Influence de la température d'endurcissement sur la tolérance au gel de la luzerne et sa teneur en sucres, amidon et proline

Abstract

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 °C or −2 °C during the day and at 10 °C or 15 °C during the dark while control plants were kept continuously at 1 °C or −2 °C. Hardening alfalfa at 10 °C and 15 °C during darkness decreased the total sugar content while it increased at 1 °C and −2 °C. However, frost tolerance and growth of the plants were not changed by raising the night temperature to 10 and 15 °C. The sugar increase corresponded in general to a starch degradation, mainly in the crowns. In plants hardened at −2 °C the proline content was higher in the leaves and lower in crowns. The opposite was observed in plants hardened at 10 and 15 °C during darkness. However, during hardening at −2 °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. Key words: Alfalfa, frost tolerance, sugars, starch, proline, hardening