High tolerance to high-light conditions for the protected species (Cactaceae).

We determined the seasonal ecophysiological performance under perennial plants and under high solar radiation for adult individuals from the 'living rock' cactus , which occurs equally under nurse plants and in open spaces. We evaluated the effective quantum yield of photosystem II (Φ) and the dissipation of thermal energy [non-photochemical quenching (NPQ)] thorough the year. The maximum apparent electron transport rate (ETR) and the saturating photosynthetically active photon flux density for PSII (PFD) were also determined from rapid light curves. We found that although the Φ was higher in shaded sites under potential nurse plants than in exposed sites, all values were close to the optimal value of 0.83. The high Φ found for plants suggests that they use a great proportion of the absorbed light for photosynthesis, under nurse plants as well as in open spaces. We also found higher NPQ values in exposed sites than in shaded ones but only in Autumn, thus reducing the risk of photoinhibition. In addition, the PFD was higher in exposed sites than in shaded ones in Spring, Summer and Autumn, but in Winter there were no differences between treatments. We also found high saturating light levels for ETR (PFD higher than 1378 μmol m s) in all seasons but in winter for shaded and non-shaded plants. Our findings indicate that plants use a great proportion of the light that they absorb for photosynthesis. This high tolerance to high-light conditions could explain why do not show preferences for protected sites under nurse plants.