RESPONSE OF THE TUSSOCK GRASS STIPA-TENACISSIMA TO WATERING IN A SEMIARID ENVIRONMENT

1. Stipa tenacissima is a perennial tussock grass of arid and semi-ari d zones around the Mediterranean basin that occupies extensive areas i n south-eastern Spain. The environment of this region is extreme, with low irregular rainfall, high temperature and high irradiance, so that S. tenacissima tussocks are subjected to a high degree of environment al stress, especially during the summer months. 2. The ability of S. t enacissima to use a pulse of water applied in midsummer was tested in a field experiment in which extension growth and physiological perform ance of leaves of watered and unwatered plants were measured. 3. In un watered plants, when leaf extension had ceased, leaf water potential, relative water content, leaf conductance and net photosynthetic rate w ere low and minimum fluorescence was high. 4. Leaf conductance and net photosynthetic rate doubled, leaf extension resumed, minimum fluoresc ence (F-o) fell and photosynthetic efficiency of photosystem II (F-v/F -m) rose with watering. As the soil dried out again, values of these v ariables approached or equalled those of unwatered plants. 5. These ch anges in F-o and F-v/F-m suggested that the photosynthetic apparatus h ad not been damaged permanently, i.e. that S. tenacissima has reversib le photoprotective mechanisms. 6. We conclude that the decreased photo synthetic rate of unwatered plants in summer results from non-damaging photoinhibition and low leaf conductance. The latter is partly a cons equence of the folding of leaves, which was inversely proportional to relative water content. 7. The arrested development of S. tenacissima leaves was apparently not due to summer dormancy because leaves respon ded opportunistically within days to an applied pulse of water.