Possible roles of sugars on leaf senescence

Abstract

Availability of nitrogen almost always limits plant growth. In plants, especially when they form dense stands, old, lower leaves are shaded with the development of younger upper leaves. Nitrogenous compounds in such shaded leaves are degraded and re-allocated to upper leaves. This process referred to leaf senescence and important for efficient use of nitrogen in photosynthetic production. Hypotheses (such as sugar repression) concerning mechanisms how leaves sense their photosynthetic status in a plant individual and regulate senescence processes were examined using Phaseolus vulgaris plants. Plants were grown under medium light intensity (300 m mol m-2 s-1) with nutrient supply (6 mM NO3-). In one group of plants, upper trifoliate leaves were shaded with primary leaves being exposed. Control plants were unshaded. In primary leaves of the plants with upper leaf shading, photosynthetic activity retained and sugar (glucose, sucrose, starch) accumulation was repressed. On the other hand, in unshaded plants, photosynthetic activity of the primary leaves decreased after full expansion. To examine whether sugar repression involves in onset of senescence, plants were grown under different nutrient supply in combination with or without shade treatments (whole plant or partial shading), and the amounts of Rubisco, sugar and transcript of Rubisco small subunit (rbcS) in the primary leaves were measured in the course of leaf development and senescence. Leaves were sampled about 1h and 12 h after the onset of the light period to estimate sugar accumulation. Possible roles of sugar and other factors in onset of leaf senescence are discussed.