Effects of plant growth regulators on number of spikelets per panicle in rice (Oryza sativa L.) under Saline flooding conditions

The changes in number of spikelets per panicle (SN) under saline conditions as affected by the application of plant growth regulators were determined. A lowland rice cultivar IR28 whose SN is reduced by salinity was grown in pots filled with the paddy soil and submerged under the three saline solutions of 50 mM, 100 mM and 150 mM sodium chloride (NaCl) and tap water during panicle development. Indole acetic acid (IAA), kinetin or gibberellin (GA₃) (0.5 mM) were foliar-sprayed. Independently, plants were grown under low solar irradiance (40% to 85% of transmittance) without NaCl application. Control plants were grown with tap water and not subjected to the spray and low solar irradiance. SN decreased because of the occurrence of spikelet malformation and degeneration under saline conditions as a result of only spikelet degeneration under low solar irradiance. The degeneration of spikelets under saline conditions was suppressed by the GA₃ spray, though a different type of malformation occurred, resulting in a small SN. The SN decreased in proportion to the decrease of panicle dry weight at panicle emergence with different effects of salinity, GA₃ and low irradiance. The effect of the spray of IAA and kinetin was not detectable. From these results, SN decreased under saline conditions because of the occurrence of spikelet malformation as well as the degeneration, and the GA₃ spray may suppress the degeneration of spikelets, suggesting that it may alleviate the adverse effect of salinity on SN.