Interaction between Phytochrome B and Gibberellins in Thermoperiodic Responses of Cucumber

in Journal of the American Society for Horticultural Science
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  • 1 Department of Horticulture and Crop Sciences, Agricultural University of Norway, P.O. Box 5022, N-1432 Ås, Norway
  • | 2 Department of Applied Biology, University of Helsinki, P.O. Box 27, FIN-00014, Finland

The role of phytochrome in control of stem elongation by daily temperature alternations is unclear. The aim of this work was to study the involvement of phytochrome B in thermoperiodism in cucumber (Cucumis sativus L.), and the interaction with gibberellin (GA). The wild type and the phytochrome B deficient, long-hypocotyl (lh) cucumber mutant were grown under alternating day (DT) and night temperature (NT) and either with or without an exposure to end-of-day far-red light (EOD-FR). Without EOD-FR, hypocotyl and internodes of the wild type plants were shorter under a low DT (19 °C)/high NT (25 °C) (negative DIF) compared with a high DT/low NT regime (positive DIF), while the number of leaves was reduced by 12%. EOD-FR enhanced elongation of hypocotyl and internodes. However, EOD-FR reduced the effect of alternating temperature on hypocotyl elongation. The lh cucumber mutant did not respond to EOD-FR treatments, but internode length was slightly increased by positive compared with negative DIF. The results suggest that phytochrome B is required for a maximum effect of daily temperature alternations on stem elongation in cucumber. Additional GA4 reduced the difference between positive and negative DIF, but it had a minor effect only on the difference between EOD-FR and EOD red light (EOD-R) in the wild type. Plants depleted for endogenous GA by the GA biosynthesis inhibitor paclobutrazol, did not respond at all to DIF or EOD treatments. When seedlings were treated with prohexadione-calcium, which blocks both biosynthesis and inactivation of GA4, response to applied GA4 was enhanced by EOD-FR. The present results suggest that, in cucumber, EOD-FR, and probably also positive DIF, enhances tissue sensitivity to GA4. In addition, catabolism of GA4 can be enhanced by negative DIF.

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