CALCIUM AND GIBBERELLIN-INDUCED ELONGATION OF LETTUCE HYPOCOTYL SECTIONS

The relationship between Ca2+ ions and gibberellic acid (GA3)-induced growth in the excised hypocotyl of lettuce (L. 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 the 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 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 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 reversed 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. Gibberellin may control extension growth in lettuce hypocotyl sections by regulating the uptake of Ca2+ by the hypocotyl cells.