Changes of K+ leakage, Free Amino Acid Contents and Phenylpropanoid Metabolism in Water Convolvulus (Ipomoea aquatica Forsk.) with Reference to the Occurrence of Chilling Injury

Abstract

d stored at 1, 6, 9, 12, 15, 20, and 30°C. A break point of Arrhenius plot for K+leakage in young and mature leaves was found between 10 and 16°C, whereas the matured leaves showed only a little chilling-sensitivity. A marked increase of the rate of K+ leakage occurred in young leaves stored at 1°C, preceding the appearance of browning. As the contents of phenylpropanoids such as t-cinnamic, p-coumaric, caffeic and chiorogenic acids were investigated, they increased before the occurrence of chilling injury and then tended to decline almost simultaneously with the development of chilling injury. They also increased at 15°C, but the rise delayed and the extent was considerably lower than that at 1°C. Among the enzymes relating to the biosynthesis of chlorogenic acid, a main substrate of browning, hydroxycinnamoyl CoA ligase (CL) activity increased in both of 1°C and 15°C storages, then the increase was faster at 1°C reaching to a peak before the occurrence of chilling injury. Hydroxycinnamoyl CoA : quinate hydroxycinnamoyl transferase (CQT) activity showed a markedly higher increase in the 1°C storage. On the results, we present a conclusion of the low temperature induced browning mechanism as follows ; exposure to low temperature leads to a rise of phenolic contents through activation of the enzymes such as CL and CQT. The accumulated phenolics induce the browning after its enzymic oxidation. 緒