Frost Injury in Wheat: Ice Formation and Injury in Leaves

Abstract

Freezing of water in leaves of a number of wheat cultivars was studied using pulsed nuclear magnetic resonance (n.m.r.) spectroscopy; these data were coordinated with measurements of the temperature dependance of freezing injury, and used to analyse the freezing process and its effects. The freezing temperature which resulted in 50% loss of cell electrolytes on thawing. T₅₀, varied from -7 to -12.1°C in the material studied. Hardening treatments decreased T₅₀ by 1.8° in the least hardy cultivar, Kite, and 3.3° in the most hardy cultivar, Cheyenne. Despite the contrast in hardiness level, freezing curves determined by n.m.r. were similar, the fraction of water remaining unfrozen, L_T/L₀, at temperature T having the relationship L_T/L₀ = ΔT_m/T + b, where L_T, L₀ are the amounts of liquid water per unit dry weight at T and 0°C, respectively, ΔT_m is the melting point depression and b the liquid water that does not freeze.

The freezing curves were similar because ΔT_m varied little among hardy and less hardy samples. At their T₅₀, the least hardy sample of Kite had lost 77.2% of its freezable water to extracellular ice at - 7°C, whereas the corresponding value for the hardy Cheyenne was 86.8% at its T₅₀ of - 12.1°C. Greater hardiness in samples was closely associated with an ability to withstand greater amounts of freeze-induced dehydration, and not with alterations in °T_m.