Photostress, photoprotection, and water soluble antioxidants in the canopies of five Canarian laurel forest tree species during a diurnal course in the field

The Canarian laurel forest ecosystem is composed of several co-dominant evergreen trek. species including Ilex perado, I. canariensis, Myrica faya, Laurus azorica,and Persea indica. With leaves of these trees the diurnal course of stress parameters (chlorophyll fluorescence Fv/Fm, pigments, ascorbate, glutathione, gas exchange, water relations) was investigated during a mildly stressful summer day. Sun leaves generally had lower photochemical efficiencies (morning Fv/Fm in sun leaves were below 0.80 and above 0.80 in shade leaves), less chlorophyll, a larger xanthophyll cycle pool per unit chlorophyll, and more glutathione and ascorbate. Minimal relative water contents of more than 85% indicated that dehydration was not a stress factor. Stomatal conductances decreased from 150 to 200 mmol H2O m(-2) s(-1) in the morning to about 50 mmol H2O m(-2) s(-1) during the day in all species, but this did not limit CO, uptake. De-epoxidation of xanthophylls only occurred in sun leaves of I. canariensis (to more than 50%) and M. faya (more than 60%). Decreases in Fv/Fm were only found in sun leaves of P indica (from ca. 0.80 in the morning to a minimum of 0.70) and, as a trend, also in L. azorica (from ca. 0.75 to ca. 0.65). I. perado showed neither of those responses. P. indica and L. azorica exhibited the highest photosynthesis rates of about 10 mumol CO2 m(-2) s(-1) compared to 8 in the other species. The photoprotection strategy of P. indica and L. azorica admitted slow recovery from photoinhibition, did not activate protective energy dissipation through xanthophylls, and allowed highest production under these typical conditions.