Abstract
Water relation parameters, leaf CO
2
assimilation rates, stomatal conductance (g
s
), internal CO
2
concentrations (C
i
), and stomatal density were studied in two major Tunisian olive cultivars, 'Chemlali' and 'Chetoui', under rain-fed, or rain-fed plus a moderate level of supplemental irrigation, conditions. Measurements were carried out throughout the season from January - September 2005, on 1-year-old leaves. CO
2
assimilation rates showed substantial seasonal variation, but were similar in the two cultivars, with higher values during the Spring and lower values during the Summer. Stomatal conductance (g
s
) followed a similar trend to leaf CO
2
assimilation rates, with an increase in March, followed by a decrease during August. 'Chetoui' had the highest leaf CO
2
assimilation rate and g
s
value (24.26 µmol m
-2
s
-1
and 0.35 mol m
-2
s
-1
, respectively); but, compared to the supplemental irrigation treatment, 'Chemlali' had a higher resistance to drought stress than 'Chetoui'.