Plant Responses to Saline Substrates IV. Chloride Uptake by Hordeum Vulgare as Affected by Inhibitors, Transpiration, and Nutrients in the Medium

Abstract

Hordeum vulgare in the first and second leaf stage excluded chloride and sodium when these ions were at high external concentrations, even after 5 days in media of 50 and 100 m-equiv/1 sodium chloride. At high transpiration rates the ascending sap attained only between 1.5 and 4% of the medium concentration, showing that most of the water flowed through regions of low chloride permeability. Chloride uptake by both roots and shoots consisted of an active and passive component. Active uptake was determined by measurements at low temperature, and by application of nitrogen, carbonyl cyanide m-chlorophenylhydrazone, and 2,4-dinitro-phenol. Active chloride uptake reached a high rate in sodium chloride at 10 m-eqiv/l, and there was little further increase at higher concentrations; it was also reduced by high nutrient concentrations in the medium and by a high chloride status of the plants. Passive chloride uptake was not appreciably affected by any of the above-mentioned conditions and it increased linearly with the chloride concentration of the medium. Passive uptake contributed between 40 and 60% to the total chloride uptake at sodium chloride concentrations of 50 m-equiv/l, and 70-80% at concentrations of 150 m-equiv/l. Increased transpiration increased both total and passive chloride uptake but this increase was not proportional to transpiration rates, i.e. chloride concentrations of the ascending sap decreased strongly with increasing transpiration. Passive chloride flow from the medium to the shoot by-passed the major pool of chloride in the root. Chloride exclusion from the Donnan free space of the root appeared insufficient to account for the pronounced chloride exclusion from the root xylem. The data can be interpreted according to the two following models (1) Passive chloride flow to the xylem was through water and Donnan free space, but then a large proportion of the water must have followed another path through membranes. (2) Both chloride and water moved through membranes at some stage during movement to the root xylem. Comparisons between halophytes and non-halophytes, in their active and passive components of chloride uptake, would contribute to an understanding of chloride regulation as an important facet of salt tolerance.