INFLUENCE OF NITROGEN DEFICIENCY ON SENESCENCE AND THE AMOUNTS OF RNAAND PROTEINS IN WHEAT LEAVES

Senescence-associated coordination in amounts of enzymes localized in different cellular compartments were determined in attached leaves of young wheat (Triticum aestivum L. cv. Arina) plants. Senescence was in itiated at the time of full lear elongation based on declines in total RNA and soluble protein. Removal of N from the growth medium just at the time of full leaf elongation enhanced the rate of senescence. Sust ained declines in the amount of ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco, EC 4.1.1.39), and a marked decrease in the rbcS tr anscripts, just after full leaf elongation indicated that Rubisco synt hesis/degradation was very sensitive to the onset of senescence. Rubis co activase amount also declined during senescence but the proportion of rea transcript relative to the total poly A RNA pool increased 3-fo ld during senescence. Thus, continued synthesis of activase may be req uired to maintain functional Rubisco throughout senescence. N stress l ed to declines in the amount of proteins located in the chloroplast, t he peroxisome and the cytosol. Transcripts of the Clp protease subunit s also declined in response to N stress, indicating that Clp is not a senescence-specific protease. In contrast to the other proteins, mitoc hondrial NADH-glutamate dehydrogenase (EC 1.4.1.2) was relatively stab le during senescence and was not affected by N stress. During natural senescence with adequate plant nitrate supply the amount of nitrile re ductase (EC 1.7.7.1) increased, and those of glutamine synthetase (EC 1.4.7.1) and glutamate synthase (EC 6.3.1.2) were stable. These result s indicated that N assimilatory capacity can continue or even increase during senescence if the substrate supply is maintained, Differential stabilities of proteins. even within the same cellular compartment, i ndicate that proteolytic activity during senescence must be highly reg ulated.