Ethylene inhibitors improve dry matter partitioning and development of late flowering spikelets on rice panicles

Primary branch development of the rice panicle was in the order of a basipe tal sequence from the top to the bottom at the time of anthesis. Delayed de velopment of spikelets on the proximal branches of the panicle resulted in reduced grain filling. Two experiments were carried out to manipulate growt h and development of the proximal spikelets with exogenous application of c hemicals regulating formation or action of ethylene. In the first experimen t, inhibitors of ethylene synthesis (cobalt) and action (silver) improved g rain biomass and specific gravity of the basal spikelets, while 2-chloroeth ylphosphonic acid (CEPA) depressed these parameters significantly. In the s econd experiment, the ethylene synthesis inhibitor 1-aminoethoxyvinylglycin e (AVG) promoted spikelet development on the basal primary branches and imp roved their survival and grain biomass. On the contrary, the ethylene precu rsor 1-aminocyclopropane-1-carboxylic acid (ACC) inhibited growth and devel opment of these spikelets. The action of AVG was reversed when ACC was appl ied in combination with AVG. In both experiments, the chemicals did not inf luence growth and development of the superior spikelets on the apical prima ry branches of the panicle. Depression of growth and development by CEPA or ACC coincided with a concomitant rise in soluble carbohydrate concentratio n of the spikelets, whereas treatments with ethylene inhibitors decreased t he concentration of the materials. The role of ethylene in metabolic domina nce of the apical spikelets and its impact on grain yield of rice panicles is discussed.