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In our previous experiments, greenhouse films that selectively remove far-red (FR) light from the growing environment reduced the stem elongation but delayed anthesis of long day plants. In the present research we investigated if the application of gibberellin A 4 and gibberellin (GA) biosynthesis inhibitors could overcome the delay in anthesis of petunia (Petunia ×hybrida Vilm.-Andr.), a quantitative long-day plant, under a FR light deficient environment. The GA biosynthesis inhibitors prohexadione-Ca and exo-C-16,17-dihydro GA 5 (GA 5 ) were used because of their ability to prevent catabolism of active GAs. Anthesis and stem elongation were investigated under control, red (R; 600 to 700 nm) and FR (700 to 800 nm) light-absorbing (A R and A FR ) films. The R:FR ratios of control, A R , and A FR films were 1.03, 0.71, and 1.51, respectively. Air temperatures among treatment chambers were not different. A R film did not affect anthesis or stem elongation, but A FR film reduced stem elongation and delayed anthesis by 12 days. Exogenous application of GA 5 had no effect on stem elongation, shoot dry weight or days to anthesis at any concentration (0 to 100 mg·L -1 ) tested under control, A R , or A FR films. Anthesis was delayed with increasing concentration (0 to 200 mg·L -1 ) of prohexadione-Ca under all treatments. Prohexadione-Ca at 200 mg·L -1 delayed anthesis by 11 and 7 days under the control and A FR film, respectively, suggesting an interaction between light quality and prohexadione-Ca treatment. Exogenous GA 4 accelerated anthesis under both films but the promotion was greater under the A FR films. However, GA 4 treatment increased stem elongation and the increase in stem elongation was greater under the A FR film. Addition of GA 5 to GA 4 had no added effect on flowering and failed to reduce stem elongation. Therefore, GA or GA inhibitors are not suitable for flower promotion under A FR films.

Role of Gibberellin A4 and Gibberellin Biosynthesis Inhibitors on Flowering and Stem Elongation in Petunia under Modified Light Environments