Shoot tips of 'York' and 'Vermont Spur Delicious' apples (Malus domestica Borkh.) were cultured in vitro to test the influence of K + , Mg + + and gelling agent concentrations on vitrification. These concentrations were 20.05, 14.05 and 8.05 mM K + , 1.5 and 3.0 mM Mg ~ +, 7.0 g/1 Difco Bacto agar and 1.0, 1.5 and 2.0 g/1 Gelrite. The lowest K + level produced a higher percentage of vitrified shoots, affected tissue appearance, reduced shoot number and shoot elongation and apparently altered shoot metabolic activity. Gelrite consistently produced vitrified leaves and stems, even though media gelled with 1.5 g/1 Gelrite presented the same apparent gel firmness as using 7 g/1 Difco Bacto agar, which did not induce vitrification. Less shoot elongation, fewer total shoots, and more usable shoots of 'York' were obtained on Bacto-agar, while similar but less noticeable effects were obtained with 'Vermont Spur Delicious'. The results presented here show that vitrification can be studied in a standardized system in which the only change is substitution of one gelling agent for another.

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