DIFFERENTIAL SENSITIVITY TO INDOLE-3-ACETIC-ACID AND GIBBERELLIC-ACIDFOLLOWING GRAVISTIMULATION OF THE LEAF-SHEATH PULVINI OF OAT AND BARLEY

Leaf-sheath pulvini excised from oat (Avena sativa) and barley (Hordeu m vulgare) were created with indole-3-acetic acid (IAA) or gibberellic acid (GA(3)), delivered apically from agar blocks, and either gravist imulated at 45 degrees or left upright. Control (upright) pulvini from both oat and barley showed similar responses to exogenous IAA, with g rowth stimulation showing a threshold of 10 to 30 mu M and a maximum r esponse at 100 to 300 mu M. When gravistimulated, the upper side of pu lvini from oat showed no response to IAA at any level, while the upper side of pulvini from barley responded like control tissues. The lower side of gravistimulated pulvini from both oat and barley showed stron g inhibition by exogenous IAA, but higher levels of IAA became less in hibitory in barley pulvini. Control pulvini from both oat and barley w ere insensitive to exogenous GA(3). Gravistimulation made the lower si de of both oat and barley pulvini sensitive to a wide range of GA(3) c oncentrations. This effect was also observed in the upper side of oat pulvini but was virtually absent in the upper side of barley pulvini. These results indicate a general phenomenon of altered sensitivity to IAA and an initiated sensitivity to exogenous GA(3) in response to gra vistimulation of the leaf-sheath pulvini of festucoid grasses. Differe nces in the quality of response between species may be ascribed to und erlying differences in hormone dynamics within the pulvini.