Experimental study on cold tolerance thresholds in field grown subtropical fruit trees

　To evaluate suitability for growth of tropical fruits under global warming in Japan, we revealed cold tolerance threshold of subtropical fruit trees by performing field and chamber experiment and developed an extrapolatory method that can predict the cold tolerance of subtropical fruit trees without conducting extensive field experiments. Results of the field experiments estimated cold tolerances of -1.5 and -2°C for ‘Summer Queen’ passion fruit and ‘Sata’ lychee, respectively, and -5, -4.5, and -4°C for ‘Mexicola,’ ‘Bacon,’ and ‘Fuerte,’ avocados, respectively, indicating the dependence of cold tolerance on the variety of the tree species. The cold tolerance obtained from the field experiment was higher than the chamber experiment by 1°C for passion fruit and avocado and 2°C for lychee, which is explained by the difference between air and leaf temperature of field trees caused by radiative cooling on a clear and calm night. Therefore, cold tolerances of passion fruit and avocado can be estimated by adding 1°C to the cold tolerances obtained from the chamber experiment and that of lychee can be estimated by adding 2°C to the cold tolerance obtained from the chamber experiment. Moreover, even for other tree species, which were not consider in this study, the cold tolerance of the trees grown in fields can be estimated based on the results of the chamber experiment and using estimations of the difference between air and leaf temperatures on a clear and calm night.