Increased Proteolysis of Senescing Rice Leaves in the Presence of NaCl and KCl

NaCl and KCl enhanced the degradation of chlorophylls and proteins in detached rice (Oryza sativa) leaves in a concentration-dependent manner. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) accounted for 73 to 80% of the protein lost by day 4 in the light. NaCl at 50 millimolar increased proteolysis by 21% over the control in 4 days, but the addition of cycloheximide reduced the increase to about one-half. Cycloheximide alone had no effect on proteolytic activity during this period. Leaf segments taken from 10-day-old seedlings contained the highest proteolytic activity. Both NaCl and KCl increased the activity of Rubisco-degrading endoproteinases (the amount of ninhydrin-positive compounds measured from HCl-hydrolyzates of trichloroacetic acid-soluble supernatant), but decreased the activity of hemoglobin- and Rubisco-degrading exoproteinases (the amount of ninhydrin-positive compounds measured directly from trichloroacetic acid-soluble supernatant). Efflux of amino acids from senescing leaf segments into the incubation media increased 7- and 12-fold in the presence of KCl and NaCl, respectively. The increased efflux resulted in a negative correlation between salt concentration and amino acid content of leaf segments at the later stage of senescence. It is concluded that, in addition to the induction of new proteinase synthesis, the increased efflux of protein hydrolyzates may play a significant role in increasing proteolysis of salt-treated leaves, especially at the later stages of senescence.