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 A4 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 GA5 (GA5) 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 (AR and AFR) films. The R:FR ratios of control, AR, and AFR films were 1.03, 0.71, and 1.51, respectively. Air temperatures among treatment chambers were not different. AR film did not affect anthesis or stem elongation, but AFR film reduced stem elongation and delayed anthesis by 12 days. Exogenous application of GA5 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, AR, or AFR 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 AFR film, respectively, suggesting an interaction between light quality and prohexadione-Ca treatment. Exogenous GA4 accelerated anthesis under both films but the promotion was greater under the AFR films. However, GA4 treatment increased stem elongation and the increase in stem elongation was greater under the AFR film. Addition of GA5 to GA4 had no added effect on flowering and failed to reduce stem elongation. Therefore, GA or GA inhibitors are not suitable for flower promotion under AFR films.
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