INFLUENCE OF DROUGHT, RAIN AND ARTIFICIAL IRRIGATION ON PHOTOSYNTHESIS, GAS-EXCHANGE AND WATER RELATIONS OF THE FYNBOS PLANT PROTEA-ACAULOS(L) REICH AT THE END OF THE DRY SEASON

Protea acaulos, a prostrate fynbos shrub, often experiences very low a ir humidity at leaf temperatures over 10 degrees C higher than mean ai r temperature. We determined to what degree this particular microclima te influenced photosynthet;ic performance, leaf conductance and water relations of non-irrigated and trickle-irrigated plants. Measurements were made at the end of the dry summer season in the sand plain lowlan d fynbos on the west coast of South Africa. Independent of water suppl y, plants showed a pronounced midday depression of gas exchange. While in non-irrigated plants leaf water potential dropped to -2.0 MPa arou nd noon, it never fell below -1.0 MPa in irrigated plants. On the othe r hand minimum pressure potential was similar in irrigated and non-irr igated plants. The latter showed higher turgor after rain, due to osmo tic acclimation, which resulted from a reduction in maximum water volu me. The main osmoticum was 1,5-anhydro-D-glucitol. Leaf temperature, d irectly or via the vapour pressure deficit between leaf and air (Delta w), rather than plant water status, was the determinant of the midday depression of gas exchange. High Delta w caused stomatal closure duri ng times of saturating light, thus limiting photosynthetic CO2 uptake and availability and enhancing the susceptibility for photoinhibition. This, as well as high leaf temperature per se, decreased the efficien cy of photochemistry of photosystem II. Initial fluorescence remained constant until temperatures exceeded 35 degrees C, above which changes in fluorescence indicated both photoinhibition and heat stress. Unlik e other fynbos plants, Protea acaulos could not use the improved soil water supply to increase carbon gain under hot summer condition.