Plant growth conditions and treatments
The experiment was carried out at the farm of Yangzhou University (32°30’N, 119°25’E) during the rice (Oryza sativa ) growing season, and in Hainan in winter seasons. At the heading stage, 200 uniformly growing and headed panicles (1-2 panicles per plant) were chosen, and spikelets on the selected panicles with the same flowering date were labeled for each cultivar/line. The flowering date and position of each spikelet on the labeled panicles were recorded. Approximately 30 labeled panicles were sampled each time from flowering to maturity. Half of the sampled grains were frozen in liquid nitrogen for at least 2 min and then stored at -80°C for further analyses. The other half of the grains were dried at 80°C for approximately 72 h to a constant weight and used for starch analyses.
For exogenous NAA application, rice plants growing in the plastic pots (three hills per pot) under open field conditions were used. Each pot (0.6 m in height, 0.5 m and 0.3 m in diameter at the top and bottom, respectively) was filled with sandy loam soil that contained the same nutrient contents as the field soil. The sowing date and cultivation were the same as those for the field experiment. After flowering, 10 μM NAA was sprayed at a rate of 5 ml per pot on the top of the plants (spikes) every 5 days until 30 DAF. The solutions contained ethanol and Tween-20 at final concentrations of 0.1% (v/v) and 0.01 (v/v), respectively. Control plants were sprayed with the same volume of deionized water containing the same concentrations of ethanol and Tween-20.