Abstract
We determined the seasonal ecophysiological performance under perennial plants and under high solar radiation for adult individuals from the 'living rock' cactus Ariocarpus kotschoubeyanus, which occurs equally under nurse plants and in open spaces. We evaluated the effective quantum yield of photosystem II (ΦPSII) and the dissipation of thermal energy [non-photochemical quenching (NPQ)] thorough the year. The maximum apparent electron transport rate (ETRmax) and the saturating photosynthetically active photon flux density for PSII (PFDsat) were also determined from rapid light curves. We found that although the ΦPSII 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 ΦPSII found for A. kotschoubeyanus 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 PFDsat 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 (PFDsat higher than 1378 μmol m-2 s-1) in all seasons but in winter for shaded and non-shaded plants. Our findings indicate that A. kotschoubeyanus plants use a great proportion of the light that they absorb for photosynthesis. This high tolerance to high-light conditions could explain why A. kotschoubeyanus do not show preferences for protected sites under nurse plants.
Highlights
Materials and methodsIn response to harsh conditions, many species appear to be more frequent under canopies of adult plants of other species which provide a less stressful micro-environment (Ellner and Shmida, 1981)
The high ΦPSII found for A. kotschoubeyanus plants suggests that they use a great proportion of the absorbed light for photosynthesis, under nurse plants as well as in open spaces
The electron transport rate (ETR) across the electron chain of chloroplasts was estimated as ETR = ΦPSII × Photon flux density (PFD) × 0.84 × 0.5, where ΦPSII is the effective quantum yield of photosystem II, PFD is the photosynthetic PFD recorded by the sensor in the leaf clip, 0.84 is the estimated mean proportion of incident light absorbed by the photosystems (Ehleringer, 1981) and 0.5 is the required factor for both photosystems to account for absorbed photons (Roberts et al, 1996)
Summary
In response to harsh conditions, many species appear to be more frequent under canopies of adult plants of other species which provide a less stressful micro-environment (Ellner and Shmida, 1981). Schum (Cactaceae) is an especially protected species in the framework of the environmental laws and regulations of México (Semarnat, 2010), and as near threatened in the framework of the international regulations of IUCN (Gómez-Hinostrosa et al, 2013), as well and is listed under Appendix 1 of CITES (Sajeva et al, 2012) This species occurs under nurse plants and in open spaces at the southern part of its distribution (Suzán-Aspiri et al, 2011). The RLC were produced using two scales of the Mini-PAM light curve program to obtain a sequence of 0, 255, 399, 590, 807, 1184, 1587 and 2372 μmol of PFD m−2 s−1 These RLC were obtained in exposed plants (n = 6) and under the more frequent nurse plant found (n = 6), once each year season. If required to comply with the assumption of normal distribution (Sokal and Rohlf, 1995)
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