Effect of cold hardening, wilting and exogenously applied proline on leaf proline content and frost tolerance of several genotypes of Solanum

Abstract

Frost tolerance and leaf proline content were examined in a number of potato hybrids selected for frost tolerance and in the cv. Astarte before and after hardening. Cold hardening (2°C for 20 days) in a dry environment (50/90% relative humidity, day/night) resulted in decreased water content, increased proline content and increased frost tolerance of the leaves of all genotypes. Frost tolerance before and after hardening was positively related to leaf proline content, but not to leaf water content. Drought stress alone, imposed by wilting excised leaves for 4 days, resulted in an accumulation of proline comparable to that after hardening in a dry environment, but the increase in frost tolerance was smaller. Cold hardening in a humid environment (90% relative humidity continuously) only caused a minor accumulation of proline and a small increase in frost tolerance, but the increase in frost tolerance was high in relation to the amount of proline accumulated. Proline, exogenously applied to one of the genotypes, was accumulated in the leaves of shoot cultures, resulting in an increase in frost tolerance. A possible role of proline in frost tolerance is discussed.