Der CO2- und H2O-Gasaustausch von Pflanzen in der Krautschicht eines Kalkbuchenwaldes in Abhängigkeit von Standortsfaktoren. I. Lichtabhängigkeit der Photosynthese im Jahresgang

Abstract

In the years 1982, 1983 and in spring 1984 the CO 2 exchange of the leaves of six understorey plant species as well as light intensity (PAR), vapor concentration gradient between the leaf and the surrounding air (ΔW) and temperature of leaves (T 1 ) and ambient air (T a ) were measured continuously in the herb layer of a beach forest on limestone. The measurements were carried out with a climatized gas exchange chamber. In this paper the relationship between leaf photosynthesis and light intensity and its variation during the course of the year were studied using two spring geophytes (Allium ursinum, Arum maculatum), a summergreen herb ( Mercurialis perennis ), a wintergreen herb ( Asarum europaeum ) and two summergreen grasses ( Hordelymus europaeus, Mélica uniflora. ) In spring the photosynthetic capacity (Pn max ) and the light saturation of leaf photosynthesis (Is) is high and increased in the course of leaf development. The apparent quantum yield (φ) is low. Only the old leaves of the wintergreen herb A. europaeum have low Pn max - and Is-values in spring. The values of the spring geophytes are on a level similar to those of light plants, whereas the values of M. perennis and of the young leaves of A. europaeum are lower. After overstorey canopy closure, Pn max and Is decreased while φ increased. The change in the photosynthetic activity is a result of the low light intensity in the understorey layer during the summer. In spring the increase of Pn max is well correlated with the total chlorophyll content (Chl a + b). With increasing darkness Chl a + b increased distinctly. Whereas Pn max and I s decreased probably due to the reduction of the protein content in the leaves, the increase of Chl a + b improved the quantum yield efficiency of the photosynthesis. The decrease of Pn max and I s as well as the increase of φ are important adaptations for maximizing the photosynthetic CO 2 gain during the dark summer conditions. Especially in the summer 1982, the poor water supply measured by low water potentials of the leaves (φ L ) lowered φ. In the autumn the water potentials rose together with φ as a result of high precipitation. Dark respiration (R D ) and light compensation point (I K ) are distinctly influenced by leaf temperature. That is why both parameters have to be temperature corrected if they shall be compared over the whole vegetation period. In spring I K and R D are high due to high photosynthetic activities and/or due to the development of young tissue. After overstorey canopy closure, R D and I K decreased similarily to photosynthetic capacity. Reductions of R D and I K are also an adaptation to the low light intensities in the understorey layer, favored by little photosynthetic activity in summer. The small CO 2 losses because of low respiration rates coupled with a low light compensation point and high apparent quantum yield efficiencies permit reaching a positive CO 2 balance in summer, in spite of the low light intensity in the understorey layer.