Effects of pH Changes on Ion and 2,4‐D Uptake of Wheat Roots

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Abstract
The quantitative relationships between pH‐dependent ion and 2,4‐D uptake in winter wheat seedlings (Triticum aestivum L. cv. Yubileynaya 50) have been investigated. The movement of various ions (potassium, phosphate, nitrate and ammonium) and 2,4‐D across the root membranes was monitored with radioactive and stable isotope tracer methods. It was found that 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. The results indicate that 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.