果樹の気象的適地に関する研究

(2) ナツミカン果実の凍害温度

抄録

This paper presents data on the degree of subcooling and critical air temperature for natural and artificial freezing of the fruit on the tree, in Citrus natsudaidai together with the effects of freezing on the weight and volume of fruits.
The results obtained are summarized as follows;
(1) The freezing points of the pulp, indicated by the maximum temperature after ice formation, ranged from -1.0°C to -2.7°C with the average of -1.7°C as shown in Table 1. The pulp was subcooled as much as 2.2°C before freezing occurred.
(2) When freezing occurred naturally, the pulp temperature increased suddenly to its freezing point (-1.5°C-1.6°C) and remained within 0.1°C at this temperature till morning (Figure 1.) This increase in temperature on freezing and the maintenance of the higher temperature than air are explained by the release of heat of fusion.
(3) In general, there was a tendency that the peel freezing occurred before the pulp freezing (Table 3.)
(4) Figure 2 represents that the lower the minimum air temperature at night, the longer the frozen fruit required to thaw.
(5) The injured fruits which had been exposed to -11°C before ten days of the second treatment were subcooled by no more than 1.2°C (Figure 3). while non-damaged fruits were subcooled as much as 2.2°C described before.
(6) No freezing of the fruits occurred when the fruits were exposed to -4.9°C or higher temperature (Table 1). However, air temperature of -5°C or lower resulted in 100per cent freezing of the fruits. This showed that air temperature near -5°C was found to be critical for freezing of the fruit. From the comparison of this result with the report by HENDERSHOTT (1962), it may be said that the freezing of the fruit of Citrus natsudaidai in our country is harder than the orange fruits in Florida.
(7) In early period of Fecruary, fruits on the tree were set and tested in a portable freezing chamber (see Fig. 4) in order to make clear the dependence of the degree of cold damage on the length of freezing.
In Table 4 and Figure 5, it was recognized that the frozen fruits, harvested in late March, diminished in size and decreased considerably in specific weight, especially in treatment of longer freezing.
(8) The climatological data at Hagi, Uwajima and Sumoto, the major districts of production of Citrus natsudaidai in our country were analysed. The minimum temperature of -4.5°C occurred every 20 years at Hagi. The minimum of -5°C occurred every 15 years at Uwajima and 15-20 years at Sumoto (Figure 6).
(9) It would be desirable to culture Citrus natsudaidai at meteorologically favorable places where the occurrence of the minimum air temperature of -5°C or lower is every 15-20 years.