Biphasic Fluence-Response Curves for Phytochrome-Mediated Kalanchoë Seed Germination

Abstract

The fluence-response curves for the effect of two red pulses separated by 24 hours on the germination of Kalanchoe blossfeldiana Poelln. cv Vesuv seeds, incubated on gibberellic acid (GA3) are biphasic for suboptimal concentrations. The response in the low fluence range corresponds with a classical red/far-red reversible phytochrome mediated reaction. GA3 induces an additional response in the very low fluence range, which is also phytochrome mediated. The sensitivity to phytochrome-far-red absorbing form (Pfr), however, is increased about 20,000-fold, so that even far-red fluences become saturating. Both in the very low and low fluence response range, the maximal responses induced by saturating fluences are modulated by the GA3 concentration. GA3 having no direct influence on the phytochrome phototransformations, alters the Pfr requirement and determines the responding seed population fraction in the very low and low fluence range. The effet of GA3 appears to be on the transduction chain of the phytochrome signal.The germination of Kalanchoe seeds is light-requiring. Incubated on water or KNO3 solution, germination requires several daily light pulses given from the first day on after sowing, to prevent induction of secondary dormancy (8,20). The seed population responds in the LF2 range (3xl0-5 to 3x10-2 mol/ mi2, R) and reversion ofa promoting light pulse by FR is complete (8, 11). The KNO3 concentration determines the responding population fraction in the LF range (8).In the presence of GA3, germination can be induced with a single light pulse. This irradiation was given on d 7 after sowing, the moment of highest response in the presence of GA3 (6). The seed population responds in the VLF range (10-9_10-6 mol/m2, R) and a single FR pulse is inductive (8). For a single R or FR pulse, the GA3 concentration determines the responding population fraction in the VLF range, in an identical way (8). Both in the presence and absence of GA3, preliminary action spectra point to phytochrome as the photoreceptor (7). GA3 increases ' Supported by the Belgian National Science Foundation Grant FKFO-2.0083.83. GA3 concentrations, the effect of R is higher than that of FR in the 0.1 to 0.01mM concentration range (6). These results suggest the possibility of a LF response, additional to the VLF response, which is induced by two R but not by two FR irradiations at suboptimal GA3 concentrations.Fluence-response curves for two R or FR pulses are presented, allowing the quantification and correlation of both the VLF and the LF response types in the same experimental conditions.We also studied the relation between exogenously applied GA3 and the maximal germination induced with VLF of R and FR, saturating the VLF response and with low R fluences, saturating the LF response. Probit analysis is used to calculate the population parameters (see "Materials and Methods"). From the LDP equations and from the zero response and the maximal response, the sigmoid curves fitting the experimental data are generated. The phytochrome...

Phytochrome-Mediated Germination of Very Sensitive Oospores

“…Activation by exposure to FR (730 nm) is consistent with the VLFR mode of phytochrome function in seeds (18). Activation is attributed to the effectiveness of the very low levels of Pfr/PotO (approximately 2%)…”

Fluence-Response Curves and Action Spectra for the Very Low Fluence and the Low Fluence Response for the Induction of Kalanchoë Seed Germination

“…The variation in the response range (RI), from about 50% to 100%, is only due to seed variation, since a correlation with the wavelength used is never observed (Fig. 4 20,000 times less R fluence than the LFR (19).…”

“…To prevent induction of secondary dormancy, irradiations are to be given from the 1st d on after sowing (18). In the presence ofGA3, germination can be the result of a VLFR when the seeds are induced with one single light pulse (12) or the result of a VLFR plus LFR when the seeds are induced by repeated irradiations (19). Repeated irradiation is required since no clear-cut LFR occurs when only one light pulse is given (12).…”

Phytochrome A Mediates the Promotion of Seed Germination by Very Low Fluences of Light and Canopy Shade Light in Arabidopsis

“…Under certain conditions, germination responses to the fluence of a R pulse are biphasic (Cone et al, 1985;VanDerWoude, 1985;Rethy et al, 1987;Scopel et al, 1991;Derkx and Karssen, 1993). The first phase, called VLFR, shows a plateau that coincides with germination levels induced by a saturating FR pulse.…”

“…Sensitization can be achieved by laboratory treatments, including chilling (VanDerWoude and Toole, 1980;VanDerWoude 1985), heating (Taylorson and Dinola, 1989), temperature shifts (Cone et al, 1985), GA, (Rethy et al, 1987), anesthetics (VanDerWoude, 1985), or incubation in water vapor phase (A.L. Scopel, unpublished data).…”