PARENT SESSION
Posters P3B Photosynthetic acclimation: Ecophysiology, diverse environments. Abstracts (479-522)

Photosynthesis limitation in Quercus ilex under drought: stomatal and mesophyll conductance, and leaf morphology contribution. Karen Peña-Rojas¹, Xavier Aranda^{1, 2}, Isabel Fleck^*,1, ¹ Departament de Biologia Vegetal. Unitat Fisiologia Vegetal., Barcelona., Spain² Departament de Tecnologia Hortícola., Cabrils., Barcelona., Spain

ABSTRACT- Holm-oak (Quercus ilex L.), a dominant species in the Mediterranean forest, has a great resprouting capacity following fire, clear-cut, grazing and other perturbations. This evergreen oak is often exposed to low temperatures in winter, high irradiance, heat and drought in summer and disturbances like forest fire. We report variations in photosynthetic parameters from winter (W) to summer (S) in undisturbed holm-oak (C) and resprouts (R) after clear-cut, and how morphological characteristics and environmental changes limit CO₂assimilation. Particularly, we studied the contribution of mesophyll conductance to CO₂ (g_mes) limiting assimilation in this species and its effects on leaf carbon isotope discrimination. Resprouts showed similar gas-exchange and chlorophyll fluorescence values to undisturbed control plants in W. From W to S, leaf density decreased and thickness increased in R, while they remained constant in C, resulting in higher mesophyll conductance to CO₂ (g_mes) and higher photosynthesis in R with respect to C, both influencing the absolute values of leaf carbon isotope discrimination. Daily variations in g_mes in S were not associated with morphological changes, thus suggesting that g_mes was under physiological control. A predominant role of g_mes in the overall CO₂ leaf diffusion during drought was indicated by a large decrease (6-fold) from W to S in the ratio of g_mes/ g_s. Seasonal and diurnal variations in g_mes paralleled decreases of net photosynthesis (A) and photosynthetic capacity (A_max), which indicates down-regulation of A in response to sustained low chloroplast CO₂ levels. Low values of potential quantum yield of PSII, indicating photodamage to PSII centres, were also observed in some situations. We conclude that g_mes, and its short-term regulation, might be more important than stomatal closure in limiting A during drought.

KEY WORDS: leaf morphology, mesophyll conductance, gas-exchange, carbon isotopic composition