Effects of Photoperiod on Growth Rate and Endogenous Gibberellins in the Long-Day Rosette Plant Spinach

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Abstract
The earliest visible responses of spinach plants (Spinacia oleracea L., cv. Savoy Hybrid 612) transferred from short to long days (8 hours of high intensity light supplemented with 16 hours of low intensity illumination from incandescent lamps) were upright leaf orientation and increased elongation of the petioles. The effect of long days on growth rate was direct; i.e., there was no after-effect if the plants were transferred to short days. Gibberellin A3 applied to plants under short days had an effect similar to that of long days, whereas application of the growth retardant AMO-1618 [2′-isopropyl-4′-(trimethylammonium chloride)-5′-methylphenyl piperidinel-carboxylate] under long days caused a growth habit typical of short-day conditions. Gibberellin A3 caused more stem growth in plants under long days in which the endogenous gibberellin content had been reduced by AMO-1618 than in plants under short days not treated with the growth retardant.