Prohexadione-calcium affects growth and flowering of petunia and impatiens grown under photoselective films

The response of petunia (Petunia x hybrida Vilm.-Andr. 'Countdown Burgundy') and impatiens (Impatiens wallerana Hook 'Accent Orange Tempo') to Prohexadione-calcium was evaluated under a clear and a far-red light absorbing greenhouse (AFR) film to investigate the dosage effect of Prohexadione-Ca and to determine if it can overcome the flowering delay under FR deficient greenhouse environments. Prohexadione-Ca reduced stem elongation of petunia and impatiens under AFR and clear films when applied 3 weeks after germination. Late applications were less effective. In both crops, main stem length decreased in a quadratic pattern as the concentration of Prohexadione-Ca increased. Under both films, 50-100 mg l−1 Prohexadione-Ca resulted in ≈30% shorter petunia plants. Greater concentrations (500 and 1000 mg l−1) resulted in excessively short plants (over 70%). Prohexadione-Ca at 100 mg l−1 delayed anthesis of petunia by 8 and 3 days under the clear film and the AFR film, respectively during less inductive photoperiods but had no effect during inductive photoperiods. In impatiens, Prohexadione-Ca at 100 mg l−1 delayed anthesis over 10 days under clear or AFR film. Greater concentrations (200 and 300 mg l−1) inhibited flowering of impatiens. Prohexadione-Ca treatments significantly affected flower color development. Untreated petunia plants had dark burgundy flowers. Prohexadione-Ca treatment increased L*, a*, and C* values and decreased hue angle indicating that the flowers were faded. Flowers of untreated impatiens plants were bright orange color. Prohexadione-Ca at 100 mg l−1 increased L* value and decreased a*, b*, and C* values indicating that significant petal fading had occurred. Flowers of treated plants were nearly white under both films. Although effective in height control, loss of color would be a major limitation to the use of Prohexadione-Ca on flowering crops.