Phosphofructokinase in Relation to Sugar Accumulation in Cold-hardened Lolium perenne L. Cultivars

Two cultivars of perennial ryegrass (Lolium perenne L.), the extreme winter hardy cv. Mondial and the moderate winter hardy cv. Yatsyn 1, were hardened under controlled conditions at 2 °C. The content of the carbohydrates, except for starch, increased in both cultivars during hardening, the effect being more pronounced in cv. Mondial. The activity and properties of phosphofructokinase (PFK) were studied in order to establish whether this enzyme plays a role in the accumulation of the carbohydrates. Two forms of PFK, differing in molecular weight, are present in the leaves. An increase in the maximum catalytic activity of the high molecular weight PFK form was the only clear change found when activities and properties of PFK were studied during hardening. The maximum activity of this PFK form is highest in cv. Mondial, the cultivar that accumulates the highest sugar levels. Lolium perenne PFK is cold-labile, but no differences were found in temperature coefficients of this enzyme between the two cultivars, indicating that also at low temperatures the enzyme activity in cv. Mondial is higher than in cv. Yatsyn 1.