Comparison of Some Photosynthetic Traits in Olive (Olea europaea L.) Genotypes under Heat Stress Conditions

High temperature has negative effects on olive physiology, especially flowering, fruit growth and oil production. To identify olive cultivars/genotypes tolerant to heat stress, 12 olive cultivars and genotypes with four temperature treatments T1: (34 ± 1.5 ˚C), T2: (37 ± 1.5 ˚C), T3: (40 ± 1.5˚C) and T4: (43 ± 1.5 ˚C) were evaluated at Tarom olive research station, Iran, in 2016. The experiment was conducted as factorial arrangement in randomized complete block design with three replications. The results showed that the photosynthesis rate and water use efficiency decreased with increasing temperature. The results showed that the highest photosynthesis rate was obtained in T1 with 11.84 μmol m-2 s-1 and the lowest in T4 with 4.87 μmol m-2 s-1. In T4 temperature treatment, cv. Manzanilla maintained more than 50% and cv. Amin 44% of their photosynthesis efficiency compared to T1 temperature treatment. The of cultivar temperature × treatment interaction effect showed that the highest photosynthesis rate in T1 was 17.42 and 17.25 μmol.m-2s-1 in cv. Amin and cv. Deira, respectively. At temperatures > 40° C, there was a sharp decrease in the stomatal conductance of the leaves. Cv. Arbequina and Ozine genotype had the highest and lowest water use efficiency with 2.63 and 1.84 mmolCO2 molH2O-1, respectively. Also, the highest water use efficiency was obtained in KH15 genotype and cv. Deira in T1 temperature treatment. Due to the considerable decrease in photosynthesis rate at temperatures above > 40 °C, olive cultivation in areas with hot summers will cause significant reduction in fruit yield and in fruit oil content.