ABSTRACT Understanding how olive cultivars coordinate diffusional, biochemical and respiratory processes under drought is essential for identifying traits associated with resilience in Mediterranean agroecosystems. Here, we analysed 204 photosynthetic CO₂ response curves (A–Cᵢ) from 14 olive cultivars subjected to well-watered conditions, moderate drought (MD) and severe drought (SD). Curves were fitted using a temperature-adjusted Farquhar–von Caemmerer–Berry framework to estimate maximum Rubisco carboxylation capacity (Vcmax), maximum electron transport rate (Jmax), dark respiration (Rd), CO₂ compensation point (Γ*), mesophyll conductance (gₘ), and the ratio between mesophyll and gas-phase CO₂ conductance (gₘ/gₜc). Moderate drought did not cause a generalized biochemical impairment. Instead, mean Vcmax and Jmax increased from 26.9 and 54.2 µmol m⁻² s⁻¹ under control conditions to 35.8 and 63.4 µmol m⁻² s⁻¹ under MD, respectively, suggesting partial acclimation of photosynthetic capacity. However, gₘ declined by nearly half under MD, indicating that internal CO₂ diffusion was already constrained before major biochemical collapse occurred. This phase was accompanied by increased Rd and elevated Γ*, consistent with a metabolically costly acclimation response involving higher maintenance and photorespiratory demands. In contrast, SD triggered a strong decline in photosynthetic functionality, reducing Vcmax, Jmax and gₘ to 18.1 µmol m⁻² s⁻¹, 29.3 µmol m⁻² s⁻¹ and 0.00335 mol m⁻² s⁻¹ bar⁻¹, respectively. The simultaneous increase in gₘ/gₜc under SD did not indicate improved mesophyll performance, but rather a proportionally stronger decline in gas-phase conductance, most likely associated with severe stomatal closure. Multivariate integration of diffusional and biochemical traits separated cultivars into contrasting resilience strategies, with Cornicabra, Koroneiki and Frantoio showing the strongest integrated performance, while Sikitita2 maintained particularly high biochemical capacity under severe stress. Overall, our results support a two-stage drought response in olive, in which moderate drought induces costly acclimation dominated by diffusional constraints, whereas severe drought drives impaired chloroplastic CO₂ supply, reduced RuBP regeneration, lower carboxylation capacity and loss of physiological coordination.
Martos-Fuente et al. (2026) studied this question.