Changes in Enzyme Activities Involved in Starch Synthesis and Hormone Concentrations in Superior and Inferior Spikelets and Their Association with Grain Filling of Super Rice

The changes in activities of key enzymes involved in sucrose-to-starch conversion and concentrations of hormones in superior and inferior spikelets of super rice were investigated and their association with grain filling was analyzed.Four super rice cultivars,Liangyoupeijiu,IIyou 084,Huaidao 9 and Wujing 15,and two high-yielding and elite check cultivars,Shanyou 63 and Yangfujing 8,were used.The activities of sucrose synthase （SuSase）,adenosine diphosphoglucose pyrophosphorylase （AGPase）,starch synthase （StSase） and starch branching enzyme （SBE）,and the concentrations of zeatin ＋ zeatin riboside （Z ＋ ZR）,indole-3-acetic acid （IAA） and abscisic acid （ABA） in superior and inferior spikelets were determined during the grain filling period and their relationships with grain filling rate were analyzed.Maximum grain filling rate,the time reaching the maximum grain-filling rate,mean grain filling rate and brown rice weight for superior spikelets showed a slight difference between the super and check rice cultivars,but were significantly lower in the super rice than in the check rice for inferior spikelets.Changes of enzyme activities and hormone concentrations in grains exhibited single peak curves during the grain filling period.The peak values and the mean activities of SuSase,AGPase,StSase and SBE were lower in inferior spikelets than in superior ones,as well as the peak values and the mean concentrations of Z ＋ ZR and IAA.However,the peak value and the mean concentration of ABA were significantly higher in inferior spikelets than in superior ones and greater in the super rice than in the check rice.The grain filling rate was positively and significantly correlated with the activities of SuSase,AGPase and StSase and the concentrations of Z ＋ ZR and IAA.The results suggested that the low activities of SuSase,AGPase and StSase and the low concentrations of Z ＋ ZR and IAA might be important physiological reasons for the slow grain filling rate and light grain weight of inferior spikelets in super rice.

Dissection of environmental and physiological effects on the temperature difference between superior and inferior spikelets within a rice panicle

Temperature of a plant organ constitutes an integrative index to its eco-physiological properties and status.However,little attempt has been made to dissect the combined effect of ecological and physiological factors on the surface temperature of a plant organ such as the rice spikelet.In this study,using a deactivated plant as reference,we developed a novel comparison method to dissect the environmental and physiological effects on temperature of rice spikelet.Three japonica rice cultivars with contrasting canopy features were used as testing materials.Temperatures of flag leaf,superior and inferior spikelets and their diurnal rhythm during grain filling stage were precisely measured by a hand-held infrared thermometer.The results showed that the variation of environmental conditions within a panicle was relatively minor,posing a limited influence on temperature difference between the superior and inferior spikelet.On the other hand,it was the intrinsic physiological properties that considerably affected the spatial variations of spikelet temperature within a panicle.Chemical analysis of sucrose and starch in grains and bracts indicated that the superior spikelet is more physiologically active at photosynthetic assimilation and starch biosynthesis.Interestingly,sugar in bracts exhibited a pattern of diurnal changes similar to the source leaf but different from the sink grain,confirming that bracts are source organs for grain filling.Our findings yield penetrating insight into the eco-physiological foundation of spikelet temperature,thus being helpful for the application of physiological approaches in crop breeding for cooler canopy.