INFLUENCE OF HARDENING TEMPERATURE ON FROST TOLERANCE OF ALFALFA AND ITS SUGAR, STARCH AND PROLINE CONTENTS

The increased sugar and starch in crowns and roots during hardening is considered a limiting factor for the acquisition of cold tolerance in alfalfa (Medicago sativa L.). We wanted to determine the effect of a decrease in carbohydrate on cold tolerance. Alfalfa plants (Medicago sativa L.) were hardened at 1-degrees-C or -2-degrees-C during the day and at 10-degrees-C or 15-degrees-C during the dark while control plants were kept continuously at 1-degrees-C or -2-degrees-C. Hardening alfalfa at 10-degrees-C and 15-degrees-C during darkness decreased the total sugar content while it increased at 1-degrees-C and -2-degrees-C. However, frost tolerance and growth of the plants were not changed by raising the night temperature to 10 and 15-degrees-C. The sugar increase corresponded in general to a starch degradation, mainly in the crowns. In plants hardened at -2-degrees-C the proline content was higher in the leaves and lower in crowns. The opposite was observed in plants hardened at 10 and 15-degrees-C during darkness. However, during hardening at -2-degrees-C, we observed the desiccation of leaves and an increase in the proline content. We discuss the role of sugars as promoting agents for the acquisition of frost tolerance.