Showing papers by "Kenneth V. Thimann published in 1983"

Metabolism of Oat Leaves during Senescence : VII. The Interaction of Carbon Dioxide and Other Atmospheric Gases with Light in Controlling Chlorophyll Loss and Senescence.

Abstract: In air largely freed from CO2, senescence of isolated oat (Avena sativa cv Victory) seedling leaves is no longer prevented by white light; instead, the leaves lose both chlorophyll and protein as rapidly as in the dark. Senescence in light is also accelerated in pure O2, but it is greatly delayed in N2; 100% N2 preserves both protein and chlorophyll in light and in darkness. In light in air, most of the compounds tested that had previously been found to delay or inhibit senescence in darkness actually promote the loss of chlorophyll, but they do not promote proteolysis. Under these conditions, proteolysis can therefore be separated from chlorophyll loss. But in light minus CO2, where chlorophyll loss is rapid in controls, two of these same reagents prevent the chlorophyll loss. Unlike the many reagents whose action in light is thus the opposite of that in darkness, abscisic acid, which promotes chlorophyll loss in the dark, also promotes it in light with or without CO2. Kinetin, which prevents chlorophyll loss in the dark, also prevents it in light minus CO2. In general, therefore, the responses to light minus CO2 are similar to the responses to darkness, and (with the exception of abscisic acid and kinetin) opposite to the response to light in air.

Relation between Respiration and Senescence in Oat Leaves

Abstract: The respiration of excised oat (Avena sativa cv Victory) leaves and their sensitivity to inhibitors was followed during senescence under varied conditions. The respiration rate, which in controls reaches its peak on the third day in darkness, is lowered at the time of fastest loss of chlorophyll (as reported earlier) by seven unrelated reagents that all delay dark senescence. When senescence is delayed by white light or by cytokinins, the respiratory rise is correspondingly delayed. Kinetin and l-serine, which act as antagonists on senescence, also act as antagonists on the respiratory rate. However, an exception to this close correspondence between senescence and the respiratory rise is offered by the lower aliphatic alcohols, which delay dark senescence and yet accelerate the onset of the respiratory rise.The respiration of freshly cut leaves is insensitive to KCN up to 8 millimolar, but sensitive to benzhydroxamate (BAM), 1 to 2 millimolar BAM causing 25% promotion and higher concentrations inhibiting. At the respiratory peak, however, part of the respiration becomes KCN-sensitive. Low concentrations of alcohols in darkness, or 3-(3,4-dichlorophenyl)-1,1-dimethylurea, diuron, in light, also render part of the respiration KCN-sensitive, but this sensitivity soon disappears again. Some 10 to 15% of the respiration is insensitive to both inhibitors. Thus, cyanide sensitivity comes and goes, while BAM sensitivity is always present. The current concept of the cyanide-resistant pathway as an overflow, therefore, does not fit well with behavior of these leaves. The respiratory rise in leaf senescence is similar to, but not identical with, the climacteric in ripening fruits and the aging phenomenon in tuber slices.