Spatial distribution of photosynthetic response to long-term influence of elevated CO 2 in a mediterranean macchia mini-ecosystem

Abstract

During an open-top chamber experiment performed in evergreen ‘ macchia’ ecosystem, which was represented by the clumps of natural vegetation dominated by Quercus ilex trees, the trees were exposed to one of two CO 2 concentrations (ambient CO 2, AC-variant and elevated CO 2, i.e. ambient plus 350 μmol CO 2 mol −1, EC-variant) continuously over 5 years. Clumps of natural vegetation were enclosed in open top chambers (OTCs). Within the crowns of investigated Quercus ilex trees in OTCs, two crown layers i.e. sunny (E-leaves) and shaded (S-leaves), were identified as differing in solar radiation environment. To evaluate the effect of elevated CO 2, as well as the functional differentiation in assimilation activity of E- versus S-leaves, gas exchange and chlorophyll a̱ fluorescence techniques were used. The stimulatory effect of the long-term elevated CO 2 on the A N –PPFD relation was evident in E- and S-leaves of investigated Quercus trees. The A Nmax sensitivity of AC-variant leaves to the sudden application of elevated CO 2 was higher for S-leaves (42%) than for E-leaves (24%). The PPFD saturated rate of regulated thermal energy dissipation (ERD) confirmed the foliage differentiation caused by the long-term influence of elevated CO 2. The ERD of E- and S-leaves in the AC-variant were 1.11 times of that in the EC-variant. However, the estimated rates of photochemistry (ERP) of E- and S-leaves in the EC-variant were 1.35 and 1.22 times of E- and S-leaves in the AC-variant. The achievement of the critical value of q P=0.4 in E- and S-leaves from the EC-variant under lower values of PPFD compared to the AC-variant indicates a higher degree of PSII over-reduction. Thus, elevated CO 2 can be responsible for an increased susceptibility of photosynthetic apparatus to high irradiance. The obtained results support the hypothesis on the foliage vertical distribution effect on the whole canopy response to elevated CO 2. The S-layers of the canopy can play an important role in providing storage space for photosynthesis under elevated CO 2.