The study of resistance to freezing temperatures in Maclura pomifera for us in urban landscaping

Low temperature is one of main factors limiting distribution of woody plants. In order to evaluate the resistance of 4-years old seedlings and adult trees of Maclura pomifera to freezing stress, a factorial experiment was carried out based on completely randomized design with three replications in the faculty of Agriculture, University of Tehran. During 2012-2013 sampling were done from the annual branches and carried out from December to March with 30-days interval. Samples were transferred to the thermogradiant freezer with the six freezing temperature of -5, -10, -15, -20, -25 and -30oC. Then, the resistance to freezing temperatures was measured based on electrolyte leakage, start to growing, relative humidity of branches, contents of proline, protein and soluble carbohydrates. The results showed that with the decrease in freezing temperature, electrolytes leakage was significantly increased in March taken samples in both adult and 4-years old trees; However, they showed high resistance in other months, and there were a significant correlation between electrolyte leakage and viability of samples (r=-0.88**). Maximum and minimum soluble carbohydrates contents were related to December and March, respectively, which have high negative correlation (r=0.98**) with electrolyte leakage during this months. Minimum content of proline in shoots was observed in March, which was correlated to highest electrolyte leakage and minimal re-growth of shoot samples during this month. Since the major freezing damages in Maclura pommifera occur in March, and in temperatures below the -20°C, this species seems to be resistant or relatively resistant to freezing temperatures.