低温ショックによるウメ果実の低温障害抑制効果と膜透過性及び膜脂質構成脂肪酸の変化との関係

Abstract

It has been reported that chilling injury in mume (Japanese apricot) fruits usually occurs much sooner at 5-8°C (moderate low temperature) than at 0-1°C, and that a cold-shock treatment by hydrocooling in ice water immediately after harvest is effective in retarding the development of the chilling injury. This paper deals with the mechanism of the cold-shock treatment.1. As an index of membrane permeability, the rate of K+ion leakage from tissue slices into water was measured at various temperatures ranging from 1°C to 20°C. While the leakage rate of chilling-insensitive potato tuber decreased as the temperature was lowered, that of chilling-sensitive cucumber fruits showed no decrease at low temperatures, remaining at an almost constant level below 10°C. However leakage rate of mume fruits, showed an unusual increase at moderate low temperatures (5-8°C). Using 4 cultivars of mume fruits, ‘Yosei’‘Nanko’‘Gyokuei’and ‘Gojiro’it was found that the temperature at which the unusual increase of K+ion leakage occurred on harvest day was almost the same as that at which severe chilling injury occurred during storage.2. When‘Oshuku’mume fruits, a very chilling-sensitive cultivar, were given a cold-shock in ice water for 2 hours and then stored at 6°C, chilling injury was retarded. Except for a temporary rise soon after the treatment, the rate of K+ion leakage remained at a constant low level. However, in untreated fruits, the rate of K+ion leakage increaesed markedly during storage at 6°C.Fatty acid composition of membrane lipids was analyzed, and C16:0 was found to be most abundant, followed by C18:2, C18:3, C18:1, C16:1, C14:0, C18:0 in that order. The ratio of unsaturated fatty acids to total fatty acids was 52%, a relatively low value compared with other commodities, suggesting a lower tolerance to chilling temperature. Cold-shock treatment increased the above ratio.Total phenol content gradually increased both in the untreated and treated fruits during 6°C storage, the values being slightly lower in the treated fruits. Activities of polyphenol oxidase and peroxidase increased during 6°C storage. No relation tocold-shock effect was found. NADH-cytochrome C reductase activity, an index of activity of the enzymatic system involved in unsaturation of fatty acids, increased rapidly during the cold-shock treatment and remained at a higher level than in untreated fruits throughout the 6°C storage period.3. From these results, it was concluded that the effect of the cold-shock treatment was to accelerate the desaturation of fatty acids in membrane lipids.