Medium Acidification by Auxin- and Fusicoccin-Treated Peeled Stem Segments from Etiolated Seedlings ofPisum sativum

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Abstract
Several reports have suggested that peeling off the epidermis from dicotyledonous stems also removes the capacity of these segments to show an auxin-induced elongation response. This could be due either to the removal of the auxin-responsive tissue, or to the loss of H+ into the medium when the impermeable cuticle is removed. Medium acidification by peeled etiolated segments of Pisum sativum L. cv. Alaska, and the effects of indoleacetic acid (IAA) and fusicoccin (FC) on this process, were thus investigated in an attempt to resolve this problem. It was found that medium acidification occurred even with control segments, and that only in the presence of the divalent cations Ca2+ or Mg2+ did IAA give any enhancement of acidification. Under these conditions, the main effect of IAA was a reduction in the lag time between IAA addition and an observed acidification response. Addition of these ions is not, however, required to obtain normal auxin-induced growth. Similarly, peeled segments of mature non-elongating tissue also acidified the medium. In contrast, FC gave some enhancement of medium acidification even in the absence of Ca2+ and Mg2+. With both IAA and FC, lag times before observable acidification occurred were reduced (from about 40–50 min to 20 min and 2/emdash 4 min respectively) in the presence of Ca2+ or Mg2+. These ions appeared to exert similar effects. IAA, when added once a stabilized pH had been attained, had little effect on pH, whereas FC caused a further drop of about 0.5 unit. It thus appears that auxin has little appreciable effect on the acidification of the medium caused by peeled segments, either of non-elongating or of growing stems, and we conclude that peeling must therefore remove the auxin-responsive tissue in pea stem.