Investigations into the possible regulation of negative gravitropic curvature in intact Avena sativa plants and in isolated stem segments by ethylene and gibberellins

Using Avena sativa L. cv. Victory oat seedlings and excised p-1 stem segments (including the p-1 and p-2 internodes) the effect of exogenously supplied ethylene and the removal of ethylene on internodal extension and gravitropic bending was assessed. Similarly, the ability of the excised system to respond to gravistimulation was assessed in the presence of inhibitors of ethylene action (AgNO₃) and ethylene synthesis (3,5-diiodo-4-hydroxybenzoic acid and benzyl isothiocyanate; BITC). The production of ethylene from both intact and excised systems was also measured from 0 to 48 h after gravistimulation, relative to vertical controls. Although gravitropic curvature is initiated, and indeed enters the most rapid phase of upward bending during the first 6 h, there is no difference in ethylene production between vertical and geostimulated plants during this period. The ethylene production of gravistimulated plants rises sharply to a maximum at 24 h, then decreases steeply to almost the control level by 48 h, at which time the rate of upward curvature is diminishing. Neither the addition nor removal of ethylene, nor the addition of inhibitors affecting ethylene-action (AgNO₃) or synthesis (DIHB) influence gravitropic bending or internodal extension in excised segments. Although the ethylene synthesis inhibitor BITC showed down the rate of upward bending, this effect could not be reversed by addition of ethylene. We conclude that the burst in ethylene production that develops in leaf-sheath bases (pulvini) after they have started to curve upwards is not primary to the induction of curvature. We further suggest that ethylene has no major effect or role in the induction of upward bending after gravistimulation. The metabolism of high specific activity gibberellin A₁ (³H]-GA₁) in the excised system was assessed during 1, 2 and 4 h of gravistimulation. Changes in endogenous GAs and GA metabolism have been shown previously to be correlated (at the later stages) with gravistimulated bending in intact Avena shoots. The excised segments ‘leaked’ free ³H]-GAs and ³H]-GA glucosyl conjugate-like substances into the bathing medium, and this was a confounding factor. Nevertheless, gravistimulated stem segments, and especially the bottom half of the segment, were significantly less leaky then vertical segments. Thus, just 1 h after gravistimulation, bottom segment halves retained 22% more precursor ³H]-GA₁, 36% more free ³H]-GA-like metabolites, and 48% more ³H]-GA glucosyl conjugate-like metabolites than vertical segments. In contrast, the 1 h gravistimulated top halves retained slightly less (1–4%) precursor ³H]-GA and free ³H]-GA metabolites, but 21% more ³H]-GA glucosyl conjugate-like radioactivity than vertical segments.