Modulated Degradation of RibuloseBisphosphate Carboxylase in Leaves on Top-Pruned Shoots of the Mulberry Tree (Morus albaL.)

Abstract

Yamashita, T. 1987. Modulated degradation of ribulose bisphosphate carboxylase in leaves on top-pruned shoots of the mulberry tree (Morus alba L.).—J. exp. Bot. 38: 1957-1964. The effects of pruning shoot tops on the synthesis and degradation of ribulose 1,5-bisphosphate carboxylase (RuBPCase) in leaves on remaining shoots were investigated in mulberry trees. Leucine labelled with 14C was fed to leaf discs from field-grown mulberry trees and 14C incorporation into RuBPCase was examined. Proportion of 14C in RuBPCase to leucine-14C absorbed by leaf discs was remarkably lowered by top-pruning, though occasionally a slight increase was observed soon after pruning. Yet RuBPCase content in leaves on top-pruned shoots became progressively higher than that in leaves on intact shoots. Changes in 14C in RuBPCase in leaves of mulberry saplings previously fed 14C02 were followed. Following 14C02 feeding, the attainment of the maximal level of 14C in RuBPCase was retarded by top-pruning. The highest level of 14C in RuBPCase was maintained in leaves on top-pruned shoots but decreased in leaves on intact shoots. Specific radioactivity in RuBPCase continued to increase in leaves on top-pruned shoots even after attaining a maximum level in the control leaves. These facts suggest that the increase in RuBPCase by top-pruning results from a cessation of its degradation for the remobilization of nitrogen for newly developing leaves on shoot tops.