Senescence of top three leaves in field-grown rice plants

The top three leaves play important roles in biomass production and grain yield of rice (Oryza sativa L.) crop since the three leaves not only assimilate majority of carbon for grain filling during ripening phase, but also provide large proportion of remobilized-nitrogen (N) for grain development during their senescence. The objectives of this study were to (a) compare senescence of the top three leaves and (b) compare the changes in N, chlorophyll, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) contents of the top three leaves after their full expansion in field-grown rice plants. When the basis of comparison among the top three leaves was plant age in terms of days after transplanting (DAT), senescence generally started earliest in -3rd leaf, intermediate in -2nd leaf, and latest in flag leaf If the basis of comparison among the top three leaves was leaf age in terms of days after full leaf expansion (DAFE), it was not clear which leaf senesced earlier. Senescence rate was generally greatest in flag leaf, intermediate in -2nd leaf, and smallest in -3rd leaf. Ribulose-1,5-bisphosphate carboxylase/oxygenase content declined earlier, and at a faster rate than N and chlorophyll contents during the senescence of all top three leaves. Correlation analysis indicated a close relationship between N and chlorophyll contents. Ribulose-1,5-bisphosphate carboxylase/oxygenase content correlated with N content better than with chlorophyll. content. The suitability of N, chlorophyll, and Rubisco contents for quantifying the leaf senescence of field-grown rice plants is discussed.