| Abstrakt: |
Photosynthetic characteristics and leaf-water relations of 16 plant species belonging to different growth forms of mountain fynbos vegetation (South Africa) were investigated. Measurements were mainly performed at the dry border of fynbos after abundant winter rain in October 1983. Only the C3-mode of photosynthesis was found among the investigated plants. Light saturation of photosynthesis did not occur up to a photon flux density of 2 000μmolm−2s−1. Maximum rates of photosynthesis of all investigated species ranged from 2.8 to 12.7μmolm−2s−1with a mean of 5.6μmolm−2s−1. For evergreen Proteaceae, leaf age markedly determined photosynthesis and water-use efficiency. Daily CO2uptake per unit leaf area increased about 2.5-fold from young to mature leaves, while water-use efficiency increased maximally by 85%. Predawn water potential of all investigated plants ranged from 0.3 to 1.2MPa and decreased, except for Rhus dissecta(2.7MPa), until noon by only about 1MPa. In all plants, leaf water potential recovered until sunset, indicating the abundance of water in the soil. Due to the absence of a marked water deficit in most plants no midday depression of leaf conductance was found. With the exception of the geophyte Dilatris corymbosa, maximum leaf conductance was generally low, ranging from 61 to 176mmolm−2s−1and transpiration ranged from 2.2 to 9.2mmolm−2s−1. Our results of either photosynthesis or leaf conductance do not show any difference corresponding with the clear classification into growth forms or families. Due to the low mineral and nutrient content of fynbos soils, ion content in the plants was low. Generally, potassium exceeded sodium content. Calcium occurred in traces only. For proteoid plants, inorganic ions account for 20–40% of the osmotic potential and for ericoid plants, 70–90%. The results are discussed in comparison with other mediterranean vegetation and show that fynbos seems to have no exceptional position. |
