Stomatal response of two almond cultivars to fusicoccin

The leaves of two cultivars of almond (Prunas amygdalus St.), one ("Filippo Ceo") resistant and one ("Occhiorosso") susceptible to the bud canker caused by Fusicoccum amygdali Del., were painted with or allowed to absorb fusicoccin at various concentrations from 2 × 10^-5 m to 2 × 10^-7 m, or control solutions. At various times after treatment, the stomatal resistance to transpiration by the leaves was measured with a ventilated diffusion porometer. Fusicoccin stimulated stomata of both cultivars to open in both the light and dark. The stomata of the resistant cultivar opened more slowly than those of the susceptible cultivar, but little difference in stomatal transpiration between the resistant and susceptible cultivars was observed after either painting with or absorption of fusicoccin. When absorbed into the transpiration stream, low concentrations of fusicoccin accumulated in the leaf to concentrations sufficient to open stomata. Inoculation of almond branches with F. amygdali induced changes in the diffusive resistance of leaves distal to the site of inoculation similar to those induced by absorption of pure fusicoccin preparations. The results show that the resistant cultivar, "Filippo Ceo", had little capacity to limit the transfer of the toxin to the leaf or to reduce the opening effect of fusicoccin. It is concluded that the major resistance of the cultivar to the disease probably occurs in limiting the development of infection or containment of the toxin at the site of infection.