Increase in Membrane Permeability and Exudation in Roots of Zinc Deficient Plants

Abstract

The effect of Zn nutrition on the root membrane permeability and root exudation was studied in cotton (Gossypium hirsutum L. cv. Deltapine 15/21), wheat (Triticum aestivum L. cv. Cumhuriyet 75), tomato (Lycopersicon esculentum L. cv. Super marmande), and apple (Malus domestica cv. M26 rootstock) plants grown in nutrient solutions under controlled environmental conditions. Root exudation as an indicator for root plasma membrane permeability was measured by incubating roots of intact plants in aerated CaCl2 (0.5 mM) solution. In all plant species studied, Zn deficiency increased root exudation (net efflux) of K+, amino acids, sugars and phenolics. Resupply of Zn to deficient plants for 8, 12 or 27 h increased the Zn concentration in the roots and simultaneously decreased the root exudation of all solutes studied. Omission of Ca from the incubation solution increased root exudation by a factor of about two in the Zn sufficient plants but only slightly in the Zn deficient plants. The levels of fatty acids and phospholipids were depressed in Zn deficient cotton roots. The depression was particularly evident in the case of unsaturated fatty acids. The results strongly suggest a distinct role of Zn in membrane integrity and thus also in root exudation. This role seems to be independent of the role of Ca and probably has ecological implications with respect to nutrient mobilization and microbial activity in the rhizosphere.