Studies of Membrane Transport and Environmental Stress in Bulb Scale Cells of Allium Cepa

Abstract

Using onion epidermis layer a v e r y accurate method for measuring the permeability of epidermis cells to water was standardized. In this method a 1.4 cm diameter epidermis disc was soaked in tritiated water ( 5 0 0 curie/ml) fo r about 1 hour. Next the disc was mounted in a specially designed elution chamber where it was held flat and washed on the non-cuticular side with ordinary water. A constant flow rate, high enough to minimize unstirred lay er effect, was used. P e rm ea b ility was calculated in the usual way after separating different exponentials from the efflux curve of tritiated water. Turgor pressure of the c e ll was regulated by soaking the disc in mannitol solutions containing tritiated water and washing it in the chamber with same concentration mannitol solution containing no radioactivity. Water permeability values were found to decrease less than 8% when the turgor pressure was decreased from 8 atmospheres (fu ll turgor) to ze ro . Turgor pressure had no significant e ffect on the water permeability of onion epidermal c e lls . Our results are contradictory to the findings of Zimmerman and Steudle 15 (1974, J. Me mb. Biol. 16:331 -352) but are s im ila r to the findings of Tarawa and Kamiya (1966, Aust. J. Biol, Sci. 19:399-419).