Antagonistic Effects of Auxins and Abscisic Acid on Floret Closure of Rice (Oryza sativa)

Rice flowering refers to the opening and closing of the floret and takes 6–10 days for the tens-hundreds of florets within the same panicle to complete flowering. Rice flowering time occurs at a certain time of the day and varies among varieties and weather conditions. The regulation of rice floret opening and closing is very important for hybrid rice seed production. However, little is known about the effects of plant hormones on rice flowering. Our previous studies have shown that jasmonate (JA) and its methyl ester, methyl jasmonate (MeJA) play remarkable roles in promoting rice floret opening. In this study, we investigated the effects of auxins including indole-3-acidic acid (IAA), indole-3-butyric acid (IBA), 1-naphthalene-acetic acid (NAA), 2,4-dichlorophenoxy acetic acid (2,4-D) and 3,6-dichloro-2-methoxybenzoic acid (Dicamba, or DIC) and abscisic acid (ABA) on floret closure of fertile rice (Jia-zao-211, Huang-yue-zhan, Jin-you-gui-9, Ping-hui-141) and sterile rice (Wu-feng-A, Zhong-9A, Bo-you-A). The results showed that the percentages of closed florets were significantly lower in plants treated with IAA, IBA, 2,4-D, DIC, and NAA and that the durations of floret opening were significantly longer in plants treated with the same auxins. The auxins on floret closure exhibited time- and concentration-dependant effects. The higher concentrations of auxins resulted in higher percentages of closed florets and longer time of floret opening. ABA displayed opposite effects of auxins because it increased the percentages of floret closure and decreased the length of floret opening of rice varieties. Similarly, higher concentrations of ABA resulted in lower percentages of closed florets and shorter time of floret opening. The effective concentrations of auxins and ABA on floret closure of rice were 100–200 mg/mL. The effects of auxins and ABA on floret closure were varied somewhat with different rice varieties. Our results showed that auxins delayed but ABA promoted the closure of rice floret regardless of the varieties. This study suggests that auxins and ABA are antagonists on floret closure of rice.