HOW INDICATIVE ARE CHANGES IN MAJOR METABOLITES FOR FREEZING TOLERANCE OF WHEAT

Relative growth rate of winter wheat at low temperature (day/night, 4/2 degrees C) was about 0.02 g g(-1) fresh weight day(-1). There was a substantial increase in dry matter content of shoots and roots, especially during the first week of low temperature exposure, which for the greater part could be attributed to an accumulation of soluble sugars and starch. The freezing tolerance (LTS,) of the leaf tissue gradually increased with 6 degrees C during the 6 weeks exposure, whereas freezing tolerance of roots was hardly affected. Upon low temperature exposure there was an increase in the content of water-soluble non-protein thiols (mainly glutathione), soluble sugars, starch, amino acids and total nitrogen in both shoots and roots. The most substantial increases in the content of thiols, sugars and amino acids already occurred within hours or a few days after the onset of the exposure. The nitrate content in shoots showed a rapid and strong decrease during low temperature exposure, however, in roots its content remained unaffected. It is proposed that the observed rapid increases in the content of various metabolites at low temperature are primarily due to an altered balance in their synthesis and their consumption in growth. There was no direct relation between increases in the content of the various metabolites and the degree of freezing tolerance of the tissue.