Pithiness in plants: I. The effect of mechanical perturbation and the involvement of ethylene in petiole pithiness in celery

Mechanical perturbation (MP) applied to celery (Appium graveolens L. cv. Florida 683) leaf petioles or ethephon application to the plant did not induce thigmomorphogenesis (inhibition of elongation and increase in thickness of the petiole). However, the two treatments did cause the parenchyma breakdown which leads to pithiness or increased natural pithiness, mainly at the base of the petiole. Nevertheless, MP (but not ethephon) decreased the severity of drought-stress or GA3-induced pithiness. Although MP stimulates ethylene production, mainly at the middle part of the petiole, it seems that the protection by MP of the petiole may not be directly mediated by ethylene production. The exposure of the plant to drought stress brought about an increase in ethylene evolution. Upon reirrigating the plants, the first steps of pithiness were accompanied by a sharp decline in ethylene production. This decrease might be due to membrane disruption. The increase in ethylene production during drought stress may be one of the events which stimulate pithiness of the celery leaf petiole.