Photosynthese und Wasserhaushalt der immergrünen mediterranen Hartlaubpflanze Arbutus unedo L. im Jahreslauf am Freilandstandort in Portugal: I. Tagesläufe von CO2-Gaswechsel und Transpiration unter natärlichen Bedingungen

Abstract

Summary Diurnal courses of CO2 and H2O gas exchange of intact, attached leaves of the mediterranean evergreen sclerophyll Arbutus unedo were measured under natural ambient conditions during an entire vegetation period in a macchia in Portugal. The measurements were performed with a mobile field laboratory with two climatizised gas exchange cuvettes. During the entire season, measurements of leaf water potential (pressure chamber) were conducted in parallel with the gas exchange experiments. The gas exchange behaviour of well watered plants during the dry season was also studied. The primary period of photosynthetic carbon gain is between November and May. As soon as the first rains of the autumn begin, there is a pronounced increase in net photosynthesis. The maximum photosynthetic rates of the year are exhibited by the recently flushed young leaves in the spring. In contrast to other mediterranean evergreens net photosynthetic rates of Arbutus unedo leaves under normal atmospheric conditions do not decrease during the cold period in the winter. Therefore, as was also shown by factor-dependence measurements at the same time of year, the photosynthetic apparatus of these leaves seems to be relatively insensitive to low temperatures. Under water stress conditions in the summer, a marked decrease in net photosynthesis can be seen. Beginning at the end of May, a midday stomatal closure can be observed together with a simultaneous depression of net photosynthesis and transpiration. The water loss and the carbon gain of the leaves is drastically curtailed by these reactions. During the summer dry period, midday stomatal closure can also be observed with well watered plants. High CO2 loss which might be caused by a temperature-dependent increase of res piration in the summer is avoided by a corresponding temperature acclimation of dark respiration. The discussion deals first with the general influence of water and temperature stress on the annual course of photosynthetic production of mediterranean evergreen sclerophylls. Due to these stress factors, the primary production of these plants in many cases is markedly lowered during the cold season and during the dry summer period compared with the considerably higher production rates measured in the autumn and spring. In contrast to this annual pattern characteristics of many sclerophylls, the less sclerophyllous and less deeply rooted Arbutus unedo must tolerate a greater degree of water stress during the dry summer period. Thus, in order to reduce water loss Arbutus frequently closes its stomata for a longer time than the more sclerophyllous species, which grow at the same site. The prolonged stomatal closure is also correlated with a greatly reduced photosynthetic carbon gain during the summer. However, the low temperature tolerance of Arbutus and its comparatively high photosynthetic rates throughout the winter, which are also likely responsible for the comparatively early flushing of the new leaves in the spring, may allow Arbutus to compensate for this low summer carbon gain and to compete successfully with the other evergreen species. The second part of the discussion addresses possible reasons for midday stomatal closure and the concomitant depression of net photosynthesis and photosynthestic activity of the mesophyll. The diurnal changes of stomatal conductance appear to be in response to changes in air humidity. The midday depression of mesophyll photosynthetic activity may be explained as a response to the combined effect of air humidity and temperature. Increased respiration occuring in the light at high temperatures may also playa role in the observed midday depression of net photosynthesis. However, for Arbutus unedo, reduced CO2 supply to the mesophyll due to partial stomatal closure can be excluded as a cause of reduced photosynthesis in the most cases because the leaf internal CO2 partial pressure was found to remain relatively constant or even to increase during the midday period. A coordination in function of stomata and mesophyll photosynthetic reactions is discussed. The importance of a depression in transpiration at midday for high water use efficiency of these plants is shown and the possible significance of reduced mesophyll photosynthetic activity in minimizing photo inhibition is also discussed.