The changes in the antioxidant enzymes activity, total protein and proline content and their correlations with freezing tolerance (FT) (expressed as LT 50 ) were investigated at 11 different olive cultivars at cold-acclimation (CA, in February) and non-acclimation (NA, in August) stages. Leaf samples were collected from each cultivar and were divided into two groups. The first group was immediately frozen in liquid nitrogen for further biochemical analysis. The second ones was subjected to different freezing temperatures (-5, -10, -15 and -20°C) for 10 h, in order to determine their FT. The unfrozen control samples were kept at 4°C. The results showed that Fishomi, Mission and Shengeh were the most freezing tolerant among other cultivars. In contrast, Zard, Manzanilla and Amigdalolia were the most sensitive ones. The cold acclimation enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), polyphenol oxidase (PPO) and total protein content. However, proline content and phenylalanine ammonia-lyase (PAL) activity did not change or even decreased slightly at CA stage, compare to those samples at NA stage. It was found that LT 50 to be closely correlated to POD, CAT, and PPO activity at CA and NA stages. Overall, higher leaf POD, CAT, and PPO activity could be used as important selection criteria in screening tolerant olive cultivars for cold zone climatic.

The production of reactive oxygen species (ROS) in plants under low temperature stress causes damage to plants. In order to evaluate the antioxidant responses of Thomson navel young trees (two years old) on different rootstocks under low temperatures, an experiment was conducted in a factorial plan based on completely randomized design. Treatments of temperatures were at seven levels including 9, 6, 3, 0, -3, -6 , and 25 ± 2 (as control) and rootstocks were Sour orange, Citrange and Trifoliate orange. Results of interactive effects of factors showed that electrolyte leakage (EL), antioxidant capacity, catalase (CAT) and ascorbate peroxidase (APX) activity were significant at P < 0.01. Maximum EL was observed in Thomson navel on Sour orange at -6 , while the highest antioxidant capacity, CAT and APX activity respectively with mean 75.06%, 0.45 and 4.350 IU g -1 FW were observed in Thomson navel on Trifoliate orange rootstock at 0, -3 and 0 . Low temperatures increased EL, lipid peroxidation, antioxidant capacity, superoxide dismutase (SOD), APX, CAT and peroxidase enzyme activities (P < 0.01). Due to effects of rootstock, lipid peroxidation reduced to 52 % (P < 0.01). The biochemical and physiological study of this experiment indicated that Thomson navel on Trifoliate orange rootstock, had best tolerance to freezing stress.Additional key words: citrus, enzyme, lipid peroxidation Hort. Environ. Biotechnol. 52(6):576-580. 2011.

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