Interactive effects of nitrogen starvation and different temperatures on senescence of sunflower (Helianthus annuus L.) leaves associated with the changes in RNA, protein and activity of some enzymes of nitrogen assimilation

Interactive effects of N-starvation and different temperatures on senescence of sunflower (Helianthus annuus L. cv Giza) leaves associated with the changes in RNA, protein and activity of some enzymes involved in N assimilation, photosynthesis and respiration were studied. Sunflower plants were grown in controlled environment in two temperatures regimes in the presence and absence of N. It was found that senescence was initiated at time of full leaf 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 and a significant decrease was observed in the amount of RNA, protein and in terms of any indices measured of growth parameters such as leaf area, specific leaf weight and relative growth rate particularly in cool conditions. The net loss of the soluble proteins in the leaves was paralleled by an increase in the total amino acids in cool more than warm conditions. Furthermore, carbohydrates content such as starch, sucrose and glucose were accumulated in N-starved leaves either in cool or warm conditions more than in plants grown in plus NO3-. In addition, on leaf area basis, activity of photosystems 1 and 1+11 were some what greater in plants grown under cool than warm conditions and in the presence than in absence of NO3-. During natural senescence with adequate plant NO3- supply the activity of nitrate reductase (NR) increased and that of glutamine synthetase (GS) and glutamate synthase (GOGAT) was stable or increased, but decreased under N-starvation. On the other hand, the activity of NADH-glutamate dehydrogenase was relatively stable during senescence and was not affected by N starvation indicating that N assimilation could be carried out and might be increased during senescence if N was present and suggesting that proteolytic activity must be regulated during senescence. Furthermore, the activities of phosphoglycolate phosphatase (PGP) and glycolate oxidase (GO) enzymes associated with photorespiration, were decreased to low levels during senescence in the presence of NO3- either at 13 or 30 degree centigrade and decreased sharply to very small amounts in the absence of NO3-, whereas the activity of phosphoenolpyruvate phosphate (PEP) activity was relatively constant in the presence of NO3- and declined during senescence when NO3- was omitted.