Involvement of Gibberellins in Phytochrome-regulated Stem and Petiole Elongation in Watermelon Plants

Phytochrome-regulated stem and petiole elongation is of great horticultural importance due to their effects on height and compactness of plants. Although it is believed that phytochrome action during cell elongation is controlled via gibberellin (GA) biosynthesis, conclusive evidence has not been presented. In this study, watermelon plants pre-treated with GAs or GA biosynthesis inhibitors were briefly exposed to red (R) or far-red (FR) at the end of photoperiod to investigate the involvement of GAs in phytochrome-regulated stem and petiole elongation. Two-week-old watermelon plants were applied with either GAs, GA3 (25 ppm) or GA4+7 (25 ppm), or GA biosynthesis inhibitors, paclobutrazol (early stage inhibitor, 2 ppm) or prohexadione-Ca (late stage inhibitor, 200 ppm). Following chemical application, plants were treated with low-intensity R or FR light for 15 min at the end of photoperiod for 8 days. The increased petiole length and increased dry mass partitioned to petioles caused by FR were further increased when plants were pre-treated with GA3 and GA4+7, but reduced when plants were pre-treated with paclobutrazol or prohexadione-Ca. Pre-treatment with paclobutrazol or prohexadione-Ca further reduced the petiole length and dry mass partitioned to petioles caused by R alone, but effects of R were reversed when plants were pre-treated with GA3 and GA4+7. Petiole growth changes were significant after 2 to 3 days of treatments and there was no light or growth regulator effects on stem growth. Low leaf chlorophyll content and higher photosynthesis resulted in FR-treated plants were not observed when plants were pre-treated with paclobutrazol or prohexadione-Ca. Prohexadione-Ca was more effective in reversing FR induced petiole elongation than paclobutrazol, while GA4+7 was more effective than GA3 in promoting the effects of FR and reversing the effects of R. These results provide further evidence that phytochrome regulated growth changes in watermelon are due to the changes in GA biosynthesis. It is also suggestive that phytochrome has differential regulation of GA biosynthesis.