Tissue specificity of acid action in rapid elongation responses of lupin hypocotyls

We have tested the acid‐responsiveness of hollow cylinders, peeled segments, and cores of stelar tissue of hypocotyls of light‐grown lupin seedlings (Lupinus angustifolius L. cv. New Zealand Bitter Blue), to determine how differential responsiveness of individual tissues to acid might govern the response of the intact hypocotyl. Hollow cylinders of cortex with the outer cell layers intact were less responsive to acid than was the cortex alone, which elongated strongly at pH 5. The stelar tissues were acid‐responsive (pH 5), but only if the cell layer surrounding the stele (the starch sheath) was disrupted. We conclude that all tissues were acid‐responsive, with the cortex alone most responsive, and the outer cell layers least responsive. We suggest that acid‐induced elongation of intact segments may be controlled by the response of the outer cell layers.