Calcium and gibberellin-induced elongation of lettuce hypocotyl sections

The relationship between calcium ions and gibberellic acid (GA₃)-induced growth in the excised hypocotyl of lettuce (Lactuca sativa L.) was investigated. The short-term kinetics of growth responses were measured using a linear displacement transducer. Test solutions were added either as drops to the filter paper on which the hypocotyl stood (non-flow-past) or by switching solution flowing past the base of hypocotyl (flow-past), resulting in differences in growth behavior. Drops of CaCl₂ added at a high concentration (10 mM) inhibited growth within a few minutes. This inhibition was reversed by ethylenediaminetetraacetic acid (EDTA). Drops of EDTA or ethyleneglycol-bis(2-aminoethylether)-tetraacetic acid caused a rapid increase in growth rate. Growth induced by EDTA was not further promoted by GA₃. A continuous H₂O flow resulted in growth rates comparable to those in response to GA₃. Addition of CaCl₂ to the flow-past medium inhibited growth and this inhibition was reversed by a decrease in CaCl₂ concentration. The growth rate was found to be a function of CaCl₂ concentration. When a constant CaCl₂ concentration was maintained by the flow-past medium, a shift in pH from 5.5 to 4.25 had no obvious effect on hypocotyl elongation. Gibberellic acid was found to reverse the inhibitory effect of CaCl₂, causing an increase in growth rate similar to that found previously when GA₃ was added to hypocotyls grown in H₂O under non-flow-past conditions. We propose that gibberellin controls extension growth in lettuce hypocotyl sections by regulating the uptake of Ca²⁺ by the hypocotyl cells.Abbreviations EDTA ethylenediaminetetraacetic acid - EGTA ethyleneglycol-bis(2-aminoethylether)-tetraacetic acid - GA gibberellin - GA₃ gibberellic acid - IAA indole-3-acetic acid