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

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 internodesof Pisum sativum, (b) the top and base of the hypocotyl of Gossypiumhirsutum, and (c) the mesocotyl of Avena sativa. For all species,concentrations (10^–5–10^–3 M) and times upto 24 h, there is a steady accumulation of radioactivity inthe segments. For equal volumes of tissue uptake is inverselycorrelated with segment length but for extending tissues theinitial enhanced extension growth is independent of length;that is there is no direct linkage between the rate of extensionand auxin content. Comparisons between segments with free andsealed ends established that over 24 h some 57–73 percent of the IAA enters via the cut surfaces. Initially, thepercentage is greater; expressed as a rate per unit of surfacethe differences between cut and epidermal surfaces can reach28-fold. The rate of entry through the epidermal surface islinearly proportional to the external concentration but thisdoes not hold for cut surfaces. The addition of streptomycin,synthalin, cetyltrimethylammoniumbromide (CTAB), and chitosanto the external medium does not promote uptake of IAA by Pisumsegments; indeed synthalin is markedly inhibitory. With Gossypiumsynthalin causes little inhibition. Larger depressive effectswere induced for entry via the cut surfaces. On entry the IAAis rapidly metabolized and the rate of conversion is higherfor segments with sealed ends. These findings are discussedin relation to (a) differences in the mechanisms determiningthe uptake of IAA and other auxins, (b) cell extension and thedistribution of auxin in the tissues.