Study of physiological and biochemical responses to freezing stress in pomegranate (Punica granatum L.) trees during acclimation and deaclimation cycle

Abstract

ABSTRACTChanges in freezing tolerance of four pomegranate (Punica granatum L.) cultivars, including ‘Alak Torsh’ (AT), ‘Tabestaneh Torsh’ (TT), ‘Poostsefid Torsh’ (PT), and ‘Poostsyah Shirin’ (PS), and its correlation with physiological and antioxidant responses were investigated in shoot samples during cold acclimation and deacclimation. Half part of shoot samples exposed to controlled freezing temperatures (−11,-14, and −17ºC) to determine lethal temperature (LT50) values based on relative electrolyte leakage method, and the other part was used for measuring physiological parameters at each time-point from September 2016 to March 2017. Totally, seasonal patterns were similar in four cultivars and their freezing tolerance changed depending on air temperatures and acclimation state alteration. In pomegranate shoots, LT50 values showed a positive correlation with MDA, starch, and antioxidant enzyme activity and a negative correlation with soluble carbohydrates, proline, and total phenolic content. The highest SOD, POD, and PPO activity for countering the stressful effect of low temperatures during acclimation and deacclimation, and the highest osmoregulant and total phenolic content coincided with the maximum cold hardiness in mid-winter. Of all studied cultivars, ‘PS’ started the acclimation phase later, but it showed the highest freezing tolerance (−16.98ºC) in mid-winter and resisted entering deacclimation phase to a greater extent (−15ºC).