Genotypic differences in the response to elevated CO₂ concentration of one-year-old olive cuttings (Olea europaea L. cv. Frantoio and Moraiolo)

ABSTRACT

One-year-old olive (Olea europaea L) cuttings of two cultivars (Frantoio and Moraiolo) were exposed for five months to ambient (360 ± 16 μmol mol^-1) or elevated (560 ± 52 μmol mol^-1) CO₂ concentration in a free-air CO₂ enrichment facility. Current-year leaves were used for gas exchange measurements, chlorophyll and nutrient determinations. Stomatal density was measured on frozen hydrated current-year leaves by low temperature scanning electron microscopy. Above-ground dry mass (stem and leaves) and leaf area were determined, and leaf mass ratio and specific leaf area calculated. Results showed that exposure to elevated CO₂ enhanced rates of net photosynthesis and decreased stomatal conductance, leading to higher water use efficiency; this was not translated in increased growth rates (particularly in the cultivar Frantoio, which showed reductions in above-ground plant dry mass compared with the cultivar Moraiolo). Chlorophyll concentrations (chlorophyll content per unit leaf area) decreased only in Frantoio leaves of plants grown in elevated CO₂. Stomatal density and leaf nutrients did not differ between treatments and cultivars. Some adjustment to elevated CO₂ was observed for the cultivar Frantoio, which showed strong reductions in leaf area, thus counterbalancing increased photosynthetic rates per unit leaf area (but also decreased stem dry mass). The decrease in total leaf dry mass in response to elevated CO₂ was accompanied by a decrease in the fraction of aerial plant dry mass belonging to leaves (elevated CO₂ changed dry mass allocation by decreasing leaf mass ratio), suggesting that canopy-level adjustment in carbon assimilation may occur in both cultivars.

KEY WORDS:

• chlorophyll
• elevated CO₂ concentration
• gas exchange
• Olea europaea
• stomatal density