Senescence of Rice Leaves VI. Comparative Study of the Metabolic Changes of Senescing Turgid and Water-Stressed Excised leaves

Some metabolic changes of senescing turgid and water-stressedexcised rice leaves were compared under incubation in the dark.The decrease of the chlorophyll and protein level and the increaseof the -amino nitrogen were faster in the water-stressed leavesthan in the turgid leaves during the first two or three daysof incubation. However, the changes in levels of chlorophyll,protein and a-amino nitrogen were later retarded by water stress.The rate of decline in soluble sugar was more rapid in the turgidthan in the water-stressed leaves. In turgid leaves, there wasa slight but significant increase in the proline content inthe first day of incubation; subsequently, proline accumulatedrelatively rapidly, yet at a slower rate than in the stressedleaves. The activity of acid inorganic pyrophosphatase increased,but that of alkaline inorganic pyrophosphatase decreased duringthe senescence of both turgid and water-stressed leaves. Theenzyme activities are, therefore, due to two separate enzymeproteins. Water stress enhanced the increase of acid inorganicpyrophosphatase activity only during the first two days; itretarded the decrease of alkaline inorganic pyrophosphataseactivity at later stage of incubation. It is concluded that water stress does not accelerate all themetabolic changes associated with the senescence of excisedleaves. (Received January 24, 1981; Accepted March 27, 1981)