Changes in the Freezing Tolerance of Spinach (Spinacia oleracea L.) and Komatsuna (Brassica campestris L.) from Autumn to Early Spring in an Unheated Greenhouse

Abstract

The relationships between freezing tolerance of spinach (Spinacia oleracea L.) and komatsuna (Brassica campestris L.) plants and air temperature, water content, and sugar concentration were investigated in an unheated greenhouses from autumn to early spring in 1996-99. Freezing tolerance was expressed as the temperature at which 15% (T_EL15) or 50% (T_EL50) of electrolyte leakage from leaf blade tissues occurred. The T_EL15 and T_EL50 of spinach and komatsuna decreased gradually from October to January. In the coldest season, the T_EL15 and T_EL50 were decreased to -12--16°C and -13--18°C in spinach, and -12--14°C and -13--17°C in komatsuna, respectively. From mid-February, the T_EL15 and T_EL50 of spinach and komatsuna increased with a rise of air temperature. There was a high correlation between the freezing tolerance and the mean minimum air temperatures for 7 days before the measurement were made. The freezing tolerance of spinach and komatsuna increased only slightly when the mean minimum air temperature was between 8 and 2°C, but increased considerably when the temperature decreased below 2°C. The changes in the freezing tolerance of spinach and komatsuna also correlated with leaf water content. The freezing tolerance increased as the leaf sugar content increased from 10 to 20 mg·g^-1 FW in spinach and from 10 to 35 mg·g^-1 FW in komatsuna. Further increases in sugar content had no effect on freezing tolerance in both species.