Effect of gibberellic acid (GA3) as a pre-flowering treatment in hybrid rice seed production

Hybrid rice seeds are produced by cross pollination between cytoplasmic male sterile (CMS) lines and the restoring (R) lines, but the panicle of CMS lines is partly enclosed by the flagleaf sheath severely restricting cross pollination and limiting seed yield. Foliar application of gibbberellic acid (GA3) at the start of panicle emergence is widely adopted in China as an essential technique for promoting panicle exsertion to obtain high cross-pollinated seed set. However, knowledge of the GA3 dosage and splits of application in hybrid rice seed production, which may enhance outcrossing rates are not well understood under tropical conditions. In order to assess the effect of GA3 as a pre-flowering treatment for CMS populations in hybrid rice seed production, one pot study and a field experiment were therefore conducted during the dry season of 2002. The pot experiment determined variations in percentage of exerted panicle segments and other morphological characteristics among ten CMS lines applied with five levels of GA3 at 5-10% heading. The field experiment which were conducted simultaneously determined the outcrossing characteristics and F1 seed yield of CMS line IR58025A under various rates and splits of GA3 application to identify the optimum rate and the most effective splits of GA3 application in hybrid rice seed production. Results of the pot experiment showed that the percentage of exerted panicle segments and morphological characters critical to outcrossing such as the rate of exposed spikelets, and the distance between the panicle tip and the flagleaf tip (DPF) were significantly influenced by GA3 application and CMS lines used, but not with their interaction. The flagleaf angle was influenced by interaction of CMS line and rate of GA3 application. The percentage of spikelets with exerted stigma was only influenced by the rate of GA3 application but not with CMS line and their interaction. Other morphological characters such as length of panicle, flagleaf and spikelet, and the angle of spikelet opening differed only among CMS lines used, but not among rates of GA3 applied. In field experiment, the percentage of exerted panicle segment, percentage of seed set and F1 seed yield of IR 58025A were significantly increased by GA3 application, but the rate of increase was significantly higher at higher rates of GA3 application between 100 to 200 g/ha than at 50 g GA3/ha application. F1 seed yield was significantly and linearly increased up to 150 g GA3/ha treatment and began to level off with further increase in rate of GA3 application. Moreover, the increase in seed yield was highly influenced by the increase in percentage of seed set of the female parent. Significant improvement of percentage of seed set was highly influenced by wider flagleaf angle, good panicle layer carriage (high DPF values), and higher percentage of exerted panicle segment and higher degree of spikelet openings but not with percentage of spikelets with exerted stigma. The panicle segment exsertion characteristic was found to be a function of the elongating topmost internode but not of the panicle. Splitting the total amount of GA3 applied increased percentage of seed set of the female parent but did not significantly increase seed yield, although it was observed that more number of splits produced higher seed yield. Based on the seed yield and economic profitability, the optimum rate of GA3 application as a pre-flowering treatment in hybrid rice seed production is 150 g/ha which can be applied in three splits at a ratio of 25:50:25 beginning at 5-10% panicle heading at the female parent population