Photosynthetic CO 2-Assimilation, Chlorophyll Fluorescence and Zeaxanthin Accumulation in Field Grown Maple Trees in the Course of a Sunny and a Cloudy Day

Abstract

Changes in photosynthetic CO 2 fixation, variable chlorophyll fluorescence ratios, carotenoid composition, zeaxanthin accumulation and the de-epoxidation state DEPS of the zeaxanthin cycle were simultaneously determined in leaves of field-grown maple trees (Acer platanoides L.) on a sunny and a cloudy day in August. In the course of the sunny day the photosynthetic net CO 2 assimilation rates P N rose with the PPFD in the morning and were then predominantly determined by the stomata opening (partial closure at noon and further closure in the afternoon as seen from the decrease in gH 2 O-values). When the rate of Co 2 -fixation began to decrease at a high sun irradiance (high PPFD) at about 10 :00 am (partial closure of stomata), large zeaxanthin amounts were accumulated in a biphasic process : 1) by a fast phototransformation of existing violaxanthin into zeaxanthin and 2) by a slow but continuous increase of the zeaxanthin level of 50 to 60% via de novo biosynthesis. A strong zeaxanthin accumulation depends on a high proton gradient and only proceeded when P N was light-saturated and was reduced by a partial closure of stomata. The zeaxanthin accumulation in the morning followed the rise in PPFD with some delay as well as the zeaxanthin retransformation to violaxanthin at the PPFD decrease in the late afternoon and evening. On the sunny day the photochemical and non-photochemical quenching coefficients of the chlorophyll fluorescence, qP and qN, showed an inverse relationship. With increasing PPFD qP declined to very low values (below 0.1) and qN increased to high values >0.9. The variable chlorophyll fluorescence ratios Fv'/Fm' and ΔF/Fm', indicators of the quantum efficiency of photosynthetic electron transport of photosystem II, declined parallel to the high PPFD-induced decline of qP and the rise in qN. There was, however, no clear correlation between the changes in chlorophyll fluorescence parameters and the rise and fall of zeaxanthin in the course of the sunny day. In fact, in the afternoon the zeaxanthin level decreased much earlier than the decline in qN and the rise and regeneration of the initial values of qP, Fv'/Fm' or ΔF/Fm'. Despite the strong increase in qN and the large decrease in qP, Fv'/Fm' and ΔF/Fm', the net CO 2 assimilation rates P N of maple leaves only declined by 22 % as compared to the maximum P N values at 10 :15 am indicating that the photosynthetic apparatus was only marginally photoinhibited. It is assumed that only a small fraction of chloroplasts at the upper leaf side became photoinhibited and functioned as a light filter for the other chloroplasts in the lower mesophyll parts which remained photosynthetically active. The results demonstrate that the changes of the chlorophyll fluorescence parameters qP and qN as well as of the ratios Fv'/Fm' or ΔF/Fm', measured with the PAM fluorometer at the upper leaf side, are not representative of the total pool of leaf chloroplasts. For this reason it is suggested to measure the chlorophyll fluorescence parameters not only at the upper but also at the lower leaf side in order to obtain more representative information on the chloroplasts of the whole leaf. In addition, chlorophyll fluorescence measurements should always be complemented by net CO 2 fixation measurements in order to investigate to which degree a presumed photoinhibition really exists at the whole leaf level. On the cloudy day up to 3 pm the PPFD was very low, yet net photosynthesis (P N ) was performed at good rates.