The Uptake of Growth Substances: XIII. DIFFERENTIAL UPTAKE OF INDOL-3YL-ACETIC ACID THROUGH THE EPIDERMAL AND CUT SURFACES OF ETIOLATED STEM SEGMENTS

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Abstract
The patterns of uptake of indol-3yl-acetic acid (IAA-2-14C) by etiolated stem segments of varying lengths have been examined, employing tissues excised from (a) the first and third internodes of Pisum sativum, (b) the top and base of the hypocotyl of Gossypium hirsutum, and (c) the mesocotyl of Avena sativa. For all species, concentrations (10–5–10–3 M) and times up to 24 h, there is a steady accumulation of radioactivity in the segments. For equal volumes of tissue uptake is inversely correlated with segment length but for extending tissues the initial enhanced extension growth is independent of length; that is there is no direct linkage between the rate of extension and auxin content. Comparisons between segments with free and sealed ends established that over 24 h some 57–73 per cent of the IAA enters via the cut surfaces. Initially, the percentage is greater; expressed as a rate per unit of surface the differences between cut and epidermal surfaces can reach 28-fold. The rate of entry through the epidermal surface is linearly proportional to the external concentration but this does not hold for cut surfaces. The addition of streptomycin, synthalin, cetyltrimethylammoniumbromide (CTAB), and chitosan to the external medium does not promote uptake of IAA by Pisum segments; indeed synthalin is markedly inhibitory. With Gossypium synthalin causes little inhibition. Larger depressive effects were induced for entry via the cut surfaces. On entry the IAA is rapidly metabolized and the rate of conversion is higher for segments with sealed ends. These findings are discussed in relation to (a) differences in the mechanisms determining the uptake of IAA and other auxins, (b) cell extension and the distribution of auxin in the tissues.

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