SALT-INDUCED INHIBITION OF PHOSPHATE-UPTAKE IN PLANTS OF COTTON (GOSSYPIUM-HIRSUTUM L)

Radio-labelled phosphate (P-32) was used to study the effect of 150 mM NaCl on P-i uptake and P distribution in the roots of cotton plants (Gossypium hirsutum L. cv Alcala SJ-2). Salinity inhibited P-i uptake more severely at low Pi concentration (30 mu M) than at high P-i concentration (1 mM). Data of P-32(i) uptake at low P-i concentration (mu M range) in the nutrient solution showed a typical Michaelis-Menten hyperbolic curve (uptake rate vs. concentration). NaCl increased the apparent K-m (from 2.23 to 4.73 mu M) and decreased the apparent V-max (from 1.18 to 0.87 mu mol g(-1) f. wt h(-1)). High NaCl inhibited P-i uptake in the mature root zone. However, salinity enhanced P-i uptake in the root tips. We speculate that salinity may induce alkalinization of the cytoplasm in root tips cells, increasing the transmembrane pH gradient and therefore P-i uptake. Our results showed an inhibition of P-32 transport within the roots and from the roots to the shoots in the salt treatment.