Photosynthese und Wasserhaushalt der immergrünen mediterranen Hartlaubptlanze Arbutus unedo L. im Jahresverlauf am Freilandstandort in Portugal: III. Einzelfaktorenanalyse zur Licht-, Temperatur- und CO2-Abhängigkeit der Nettophotosynthese

Abstract

Summary Steady state field measurements of the photosynthetic light-, temperature- and humidity-response of intact Arbutus unedo leaves were carried out parallel to the 1982/83 diurnal course measurements of CO 2 and H 2 O-gas exchange described in parts I and II of the present series. The following physiologically meaningful parameters were calculated from these single factor response curves, and subsequently related to the annual course of microclimate conditions of the growing site and plant water status: – Maximum light utilization efficiency α (initial slope of the net photosynthesis (NP) response curve to incident photosynthetically active radiation (PAR), measured under saturating CO 2 partial pressure): the annual course of this parameter (which is well known from the literature to be a general stress indicator and also an indicator for the activity of the photosynthetic light reaction) showed two marked phases of depression which occurred (i) during the low temperature period in the winter an (ii) during summer drought. The annual maximum was reached in newly flushed leaves in the spring. – Maximum photosynthetic capacity P ML (net photosynthesis rate at saturating PAR and under saturating CO 2 partial pressure, which frequently has been interpreted as a measure of the RuBP-regeneration capacity of the C alvin cycle): High values were exhibited in the late autumn, the P ML -rates then declined during winter time, with the lowest winter values occuring at the end of February, when the lowest temperatures of the year were recorded (see part I of the present series). As in the case of α there was another decline during the period of summer drought, when at all temperatures the P ML -values were found to be markedly lower than in February. The temperature optimum of P ML remained at approx. 30°C during most of the entire year. During the dry period, however, the temperature optimum was shifted to approx. 25°C. This effect was not observed in watered plants growing at the same site. Throughout the year, P ML was further found to be negatively and non-linearily correlated with the water status of the leaves (expressed as predawn water potential of comparable leaves) taken from the same tree outside of the gas exchange cuvette. – Carboxylation efficiency CE (initial slope of the photosynthetic CO 2 -response plotted vs. leaf internal CO 2 -partial pressure, Pi) which has frequently been interpreted as a measure for the activity of the CO 2 -binding enzyme RuBP-carboxylase-oxygenase: CE showed similar annual changes to those of P ml , which was also reflected in a significant linear correlation of the parameters throughout the seasons. Throughout the entire year, the leaves were always operating on the linear part of the photosynthetic CO 2 -response curve and CE was the major parameter determining the net CO 2 -uptake rate under normal ambient conditions.