Kinetics of upward bending response of gravistimulated snapdragon shoots

Conclusions

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  1. In flowering shoots of the dicot, snapdragon, the locus of graviperception continously shifts upwards during shoot elongation.
  2. When shoots of this plant respond to gravistimulation, the initial response is rapid during the first 8 hours (phase 1), resulting in an overshoot beyond 90o curvature by as much as 60o. Subsequently, the return curvature response back to a vertical position is much slower, taking as long as 50 hours (phase 2).
  3. In gravistimulated preflowering snapdragon shoots, the initial curvature is downwards for ca. 50 minutes, followed by continous upward bending. This initial downward curvature response is due to the weight of the shoots.
  4. In contrast, in preflowering shoots of cereal grasses (monocots) such as oats, the locus of gravitropic curvature is localized to discrete regions called leaf sheath pulvini (swollen joints along the stem where leaves are attached). In intact oat shoots, as many as 2 or 3 leaf-sheat pulvini may be involved in reorientation of lodged shoots to a vertical position.
  5. In gravistimulated oat shoot stem segments (with a single leaf-sheath pulvinus present), there is no initial downward curvature. Upward curvature begins within 30 to 60 minutes and continues for ca. 60 hours, reaching an ultimate angle of 50 to 60o.