Relationship between freezing tolerance and nutritional status in Citrus leaves

This study was carried out in order to obtain some basic data necessary to develop the cultural practices to increase freezing tolerance of Citrus tree in cold season. The seasonal changes in freezing tolerance of Citrus leaves, and mineral and carbohydrate contents were analyzed from Nov. 1987 to Apr. in 1988. Freezing temperature observed in leaf of C. unshiu cv. Aoshima was -7.2deg C in Nov., gradually decreased to -10.5deg C in Feb., and rapidly increased from Mar. Seasonal changes in water content and water potential of leaf showed the same pattern as those of leaf freezing temperature. The leaf temperature observed on a clear and calm winter day were higher by about 4deg C during daytime and lower by 0.5deg C at night than the ambient air temperature. Cold injury increased with longer duration of low temperature and with rapid cooling or thawing. The TTC reduction (%) remarkably decreased by incubation of leaf at -2deg C for 2 hours in Dec., while it was not affected by the treatment at -10deg C for 2 hours in Jan. and Feb. In Mar., the TTC reduction (%) decreased by treatment at -4deg C for 2 hours. Only nitrogen content among various inorganic elements in the leaf often showed a positive correlation with leaf freezing tolerance in the same season. Seasonal changes in nitrogen contents, however, were not consistent with those in freezing tolerance. Seasonal changes in the contents of total sugars, reducing sugars and total carbohydrates in the leaf were closely correlated with those in the freezing tolerance, while the correlation between the carbohydrate contents and the freezing tolerance was not recognized among trees in the same season. Application of excess nitrogen up to 24Kg/10 a (two times of standard level) increased both the leaf nitrogen content and the freezing tolerance, whereas three times of standard nitrogen didn't. Late harvest decreased leaf freezing tolerance in the trees with heavy crop