Abstract
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.
Highlights
Mediterranean-type climates are characterized by hot/dry summers and mild or cold winters that can potentially restrict vegetative activity [1,2]
), and maximum velocity of carboxylation. These results indicate that both diffusive, and maximum velocity of carboxylation (V cmax ). These results indicate that both diffusive and non‐diffusive limitations of photosynthesis increased and non-diffusive limitations of photosynthesis increased throughout the the drought drought period, period, which which helped helped explain explain the the pattern pattern followed followedby byA
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Summary
Mediterranean-type climates are characterized by hot/dry summers and mild or cold winters that can potentially restrict vegetative activity [1,2]. Stomatal closure in response to drought is a common way of regulating water consumption [8,9,10,11] This strategy is considered to be an effective mechanism that prevents xylem cavitation and runaway embolism [12,13], albeit at the expense of a reduction in net CO2 assimilation that could compromise the carbon balance of the plant [2,14]. The existence of a delay between the water potential leading stomatal closure and that inducing extensive xylem cavitation (i.e., a wide “security margin”) [15]
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