Interaction between Pfr and growth substances in the germination of light-requiring Kalanchoë seeds

Seeds of Kalanchoë blossfeldiana Poelln. cv. Feucrblüte, incubated on gibberellic acid, become very light-sensitive through a synergism between the far-red absorbing form of phytochrome and the growth substance, which results in high physiological activity of short far-red (FR) exposures. On 2 × 10^-3 M gibberellic acid (GA₃), one saturating FR pulse is somewhat more effective than one saturating red light (R) irradiation. Fluence-response curves for R and FR confirm this observation. At lower GA₃ concentrations, this difference disappears and the effects of one saturating R and FR pulse decrease in an identical way with the GA₃ concentration. When two saturating irradiations, separated by 24 h are given, the effect of FR falls off faster than that of R at low GA₃ concentrations. Consequently, the second irradiation must have a different impact in comparison with the first one. Of the other growth substances tested, only a mixture of gibberellins A4 and A₇ had an analogous, still more pronounced effect than GA₃. Abscisic acid (ABA) inhibits the phytochrome-mediated germination of Kalanchoë , both in the absence and presence of GA₃. An antagonism between ABA and GA₃ was demonstrated.