Changes in Pigment and Element Composition of Leaves of Platanus occidentalis L. during Autumnal Leaf Senescence1

The loss of chlorophyll and total leaf nitrogen during autumnal senescence of leaves from the deciduous tree Platanus occidentalis L. was accompanied by a marked decline in the photosynthetic capacity of 02 evolution on a leaf area basis. When expressed on a chlorophyll basis, however, the capacity for lightand C02-saturated O2 evolution did not decline, but rather increased as leaf chlorophyll content decreased. The photon yield of 02 evolution in white light (400-700 nanometers) declined markedly with decreases in leaf chlorophyll content below 150 milligrams of chlorophyll per square meter on both an incident and an absorbed basis, due largely to the absorption of light by nonphotosynthetic pigments which were not degraded as rapidly as the chlorophylls. Photon yields measured in, and corrected for the absorptance of, red light (630-700 nanometers) exhibited little change with the loss of chlorophyll. Furthermore, PSII photochemical efficiency, as determined from chlorophyll fluorescence, remained high, and the chlorophyll a/b ratio exhibited no decline except in leaves with extremely low chlorophyll contents. These data indicate that the efficiency for photochemical energy conversion of the remaining functional components was maintained at a high level during the natural course of autumnal senescence, and are consistent with previous studies which have characterized leaf senescence as being a controlled process. The loss of chlorophyll during senescence was also accompanied by a decline in fluorescence emanating from PSI, whereas there was little change in PSII fluorescence (measured at 77 Kelvin), presumably due to decreased reabsorption of PSII fluorescence by chlorophyll. Nitrogen was the only element examined to exhibit a decline with senescence on a dry weight basis. However, on a leaf area basis, all elements (C, Ca, K, Mg, N, P, S) declined in senescent leaves, although the contents of sulfur and calcium, which are not easily retranslocated, decreased to the smallest extent. The loss of Chl and decline in photosynthesis are two of the phenomena associated with the process of senescence in This work was supported by the Deutsche Forschungsgemeinschaft and by the Bayerisches Staatsministerium fur Landesentwicklung und Umweltfragen. W. W. A. gratefully acknowledges the support of Fellowships from the North Atlantic Treaty Organization and the Alexander von Humboldt-Stiftung. 2 Present address: Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO 80309. leaves of higher plants (6, 19, 24, 29, 31, 34). Photosynthesis at saturating photon flux densities is generally independent of Chl content throughout the greater portion of a green leaf s life span and is, instead, dependent on factors such as the capacity for the enzymatic fixation of CO2 into starch and sucrose and the capacity for photosynthetic electron transport (4, 30). At limiting PFDs, photosynthesis based on incident light (the apparent photon yield) has been found to be dependent on Chl content (4, 5, 13). When corrected for changes in absorptance, however, the photon yield is unaffected by changes in Chl content, at least in the range found in healthy green leaves (4, 5). In pale green to yellow leaves such corrections may be more complicated due to an increase in the proportion of light which is absorbed by components of the leaf other than Chl. Furthermore, during senescence, it is possible that the active breakdown of chloroplasts and catabolism of Chl and other components of the photosynthetic apparatus could result in a lowering of the capacity and efficiency of photosynthetic energy conversion for those components remaining in the leaf. We have examined the changes in a number of photosynthetic characteristics from leaves of Platanus occidentalis during the natural course of autumnal senescence to evaluate the ability of the leafto maintain high levels ofphotosynthetic efficiency in those components (particularly Chl) which remain as degradation progresses. MATERIALS AND METHODS Leaves from a large tree ofPlatanus occidentalis L., growing in the Wurzburg Botanical Garden, were examined during October of 1987 and September and October of 1988. During 1987 many leaves were collected prior to dawn, and the majority of October was overcast. In both years care was taken to use leaves which were not receiving direct illumination from the sun at the time of collection in order to avoid the possibility of any high-light induced reductions in photochemical efficiency (1, 2, 8). The leaves of P. occidentalis do not senesce synchronously, and thus leaves with different Chl contents (i.e. in various stages of senescence) could be examined at a given time during the period of this study. Furthermore, only leaves with a uniform color were chosen, and no leaves were examined after minimum nocturnal temperatures reached 0°C. 1184 PHOTOSYNTHESIS AND CHL FLUORESCENCE DURING LEAF SENESCENCE