CHLOROPHYLL FLUORESCENCE, STRESS AND SURVIVAL IN POPULATIONS OF MEDITERRANEAN GRASSLAND SPECIES

Photosynthetic stress in response to a natural episode of frost and se asonal drought was assessed in a 'dehesa' grassland of SW Spain with a portable fluorimeter. Chlorophyll fluorescence characteristics of dar k-adapted leaves of 11 abundant species of Mediterranean grassland wer e measured over the course of a growing season from November 1992 to J uly 1993. Concomitant changes in population size were estimated from c ensuses of permanent quadrats. There was a general decline in the phot ochemical efficiency (F-v/F-m) during the growing season and this was particularly evident late in the growing season (spring and early summ er) when ambient temperatures were increasing rapidly and rainfall was low; it coincided with the period of most intense mortality for most species. A frost in early March (-5 degrees C), when photosyntheticall y active radiation was relatively high, resulted in a small decrease i n F-v/F-m that was consistent across many species. The mechanisms of p rotection in species of Mediterranean grassland appear to be sufficien tly effective to avoid damage to PSII for most of the year. For most s pecies there was little evidence of photosystem II damage, as initial fluorescence (F-0) usually did not increase. Many of the effects obser ved were due to a reduction in F-m and thus were consistent with non-p hotochemical quenching. This could be adaptive in protecting PSII from damage in species that show little evidence of stress. The sharp incr ease in stress toward the end of the life cycle coincided with the fal l in net population size.