Photosynthesis and Chlorophyll Fluorescence Characteristics in Relationship to Changes in Pigment and Element Composition of Leaves of Platanus occidentalis L. during Autumnal Leaf Senescence.

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 O(2) evolution on a leaf area basis. When expressed on a chlorophyll basis, however, the capacity for light-and CO(2)-saturated O(2) evolution did not decline, but rather increased as leaf chlorophyll content decreased. The photon yield of O(2) 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.