Holm oak (Quercus ilex L.) is a Mediterranean species that can withstand intense summer drought through a high resistance to cavitation far beyond the stomatal closure. Besides stomatal limitations, both mesophyll and biochemical limitations to CO2 uptake could increase in holm oak under drought. However, no studies have addressed how hydraulic and non-hydraulic factors may limit the recovery of photosynthesis when re-watering after inducing 50% loss of hydraulic conductivity. We measured photosynthetic traits, xylem embolism, and abscisic acid (ABA) in holm oak with increasing levels of drought stress and seven days after plant re-watering. Drought stress caused a sharp decrease in net CO2 assimilation (AN), stomatal and mesophyll conductance (gs and gm), and maximum velocity of carboxylation (Vcmax). The stomatal closure could be mediated by the rapid increase found in ABA. The high level of xylem embolism explained the strong down-regulation of gs even after re-watering. Therefore, only a partial recovery of AN was observed, in spite of non-hydraulic factors not limiting the recovery of AN, because i/ABA strongly decreased after re-watering, and ii/gm and Vcmax recovered their original values. Therefore, the hydraulic-stomatal limitation model would be involved in the partial recovery of AN, in order to prevent extensive xylem embolism under subsequent drought events that could compromise holm oak survival.
Nowadays, evergreen sclerophyllous and winter-deciduous malacophyllous oaks with different paleogeographical origins coexist under Mediterranean-type climates, such as the mixed forests of the evergreen Quercus ilex subsp. rotundifolia and the winter-deciduous Quercus faginea. Both Mediterranean oaks constitute two examples of contrasting leaf habit, so it would be expected different functional strategies to cope with summer drought. In this study, we analysed photosynthetic, photochemical and hydraulic traits of different organs for Q. faginea and Q. ilex subsp. rotundifolia under well-watered conditions and subjected to very severe drought. The coordinated response between photosynthetic and hydraulic traits explained the higher photosynthetic capacity of Q. faginea under well-watered conditions, which compensated its shorter leaf life span at the expense of a higher water consumption. The progressive imposition of water stress evidenced that both types of Mediterranean oaks displayed different functional strategies to cope with water limitations. Specifically, the decrease in mesophyll conductance (gm) associated to edaphic drought seems to be the main factor explaining the differences found in the dynamics of net CO2 assimilation (AN) throughout the drought period. The sharp decline in photosynthetic traits of Q. faginea was coupled with a strong decrease in shoot hydraulic conductance (Kshoot) in response to drought. This fact probably avoided extensive xylem embolism in the stems (i.e., “vulnerability segmentation”), which enabled new leaf development after drought period in Q. faginea. By contrast, leaves of Q. ilex subsp. rotundifolia showed effective photoprotective mechanisms and high resistance to drought-induced cavitation, which would be related with the longer leaf life span of the evergreen Mediterranean oaks. The co-occurrence of both types of Mediterranean oaks could be related to edaphic conditions that ensure the maintenance of soil water potential above critical values for Q. faginea, which can be severely affected by soil degradation and climate change.
Extreme droughts and heat events, frequently produced in Mediterranean climates, induce anomalies in the ecosystem-atmosphere CO 2 fluxes. In order to mitigate the consequences on forests and agriculture, managers must have a better knowledge of the ecosystem by monitoring plant status. Water status is commonly observed measuring water potential but when the extreme event is over, this parameter cannot show managers the recovery of other physiological processes such as photosynthesis. To address this problem, we have evaluated the Quercus ilex L. water status and photosynthetic capacity throughout an intense water scarcity event and a subsequent re-watering. Photosynthetic capacity was evaluated through chlorophyll fluorescence parameters and leaf reflectance indices. We found that all fluorescence parameters changed as water potential decreased and they did not completely recover after re-watering. Among the reflectance indices, the physiological reflectance index (PRI) varied similarly to fluorescence, obtaining a strong correlation with the non-photochemical quenching (NPQ). We proposed using PRI to detect the level of photosynthetic capacity in Q. ilex, as it is much easier-to-handle. We also concluded that intense droughts and heat stress not only might reduce photosynthetic capacity through changes in Chl fluorescence parameters during the stress period, but might also affect photosynthetic capacity once the plant water status is recovered.
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