An Improved Method for Detecting Auxin-induced Hydrogen Ion Efflux from Corn Coleoptile Segments

Conditions necessary to detect maximal auxin-induced H(+) secretion using a macroelectrode have been investigated using corn coleoptile segments. Auxin-induced H(+) secretion is strongly dependent upon oxygenation or aeration when the tissue to volume ratio is high. Cuticle disruption or removal is also necessary to detect substantial auxin-induced H(+) secretion. The auxin-induced decrease in pH of the external medium is stronger when the hormone is applied to tissue in which the cuticle has been disrupted with an abrasive than when the hormone is applied to tissue from which the cuticle and epidermis have been removed by peeling. The lower detectable acidification of the external medium when using peeled segments appears to be due in part to the leakage of buffers into the medium and in part to the removal of the auxin-sensitive epidermal cells.The sensitivity of corn coleoptile segments to auxin, as measured by H(+) secretion, increases about 2-fold during the first 2 hours after excision. This change in apparent sensitivity to auxin as reflected by H(+) secretion is paralleled by a time-dependent change in the growth response to auxin. Under optimal conditions for detecting H(+) efflux (oxygenation, abrasion, hormone application 2 hours after excision), the latent period in auxin-induced H(+) efflux (about 7 or 8 minutes) is only half as great as the latent period in auxin-induced growth (about 18 to 20 minutes). These observations are consistent with the acid growth hypothesis of auxin action.