Frost Injury and Heterogeneous Ice Nucleation in Leaves of Tuber-Bearing Solanum Species

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Abstract
The heterogeneous ice nucleation characteristics and frost injury in supercooled leaves upon ice formation were studied in nonhardened and cold-hardened species [S. tuberosum and S. acaule] and crosses of tuber-bearing Solanum. The ice nucleation activity of the leaves was low at temperatures just below 0.degree. C and further decreased as a result of cold acclimation. In the absence of supercooling, the nonhardened and cold-hardened leaves tolerated extracellular freezing between -3.5.degree. and -8.5.degree. C. However, if ice initiation in the supercooled leaves occurred at any temperature < -2.6.degree. C, the leaves were lethally injured. To prevent supercooling in these leaves, various nucleants were tested for their ice nucleating ability. Aqueous suspensions of fluorophlogopite and acetoacetanilide (1%) were effective in ice nucleation of the Solanum leaves above -1.degree. C. They had threshold temperatures of -0.7.degree. and -0.8.degree. C, respectively, for freezing in distilled H2O. Although freezing could be initiated in the Solanum leaves above -1.degree. C with both the nucleants, 1% aqueous fluorophlogopite suspension showed overall higher ice nucleation activity than acetoacetanilide and was nontoxic to the leaves. The cold-hardened leaves survived between -2.5.degree. and -6.5.degree. using 1% aqueous fluorophlogopite suspension as a nucleant. The killing temperatures in the cold-hardened leaves were similar to those determined using ice as a nucleant. However, in the nonhardened leaves, use of fluorophlogopite as a nucleant resulted in lethal injury at higher temperatures than those estimated using ice as a nucleant.