Ethylene inhibitors enhanced sucrose synthase activity and promoted grain filling of basal rice kernels

At the booting stage of development, rice (Oryza sativa L.) plants were treated with chemicals that either inhibited the action or synthesis of ethylene, or produced ethylene. Inhibitors of ethylene action (AgNO3) and synthesis [uniconazole, paclobutrazol, Co( NO3)(2)] promoted grain filling and quality of the kernels of the basal spikelets of the panicle, while the ethylene-releasing substance CEPA (2-chloroethylphosphonic acid) depressed these characteristics further. The inhibitors depressed the concentration of ethylene of the basal primary branches, but CEPA increased it above the control during the period of grain filling. The treatments were not effective on the superior apical spikelets of the panicle. The ethylene inhibitors improved starch synthesis in the kernels of the basal spikelets, but CEPA reduced it significantly, resulting in accumulation of soluble carbohydrates in the kernels. During the period of grain filling, sucrose synthase activity was higher than that of invertase in the kernels. Activities of sucrose synthase and invertase were higher in the apical than in the basal kernel. The ethylene inhibitors increased activities of both enzymes only in the basal kernel, whereas CEPA reduced activities significantly. Together, the results indicate that starch filling and grain quality of the basally positioned under-developed rice kernels can be affected by ethylene, and that key enzymes of sucrose metabolism are also affected in the process.