Spikelet opening under controlled transpiration conditions in rice

Abstract

In rice, spikelet opening is an important process for anthesis to produce seeds. Spikelet opening is greatly affected by temperature, humidity, light, and physical stress. To clarify the important factors for spikelet opening, flowering panicles of Japonica cultivar Nipponbare were observed under four controlled transpiration conditions designated as submergence, shower, oil and high-humidity conditions. The submergence condition was designed to allow transpiration only from panicles. Other three conditions were designed to suppress transpiration from panicles by water, oil and high humidity. For each condition, four panicles with 15 spikelets were prepared, and flowering ratios were recorded on the following afternoon. The examination was repeated for three days, and the averages were compared. Under the submergence condition, many flowered spikelets were observed with the average value of 83.3%. On the other hand, a few or no spikelets flowered in the panicles under the shower (17.8%) and oil conditions (0.6%). Almost half of the spikelets (46.1%) flowered under the high-humidity condition. These average values were all significantly lower than the controls (ca. 92%) without any treatments. These results indicate that transpiration from lemma and palea is necessary to have spikelet opening. In addition, spikelet opening time was observed using a wild accession of O. rufipogon to examine whether transpiration from an awn (a tip organ of the lemma) is associated with flowering time or not. Eight panicles of three wild plants were prepared, and awns of the spikelets were alternatively cut based on the primary branch order. The plants were put outside under the sun, and the exact opening time was recorded in minutes for all the spikelets flowered on the following morning. As a result, average opening times for awned and awnless spikelets were not significantly different to each other. Since opening time was not associated with transpiration from the awns, a precise switch for panicle opening may synchronize with transpiration from the surface of the lemma and/or palea.