SENESCENCE PARAMETERS OF ORGANS CONSTITUTING THE PANICLE, 1ST INTERNODE, AND FLAG LEAF IN RICE

Long-lived rice (Oryza sativa L.) panicles with high physiological activities are important for maintaining translocation of assimilates and nutrients. We studied senescence parameters, N concentration, and rate of N loss from panicles, first internodes, and flag leaves from flowering until maturity. In 1988, 76 rice cultivars were started indoors in pots and grown outdoors in paddies under standard fertilization (94.5 kg N ha-1). In 1989, six cultivars were grown under 94.5, 154.5, and 214.5 kg N ha-1. The rate of Kjeldahl N loss from organs was estimated from Y = AB(x), where Y is N concentration at X d after flowering, and A is N concentration at flowering. The regression coefficient B (fraction of N retained after unit time) was significantly different from zero in all cultivars and organs in both years. The N concentration at flowering was highest in flag leaves, intermediate in rachis and rachis branches, and lowest in hulls; among cultivars, it varied largely in the first internodes. Conversely, the rate of N loss, calculated as A (1 - B), was lowest in flag leaves, intermediate in rachis and rachis branches, and highest in hulls. Fertilizer application increased N concentration at flowering, but showed little effect on the rate of N loss. The rate of N loss exhibits cultivar specificity. It seems likely that fertilizer application can change the N level of panicles and organs only at initiation of grain filling. Although there are positive correlations between the two parameters in hulls, rachis, and rachis branches, cultivars with panicles exhibiting high initial N concentration in combination with a cultivar-specific low rate of N loss may be developed through breeding.