The Stimulatory Effect of Indole-3-Acetic Acid on the Uptake of Amino-Acids by Tissue 1Helianthus annuus

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Abstract
Indole-3-acetic acid was observed to bring about a prompt and marked increase in the amount of 14C accumulated by segments of sunflower hypocotyl from solutions of labelled glutamic acid, glycine, and lysine. The curve relating magnitude of effect to indole-3-acetic acid concentration followed the comparable curves for water uptake and extension growth. The accumulation of 14C was related to the external concentration of glutamic acid by a curve which departed only slightly from linearity. The percentage increase in 14C accumulation brought about by auxin did not decline to any appreciable extent with increasing external concentration of glutamic acid. Under nitrogen the amount of 14C taken up from solutions of labelled glutamic acid in 1·75 hour was cut down by approximately one-third, and the auxin effect was abolished. The Q10 for 14C accumulation between 16° C. and 26° C. was 1·2 in the absence of indole-3-acetic acid, and was 1·3 in its presence. When net water uptake was eliminated by the addition of mannitol to the external solution, 14C accumulation in auxin-free media was not depressed. The percentage increase in 14C accumulation brought about by auxin, however, was markedly reduced. The fate of the 14C accumulated was investigated by means of chromatography on resin columns and on filter paper. About 30–40 percent, of the 14C was in the form of glutamic acid after approximately a hours' treatment. No marked difference in the level of glutamic acid was observed between auxin-treated and control segments. The effect of auxin was more evident on the amounts of other radioactive derivatives, as yet unidentified. It was observed that, not only was the amount of CO2 evolved in respiration higher in the presence of indole-3-acetic acid, but that this CO2 was richer in 14C, i.e. in auxin-treated tissue glutamic acid formed a larger proportion of the substrate respired. The possible implications of these observations are discussed. It is pointed out that indole-3-acetic acid may have achieved its effect by stimulating a transfer process, by lessening a diffusion resistance, or by promoting a process or processes which, by removing free amino-acids within the cell, maintain an inward diffusion gradient.
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