Shosuke Kaku

Analysis of freezing temperature distribution in plants.

Summary The nature of ice nucleation in plant materials was investigated through analysis of freezing temperature distributions effected by differences in leaf size and maturity. The effect of changing leaf size (or length) on freezing temperature distributions could not be observed within a certain range of size, e.g., in Pinus needles varying in length from 10 to 50 mm and Ilex leaves 3–5 to 50–60 mg in weight. In Pinus needles, however, as needle length exceeded 75 mm, freezing temperature distributions were markedly widened. These changes in longer needles appear to be dependent on the following factors: (1) a larger number of ice nucleators due to increasing needle length; (2) anatomical or histological heterogeneity of vascular systems in different areas of longer needles. Changes in freezing temperature distributions which accompany leaf maturation were markedly observed in Buxus, Ilex , and Cedrus leaves and Cedrus twigs. In these plants, mature materials appeared to have more effective or higher concentrations of ice nucleators than did the immature ones.

Deep supercooling in xylems and ecological distribution in the genera Ilex, Viburnum and Quercus in Japan

The role of deep supercooling and the occurrence of low temperature exotherms in living xylems of twigs were investigated with reference to the northern limit of natural ranges, freezing resistance, deciduousness and wood morphology in the genera ilex, Viburnum and Quercus. Low temperature exotherms for the species having greater potentialities for northern distribution and high altitudes occurred at lower subzero temperatures. The doubling of low temperature exotherms was characterized in coldhardy species, while the same phenomenon did not occur in the less hardy or more southern species. The occurrence of low temperature exotherms was widely observed in almost all tree species regardless of their different wood porosities and deciduousness. These results suggest that low temperature exotherm measurement of xylems in overwintering broad-leaved trees will be useful to evaluate the potential ranges for northern distribution and testing freezing resistance.

Cold hardiness in various organs and tissues of Rhododendron species and the supercooling ability of flower buds as the most susceptible organ.

The freezing resistance of various organs and tissues was determined in 24 Rhododendron species (mainly Subgenus Tsutsutsi) having different ecological distributions. The order of hardiness for organ or tissue is as follows: leaf bud greater than wood greater than or equal to bark greater than flower bud, and the flower bud is characterized as the most cold-susceptible organ. The relationship of killing temperature (KT) to northern distribution was the most significant in leaf buds compared to other organs and tissues. KTs of leaf buds for the most hardy species were -45 degrees C (or below) and those for the most tender species were about -23 degrees C, while KTs of flower buds were about -28 degrees C for the former and -16 degrees C for the latter. Although KTs of flower buds native to southwestern Japan were well correlated with the exothermic temperature distribution (ETD) of florets, those in the more northern species were generally lower than ETDs. The supercooling ability of flower buds appears to be sufficient to avoid the freezing stress since the extreme minimum temperature (EMT) at the northern limit of natural distribution for each tree species examined was not lower than the KT and ETD of the flower buds.