This study investigates the possible roles of calcium and ethylene in the gravitropic response of Aritirrhinum majus (snapdragon) flower spikes. Stem curvature and regions of maximum ethylene production were measured in varying levles of endogenous calcium -- regulated by calcium chelators, agonists, and ionophores -- and ehylene -- regulated by inhibitors.
Flower spikes 55 or 70 cm long were placed in 100 or 500 mL of one of the following:
Philosoph-Hadas et al. found results in three areas:
Calcium chelators inhibited gravitropic bending for approximately 24 hours, at which point the treated flower spikes exhibited the same degree of curvature as did the control. The calcium channel blocker retained the inhibitory effect; the ionophore had no effect.
Click here to view these results. The darkest bar represents the calcium channel blocker.
After four days, ethylene production was highest in the region of gravitropic curvature.
Click here to view this data. Ethylene levels are similar in whole stems (A) and vertical half cuts (B), with production tapering off further from the apex. In horizontal half cuts (C), significantly more ethylene is present on the lower side of the stem, where gravitropic growth occurs. The ratio of ethylene production in the upper and lower halfs (D) is four to one.
The presence of ethylene inhibitors affected the gravitropic response as well as the ethylene biosynthesis pathway.
Click here to view this data. Inhibitors of ethylene synthesis also inhibited gravitropic bending (A) and eliminated the ethylene imbalance between the upper and lower halves of the stem (B).
These results led Philosoph-Hadas, et al. to suggest that the signal-transduction pathway may involve calcium mediation in shoots as well as in roots, as had already been shown.
Calcium may regulate auxin action, as manifested in increased ethylene production. The question remains, however, whether ethylene asymmetry is merely a reflection of auxin asymmetry (in accordance with the Cholodny-Went Hypothesis) or is directly involved in the gravitropic response.
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