Studies on the Frost-Hardiness of Woody Plants ; 1.The causal relation between sugar content and frost-hardiness

Abstract

IntroductionIt has long been well known that frost-hardiness in woody plants, as a rule, shows remarkable periodicities through the cycle of a year.Many hardy plants, more or less increase their frost-hardiness from summer to winter.In summer the twigs of poplar and willow cannot withstand freezing at -5C for a short time; while in winter they can survive even a temperature as low as -30C for a long time l ).In natural and artificial') frost-hardening, the increase of frost-hardiness has been ascertained to be associated with the fluctuations in the content of vario~s substances, such as sugar 3 ,7\ polyhydric alcoho[8,lO), protein l1 ,lS\ glycoprotein l6 ,l7\ amino-acid l8 ), peptide l8 >, organic acids l9 ) and inorganic salts19), etc. MULLER-TURGAU (1882) demonstrated first that there was a positive correlation between sugar content and frost-hardiness").This problem has been investigated with various plaIJ.tmaterials, and a parallelism between the two factors has generally been accepted by most investigators.However, more recently it has been maintained by some workers ll ,l7,l9,Zl,Z3) that the increase of sugar content in cells is not the main cause of that of their frost-hardiness; a question has.arisen as to the role of sugars in increasing forst-hardiness.These workers have chiefly intended to find out a correlation between the variations of frost-hardiness and sugar content in the process of both natural and artificial frost-hardening.As a matter of fact, it has been impossible to make clear any causal relation between the two factors, since it is impossible to distinguish the effects of low temperature itself and that of increase in sugar concentration in frost-hardening process, especially in artificial frost-hardening.Moreover, no direct evidence has yet been shown that in a cell the increase of sugar content alone results in rise of the frost-hardiness.This may be due to the difficulties in artificially introducing an appreciable amount of sugar into a plant cell.LEVITT (1959) recently reported that hardy cabbage leaf cells showed definite increase in frost-hardiness when they were infiltrated with 0.5 M hexose solution by means of a vaccum desiccator'S).His observed increase in hardiness agrees very well with the calculated value on the basis of a purely osmotic effect.However, such an increase in the frost-71 Contribution No. 611 from the Institute of Low Temperature Science.A. SAKAI hardiness may be considered as the effect of increase of sugar content both in cells and in intercellular space, because of course in the method of vaccum infiltration, sugar is introduced into the intercellular space as well as into the cell interior.Accordingly, to distinguish these two factors, it becomes necessary to introduce sugar into the cell interior without altering the state of intercellular space.The purpose of the present study is to make clear the causal relation between frosthardiness and sugar content and to ascertain the mechanism of the protective action of sugar against frost injury in plant cells.* Percentage of total sugar expressed as glucose equivalent per wet weight of sample.** Minimum temperature at which the twig as a whole or leaf can with~tand freezing for a full day.