Genotypic differentiation in the stomatal response to salinity and contrasting photosynthetic and photoprotection responses in five olive (Olea europaea L.) cultivars

Olive tree is moderately tolerant to salinity. Differences in salt tolerance among olive tree cultivars are documented. Salt-mediated decreases of stomatal conductance and photosynthesis are common in the studied cultivars, but other photosynthetic characteristics in response to increasing salinity are much less investigated. Greenhouse experiments were conducted to investigate the responses to increasing salinity up to 200mM NaCl of five olive tree cultivars: ‘Chemlali’, ‘Chetoui’, ‘Arbequina I18’, ‘Arbosana I43’ and ‘Koroneiki’, widely used in Tunisia. Techniques used include determinations of leaf concentrations of the toxic ions Na and Cl, photosynthetic pigment analyses, gas exchange and chlorophyll (Chl) fluorescence measurements. Relationships between thermal energy dissipation and changes in pigment composition were investigated. With 100mM NaCl water, the most readily available water in arid regions of Tunisia, ‘Chemlali’, ‘Chetoui’ and ‘Arbosana’ grew similarly to control plants, which is a valuable information for horticultural purposes. In all cultivars, leaf Na and Cl concentrations increased and net CO2 assimilation rates decreased as salt concentration increased. A negative correlation was observed between photosynthesis and leaf thickness. Data revealed genotypic differentiation in the stomatal response to salinity, decreasing in ‘Koroneiki’, ‘Arbequina’ and ‘Arbosana’ but remaining fairly constant in ‘Chemlali’ and ‘Chetoui’. In those cultivars where the amount of energy thermally dissipated increased in response to salinity, it was well correlated with increases of (zeaxanthin+antheraxanthin)/(violaxanthin+antheraxanthin+zeaxanthin) ((Z+A)/(V+A+Z)) and lutein to Chl molar ratios. In summary, salinity caused marked reductions in photosynthetic rates well correlated with leaf Na and Cl accumulation. Photosynthesis limitation was partially attributed to stomata closure, except in the cultivars ‘Chemlali’ and ‘Chetoui’ in which stomatal conductances were unaltered. Possible decrease of CO2 mesophyll conductance mediated by increases of leaf thickness is discussed. Salinity increased Z+A− and/or lutein-mediated thermal energy dissipation of some of the olive tree cultivars investigated.