Profiles of leaf senescence during reproductive growth of sunflower and maize

We investigated the effect of reproductive growth on the profiles of leaf s enescence in maize (Zea mays L.) and sunflower (Helianathus. annuus L.). Le af senescence after flowering was assessed using both structural (leaf chlo rophyll, nitrogen and dry matter) and functional (photosynthesis) variables in undisturbed plants (+ G) and in plants in which grain set was prevented (- G). Two weeks after flowering, lack of grain accelerated senescence in maize and delayed senescence in sunflower as indicated by leaf chlorophyll; leaf nitrogen and dry matter were less sensitive response variables. Lack of interaction between reproductive treatment and leaf position indicates t hat the senescence signal, whatever its nature, was equally effective throu ghout the plant in both species. In both species, feedback inhibition of ph otosynthesis was first detected 30-35 d after flowering; excess carbohydrat e in the leaves was therefore an unlikely trigger of accelerated senescence in maize. As reproductive development progressed, differences between + G and -G plants were more marked in sunflower, and tended to disappear or rev erse in maize. In sunflower, interactions between leaf position and reprodu ctive treatment-attributable to the local effect of grain-were detected aro und 20-27 d after flowering. (C) 2000 Annals of Botany Company .