of dry matter with a greater value of dry matter (mg) per spikelet from heading to maturity. A line or combination with greater ripened-grain percentage showed higher grain yield. Poor grain filling as a result of low ripened-grain percentage limited the yield potential in NPT and J/IH rice.2. Both NPT and J/IH exhibited poorer remobilization of nonstructural carbohydrate (NSC) reserves from the stem (culm + sheath), a smaller transfer ratio of assimilates, less partitioning of I4C to grains from labeled flag leaves, and lower harvest indices, relative to CK. A line or combination with a greater transfer ratio of assimilates showed a greater ripened-grain percentage, indicating that poor remobilization of assimilates from vegetable tissues to grains results in poor grain filling of NPT and J/IH rice.3. NPT and J/IH rice showed a low sugar-spikelet ratio at heading (the ratio of amount of NSC in the stem at heading to spikelet number, expressed as mg/spikelet) and low physiological activities (indole-3 -acetic content and activities of adenosine triphosphate enzyme and starch synthase) in the grains at early grain filling stage (3-12d after anthesis). The physiological activities of grains at early filling stage were significantly correlated with the transfer ratio of assimilates, ripened-grain percentage, and grain plumpness(r = 0.775 to 0.954). The sugar-spikelet ratio at heading was positively correlated with physiological activities of grains during the early grain filling period (r = 0.951 to 0.961). It is suggested that low physiological activities of grains at early filling stage is attributed to low source-sink ratio at heading, and the low sink activity weakens the ability to remobilize assimilates into grains, and leads to poor grain filling in NPT and J/IH rice.4. A moderate soil drying (soil water potential at -25 kPa) imposed during grain filling period significantly enhanced a-amylase and sucrose-phosphate synthase (SPS) activities in the stem, facilitated remobilization of assimilates from stems to grains, and increased ripened-grain percentage and grain weight of NPT and J/IH rice. Exogenously applied abscisic acid (ABA) with low concentration (25 X 10-6 M) during grain filling increased ABA whereas reduced gibberellins (GAs, GA1 + GA4) contents in grains, enhanced remobilization of assimilates, increased grain filling rate and grain weight. These results suggest that either an enhancement of activities of starch hydraulic enzymes and SPS in the stem or an altered hormonal balance in the grain, especially a decrease in GAs and an increase in ABA, increases translocation of assimilates from vegetative tissues to grains, and therefore improves grain filling of NPT and J/IH rice.