Abstract
Responses of leaf water relations and photosynthesis to summer drought and autumn rewetting were studied in two evergreen Mediterranean oak species, Quercus ilex spp. rotundifolia and Quercus suber. The predawn leaf water potential (Ψ(lPD)), stomatal conductance (gs) and photosynthetic rate (A) at ambient conditions were measured seasonally over a 3-year period. We also measured the photosynthetic response to light and to intercellular CO₂ (A/PPFD and A/C(i) response curves) under water stress (summer) and after recovery due to autumn rainfall. Photosynthetic parameters, Vc(max), J(max) and triose phosphate utilization (TPU) rate, were estimated using the Farquhar model. RuBisCo activity, leaf chlorophyll, leaf nitrogen concentration and leaf carbohydrate concentration were also measured. All measurements were performed in the spring leaves of the current year. In both species, the predawn leaf water potential, stomatal conductance and photosynthetic rate peaked in spring, progressively declined throughout the summer and recovered upon autumn rainfall. During the drought period, Q. ilex maintained a higher predawn leaf water potential and stomatal conductance than Q. suber. During this period, we found that photosynthesis was not only limited by stomatal closure, but was also downregulated as a consequence of a decrease in the maximum carboxylation rate (Vc(max)) and the light-saturated rate of photosynthetic electron transport (J(max)) in both species. The Vc(max) and J(max) increased after the first autumnal rains and this increase was related to RuBisCo activity, leaf nitrogen concentration and chlorophyll concentration. In addition, an increase in the TPU rate and in soluble leaf sugar concentration was observed in this period. The results obtained indicate a high resilience of the photosynthetic apparatus to summer drought as well as good recovery in the following autumn rains of these evergreen oak species.
Highlights
In the south-western Iberian Peninsula with a Mediterraneantype climate, the landscape is dominated by evergreen oak savannas, exploited as multipurpose silvopastoral systems
The main objectives of this work were to study (i) the photosynthetic resilience of these Mediterranean evergreen oaks to seasonal drought and (ii) whether the reversibility of drought-induced downregulation of the photosynthetic machinery occurs upon the first autumn rains, and if it does, what underlying processes are involved?
This study contributed to the literature by showing the capacity of survival of holm and cork oaks under Mediterranean summer drought conditions
Summary
In the south-western Iberian Peninsula with a Mediterraneantype climate, the landscape is dominated by evergreen oak savannas, exploited as multipurpose silvopastoral systems. Covering a large area (2–2.5 million hectares), the main tree species are Quercus ilex spp. rotundifolia Quercus ilex spp. ballota) (holm oak) and Quercus suber (cork oak). Both species often coexist, Q. ilex mainly occurs in the drier, inland regions, whereas Q. suber dominates in the wetter, western coastal areas. In these ecosystems, trees must withstand a long hot and dry summer season when water deficits are associated with high light intensity and temperatures, which impose restrictions on carbon assimilation (Faria et al 1998, Chaves et al 2002, Pereira et al 2004, 2007).
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