Grain filling pattern and cytokinin content in the grains and roots of rice plants

Grain filling patterns and their relationships with zeatin (Z), zeatin riboside (ZR), indole-3-acetic acid (IAA) and gibberellin (GA) contents in the grains and roots during grain development were examined in six rice (Oryza sativa L.) genotypes grown in the field and in water culture. Three grain filling patterns based on the filling rate of superior and inferior spikelets were observed, i.e., fast synchronous: all spikelets started filling early and fast at the early filling stage; slow synchronous: all spikelets filled slowly at the early filling stage and reached the maximum filling rate late; and asynchronous: superior spikelets started filling and reached the maximum filling rate much earlier than the inferior ones. The order of grain filling percentage in the three types of grain filling patterns was: fast synchronous > asynchronous > slow synchronous. Changes in Z + ZR contents in the superior and inferior spikelets were associated with the grain filling patterns. Grain filling percentage was significantly correlated with Z + ZR contents in the grains and roots at the early and middle grain filling stages. IAA and GA (GA(1) + GA(3) + GA(4)) contents in the grains and roots were not significantly correlated with grain filling percentage. The results suggest that cytokinins in the grains and roots during the early phase of grain development play an important role in regulating grain filling pattern and consequently influence grain filling percentage.