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  • Author or Editor: Arne Sæbø x
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Treatment of young watermelon (Citrullus lanatus cv. Sugar Baby) plants with individual and multiple FR light (15 min) treatments during the dark phase of the photoperiod influenced plant growth and development (i.e., petiole elongation, internode elongation, and reduced petiole angles) as compared to plants not treated with FR signals. The timing for the most effective light signal for inducing a growth response was when the signal was delivered immediately after the plant entered into the dark phase of the photoperiod. Decreasing growth responses to FR signals were observed as the signals were delayed after the plant entered into the dark phase. Multiple FR signals during the dark phase slightly increased growth responses as compared to plants that received the signal immediately after the light period. Young watermelon plant growth responses to FR light signals do not appear to be photoperiodic as plants similarly treated with a white light signal did not generate growth responses. Tissue analysis of petioles, leaves, stems, and cotyledons from plants treated with individual and multiple FR signals suggested that carbohydrate composition, distribution, and diurnal fluctuation were affected by the light quality treatments.

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Previous research has demonstrated that watermelon plants are sensitive to changes in light quality, as suggested by an individual treatment of plants with 15-min of end-of-day (EOD) red (R)and far-red (FR) light. FR-induced growth responses (i.e., petiole elongation, internode elongation, reduced petiole angles) were reversed by immediately following the FR light treatment with R light implicating phytochrome as the light quality perception mechanism. The objective of the present experiment was to determine the influence of individual and multiple FR light treatments (each treatment of 15-min duration) during the light and dark phase of the photoperiod on photomorphogenic growth responses of young watermelon plants. Light regulated growth responses of watermelon were influenced by the timing and the number of light quality exposures during the light or dark phase of the photoperiod. Individual FR treatments during the light phase except for the EOD exposure did not affect plant growth responses. In contrast, individual FR treatments at selected intervals during the dark period affected plant development. The most effective individual FR treatment to induce growth responses was at the beginning of the dark period, with decreasing responses as the FR treatment was delayed into the dark period. Multiple exposures of FR during the dark slightly increased growth responses as compared to a single EOD FR treatment.

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