EFFECTS OF PH CHANGES ON ION AND 2,4-D UPTAKE OF WHEAT ROOTS

The movement of various ions (K, phosphate, nitrate and ammonium) and 2,4-D across the root membranes was monitored with radioactive and stable isotope tracer methods. The H+ ion concentration of the absorption solution strongly influences the 2,4-D uptake of the roots. Simultaneously, the 2,4-D uptake stimulates secretion of H+ into the absorption solution, that is, a H+ efflux can accompany the uptake of 2,4-D. This finding is consistent with the acid secretion theory of auxin and fusicoccin action. At pH 4 the 2,4-D uptake was much higher than at pH 6, thereby inhibiting the ion uptake and increasing the phytotoxicity in the plant. 2,4-D enters the root cells rapidly at the lower pH, mostly as undissociated molecules. With reference to the 2,4-D concentration in the roots at pH 4, a possible transport mechanism of the auxin herbicide is briefly discussed.