The Metabolism of Oat Leaves during Senescence

Abstract

When the detached first leaves of green or etiolated oat (Avena sativa cv. Victory) seedlings senesce in the dark, their oxygen consumption shows a large increase, beginning after 24 hours and reaching a peak of up to 2.5 times the initial rate by the 3rd day. This effect takes place while the chlorophyll of green leaves, or the carotenoid of etiolated leaves, is steadily decreasing. Kinetin, at a concentration which inhibits the decrease in pigment, completely prevents the respiratory rise; instead, the oxygen consumption drifts downwards. Lower kinetin concentrations have a proportional effect, 50% reduction of respiration being given by about 0.1 mg/l. About one-fifth of the respiratory rise may be attributed to the free amino acids which are liberated during senescence; several amino acids are shown to cause increases of almost 50% in the oxygen consumption when supplied at the concentrations of total amino acid present during senescence. A smaller part of the rise may also be due to soluble sugars liberated during senescence, largely coming from the hydrolysis of a presumptive fructosan. The remainder, and the largest part, of the increase is ascribed to a natural uncoupling of respiration from phosphorylation. This is deduced from the fact that dinitrophenol causes a similar large rise in the oxygen consumption of the fresh leaves or of leaf segments kept green with kinetin, but causes only a very small rise when the oxygen consumption is near its peak in senescent controls. The respiration of these leaves is resistant to cyanide, and 10 mM KCN even increases it by some 30%; in contrast, etiolated leaves of the same age, which undergo a similar rise in oxygen consumption over the same time period, show normal sensitivity to cyanide. The respiratory quotient during senescence goes down as low as 0.7, both with and without kinetin, though it is somewhat increased by supplying sugars or amino acids; glucose or alanine at 0.3 M bring it up to 1.0 and 0.87, respectively. N6-Benzylaminopurine and A-2-isopentenylaminopurine act similarly to kinetin in repressing the respiratory rise, the former being five times as active as kinetin, while the latter has only 1 % of the activity of kinetin. Zeatin also powerfully prevents senescence. Because the repression of the respiratory rise is shown by each cytokinin at the concentration at which it inhibits senescence, the action is ascribed in both cases to the maintenance of a tight coupling between respiration and phosphorylation. It is pointed out that such an effect would explain many features of cytokinin action.I This work was supported in part by Grants 35238X and 11867 from the National Science Foundation to K. V. Thimann.'This paper is dedicated to the memory of Solon A. Gordon because of his continued interest in plant metabolism and his many contributions to it, and in recognition of a long standing friendship.A change in the methodology of the senescence experiments is described and compared with the method previously used, and the influence of temperat...

Photosynthesis, Leaf Resistances, and Ribulose-1,5-Bisphosphate Carboxylase Degradation in Senescing Barley Leaves

“…3A). Starch does accumulate in primary leaves of barley (unpublished data) or oats (16). A basipetal translocation of amino acids, but not soluble sugars, occurs when oat leaves senesce (19).…”

“…The rapidity of the stomatal response suggests that hormonal influences may be important here. Application of hormones to senescing leaves has a pronounced effect on stomatal resistance (17) and phenomena intimately associated with CO2 assimilation or evolution, such as RuBPCase (12,25), RuBPCase activity (12), and dark respiration (16,18 …”

“…It is possible that the accumulation of nonreducing sugars results in a feedback inhibition of photosynthesis. Dark respiration increases during leaf senescence (16,18,19,21,26). The increase in soluble sugars and dark respiration during leafsenescence indicates that C is not conserved to the same extent as N.…”

The Metabolism of Oat Leaves during Senescence

“…it is dependent on some synthetic processes, probably formation of enzymes, at the start. There is evidence that the increased respiration is due to an uncoupling phenomenon and that cytokinins act to maintain the tightness of the coupling (33). It has also been found that proteolysis precedes the beginning of loss of Chl by some 18 to 20 hr at 25 C, and that levels of two proteinases in these leaves increase during senescence, an increase which is prevented by cycloheximide (21).…”

“…As will be shown, the antibiotics are not without an effect on senescence. Each of the 12 concave wells in a porcelain plate received one such 0.25-ml preparation; the plate rested on moist filter paper in a 14-cm Petri dish, and was either left in the growth chamber at 25 C in the dark or exposed to fluorescent white light of (21,33), and Michael (22) deduced that the protein-Chl ratio remained constant during leaf senescence. The probable explanation for the large difference here is that in the chloroplasts, but not in the leaf, the Chl is subjected to photooxidation, and this will be confirmed below.…”