The changes in activities of key enzymes involved in sucrosetostarch conversion and contents of hormones in superior and inferior spikelets were investigated and their relation with grain filling of super rice was analyzed with four super rice cultivars, Liangyoupeijiu and Ⅱyou 084 (indica hybrids), Huaidao 9 and Wujing 15 (japonica), and two highyielding and elite check cultivars, Shanyou 63 (indica hybrid) and Yangfujing 8 (japonica)as material in paddy field. Activities of sucrose synthase (SuSase), adenosine diphosphoglucose pyrophosphorylase (AGPase), starch synthase (StSase), and starch branching enzyme (SBE) and contents of zeatin + zeatin riboside (Z+ZR), indole3acetic acid (IAA), 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 a maximum grainfilling rate, mean grain filling rate and brown rice weight for superior spikelets slightly differed between super rice and elite check rice, but those for inferior spikelets were significantly different between super rice and check rice cultivars. Changes in enzyme activity and contents of hormones in grains followed single peak curves during the grain filling period. The peak and mean activity of SuSase, AGPase, StSase and SBE were lower in inferior spikelets than in superior ones,as well as the peak and mean contents of Z+ZR, IAA and ABA. The peak and mean contents of ABA were significantly greater in inferior spikelets than in superior ones and greater in the super rice than in the check rice. The grain filling rate was positively significantly correlated with the activities of SuSase, AGPase and StSase and contents of Z+ZR and IAA, but not significantly correlated with activity of SBE and ABA contents. The results suggested that the low activities of SuSase, AGPase and StSase and low contents of Z+ZR and IAA may be an important reason for the slow grain filling rate and low grain weight of inferior spikelets in super rice.
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