The Elongation Response of Watermelon Hypocotyls to Indole-3-Acetic Acid: A Comparative Study of Excised Segments and Intact Plants

Carrington, C. M. S. and Esnard, J. 1988. The elongation response of watermelon hypocotyls to indole-3-acetic acid: a comparative study of excised segments and intact plants.—J. exp. Bot 39: 441–450. The auxin-growth response along the hypocotyl of Citrullus lanatus (Thumb.) Mansf. seedlings was studied. In excised segments, promotion of elongation was seen in all zones at the concentrations of IAA used (10–4–10–2 mol m-3). In intact plants, only the most basal zone showed unequivocal IAA-extension while in the most apical zone elongation was inhibited by auxin. This difference between segments and intact plants for apical zones suggests a modifying effect of the apex and cotyledons on the growth response. Indeed, removal of the apex and colyledons only affected elongation in the zones adjacent to the excision but only in buffer-treated plants, not auxin-treated plants. Auxin supplied apically to the intact plant only resulted in a short-lived promotion of elongation whereas basally supplied auxin gave a longer-lasting effect Zonal differences between auxin-promoted growth of excised segments suggests that sensitivity to auxin varies in the hypocotyl. The response of intact plants to auxin was shown to be more complex than in segments. Thus, responses given by segments are poor indicators of auxin activity in intact plants.