Responses of two olive tree (Olea europaea L.) cultivars to elevated CO2 concentration in the field

Five-year-old plants of two olive cultivars (Frantoio and Moraiolo) grown in large pots were exposed for 7 to 8 months to ambient (AC) or elevated (EC) CO2 concentration in a free-air CO2 enrichment (FACE) facility. Exposure to EC enhanced net photosynthetic rate (P-N) and decreased stomatal conductance, leading to greater instantaneous transpiration efficiency. Stomata density also decreased under EC, while the ratio of intercellular (C-i) to atmospheric CO2 concentration and chlorophyll content did not differ, except for the cv. Moraiolo after seven months of exposure to EC. Analysis of the relationship between photosynthesis and C-i indicated no significant change in carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase after five months of exposure to EC. Based on estimates derived from the P-N-C-i relationship, there were no apparent treatment differences in daytime respiration, CO2 compensation concentration, CO2-saturated photosynthetic rate, or photosynthetic rate at the mean C-i, but there was a reduction in stomata limitation to P-N at EC. Thus 5-year-old olive trees did not exhibit down regulation of leaf-level photosynthesis in their response to EC, though some indication of adjustment was evident for the cv. Frantoio with respect to the cv. Moraiolo.