Grain Filling Mechanism Of The Inferior Spikelets In Super Rice And Its Regulation Techniques

Super rice varieties have numerous spikelets on a panicle with a large yield capacity, but they often fails to achieve their great yield potential due to poor grain-filling of later-flowering inferior spikelets (in contrast to the earlier-flowering superior spikelets). The reasons for poor grain-filling of super rice varieties have been studied from the carbohydrate metabolism, starch configuration, vascular bundle system and hormonal balance. However, the mechanism underlying the poor grain filling is yet to be understood. In this study, four super rice varieties were compared with two elite varieties under field-grown conditions. The content changes of indole-3-acetic acid (IAA), zeatin+zeatin riboside (Z+ZR) and abscisic acid (ABA) in both superior and inferior spikelets during the grain filling period were investigated and the relationship between the hormonal content and grain filling rate was analyzed. Effects of chemical substances on endogenous hormonal content, carbohydrate accumulation and remobilization and grain filling were observed by spraying Z, brassinolide (BR), KH2PO4and ABA during5-8days post anthesis. The main results are as follows:1. Four super rice cultivars, Liangyoupeijiu and IIyou084(indica), Huaidao9and Wujing15(japonica), and two elite cultivars, Shanyou63(indica hybrid) and Yangfujing8(japonica), were field grown. Contents of IAA, Z+ZR and ABA in both inferior and superior spikelets during the grain filling period and grain filling rate were determined. The relationship between them was also analyzed. The results showed that, compared with superior, inferior spikelets had lower grain-filling rate and grain weight, and the time reaching maximum grain-filling rate was much longer. The grain filling rate and brown rice weight for superior spikelets showed small difference between super rice and check rice, but those of inferior spikelets were significantly lower for super rice varieties than for check rice varieties. The contents of IAA, Z+ZR and ABA were greater in the superior than in the inferior at the early filling stage, while the results were opposite during the later grain filling period, and the hormonal differences between the superior and the inferior were larger in super rice varieties than in check rice varieties. The grain-filling rate and brown rice weight were positively and very significantly correlated with IAA, Z+ZR and ABA contents at0-14days after anthesis. The results suggest that hormones play a key role in regulating grain filling. Low contents of IAA, Z+ZR and ABA may be an important physiological reason for the small grain-filling rate and low grain weight of inferior spikelets in super rice. Changes in activities of sucrose synthase (SuSase), adenosine diphosphoglucose pyrophosphorylase (AGPase), starch synthase (StSase), and starch branching enzyme (SBE) were in line with the contents of hormones. It is inferred that endogenous hormones could regulate grain filling by controlling the activities of key enzymes in the sucrose-to-starch conversion.2. The super rice variety of Liangyoupeijiu (indica hybrid), and the high-yielding check variety of Shanyou63(indica hybrid) were used to investigate endogenous hormonal contents, dry matter accumulation and remobilization, leaf senescence index and grain filling by spraying Z, BR, KH2PO4and ABA at5-8days after anthesis. Z and BR treatment obviously increased SPAD value, photosynthetic rate and protein content in the flag leaves and dry matter accumulation. Grain-filling rate and grain weight were increased, due to the increase of endogenous Z+ZR, BR and IAA contents in inferior spikelets. Inferior spikelets at the early filling phase showed more spots of protein expressions by spraying Z. The eight protein spots that exhibited significant differences in the expressions between Z treatment and CK were chosen for the MALDI analysis, and their functions were identified. The identified proteins were divided into four groups according to their functions:(a) structure composition (Profilin-2);(b) pathogenesis-related protein (Probenazole-inducible protein);(c) photosynthetic metabolism (including23kD PS â…¡ protein, ribulose-1,5-bisphosphate carboxylase/oxygenase, fructose-bisphosphate aldolase, and glyceraldehyde-3-phosphate dehydrogenase);(d) detoxification enzyme (Superoxide dismutase). Spraying KH2PO4postponed the senescence of plants, but it has little influence on grain filling. When spraying ABA at the heading time, the SPAD value, photosynthetic rate and protein content of the flag leaves were decreased rapidly. Exogenous ABA accelerated the conversion of starch to sucrose in stem-sheath, and the export and transformation percentage of the matter in stem-sheath as well as endogenous ABA and ABA/GA in grains were greatly enhanced. It is suggested that ABA would promote inferior spikelets filling, and consequently improve final grain weight.All the results suggest that plant hormones play an important role in regulating grain filling through mediating activities of key enzymes involved in sucrose-to-starch synthesis and protein expressions in super rice. Increase in ABA level in inferior spikelets by chemical regulations could improve the filling of inferior spikelets in super rice varieties.