Early bending kinetics in response to geostimulation or unilateral IAA application in Lolium nodal segments

In Lolium multiflorum nodal segments, bending responses both to geostimulation and unilateral indole‐3‐acetic acid (IAA) application exhibited much variability in their lag times and speeds of early bending. Despite this variability, mean response curves to gravity and auxin stimulus were markedly similar with each having a phase of immediate, negative bending followed by phases of slow, positive bending and eventually more rapid, positive bending within 40 min of initial treatment. Comparison of lag times for response to geostimulation and unilateral IAA application, whether derived from the mean of individual replicates, or from mean curve data, showed that at least 4 min is available in this geotropic system for establishment of asymmetric auxin levels that could lead to differential growth. The hypothesis that variability in georesponse in Lolium nodal segments is linked to variable sensitivity of geosensitive tissue to auxin was tested using matching longitudinally‐halved nodal segments and evidence was obtained in support of the hypothesis from lag time but not from early bending speed data. The implications of the findings for an involvement of endogenous IAA in shoot geotropism together with the necessity to understand better the complex behaviour of bending response in individual replicates are discussed.