Frost hardiness of 10 olive cultivars after natural and controlled freezing

Selecting winter-hardy cultivars is an effective way to avoid freezing damage in olive trees. In this study, we evaluated the cold hardiness of 10 olive cultivars in the field after a natural freezing event (-12 degrees C in December 2016, Sorkhe, Semnan, Iran) and treatment with controlled freezing temperatures ranging from 0 degrees C to -21 degrees C. The 50% lethal temperature (LT50) based on electrolyte leakage (LT50-EL) and tetrazolium tests (LT50-TZ) before, during, and after dormancy were determined in each cultivar's shoots. The proline and soluble carbohydrate content of leaves and shoots also were measured during dormancy. The LT50-EL had a mean of -9.5 degrees C and ranged from -16.5 degrees C to -4.8 degrees C. The LT50-TZ had a mean of -10.2 degrees C and ranged from -16.3 degrees C to -6.4 degrees C. We classified the olive cultivars into five groups based on their lab-freezing evaluations using cluster analysis. 'Manzanilla', 'Mission', and 'Rashid' were very sensitive to freezing, while 'Roughani' and 'Zrad' were the hardiest cultivars. In Field evaluation, the recovery ability and yield of each cultivar were evaluated six years after freezing damage. Visual scoring indicated that 'Roghani' and 'Zard' had the least amount of trunk damage. However, 'Zard' showed low regrowth power in the field after freezing damage despite its high freezing tolerance in lab and field experiments. 'Roughani' had the maximum fruit yield among the 10 olive cultivars six years after freezing damage (P < 0.01). Our finding indicates that laboratory techniques be combined with field assessments to more accurately evaluate the cold hardiness of olive trees.