Effects of girdling and fruit thinning on vegetative growth and some pomological characteristics of olive cultivars 'Koroneiki' and 'Chetoui'

Olive production in Tunisia is marked by annual oscillations, as trees alternate from high to low crop loads in successive years. With the aim of modifying cultural practices so as to reduce such fluctuations, the effects of girdling and fruit thinning on pomological parameters of olive trees were studied. Three branches of each cultivar, 'Koroneiki' and 'Chetoui', in Taous (Sfax, Tunisia), were girdled on 10 April and subjected to fruit thinning on 31 July. Four thinning ratios were applied on girdled and ungirdled shoots: R0 (shoot not thinned), R3 (one fruit retained per three internodes), R4 (one fruit per four internodes) and R5 (one fruit per five internodes), with ungirdled and unthinned shoots as a control. The studied parameters were: leaf surface area, diametric and longitudinal growth of harvested fruit, fresh fruit weight, pulp/pit ratio (P/N) and ripening index (RI). The combined treatment (girdling and thinning) and especially the ratios R3 and R4, showed the highest increase in the P/N ratio, of 40% for 'Chetoui' and consecutively 16.5 and 22% for 'Koroneiki', while fruits of girdled and thinned fruit branches R5 had P/N ratios 17 and 34% higher than the controls consecutively for 'Chetoui' and 'Koroneiki'. The combined girdling and thinning treatments R4 and R5 delayed ripening of fruit of both cultivars studied by reducing the RI to half that of the controls. Further investigation will be necessary to define the impact of girdling on the anatomy of the tree and the mobilization of carbohydrates as well as on carbohydrate-related gene expression in leaves, bark and roots.