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Abstract— Kalanchoë blossfeldiana seeds are light‐requiring for seed germination. On water or KNO3 solution and irradiated with several daily red (R) irradiations, the seeds show a low‐fluence (LF) response which is far‐red (FR) reversible. Incubated on gibberellic acid (GA3) the seeds show a very‐low‐fluence (VLF) response which can be saturated with red as well as with far red light. As germination is a quantal response, the sub‐optimal segments of the dose‐response curves are analysed by means of probit analysis in order to calculate the seed population parameters. There is a linear relation between the probit of the germination response and the logarithm of the fluence. Moreover, the slope for the VLF as well as for the LF response is the same. The VLF requires about 8 × 104 times less fluence than the LF. VLF saturation with FR requires about 200 times more fluence than with R. Although, GA3 and KNO3 modulate VLF and LF, respectively, there is no direct influence on the phytochrome‐phototransformations. Once Pfr is formed (in VLF or LF, or preserved in dry seeds) germination is proportional to the GAS concentration (for VLF and dark germination) or proportional to the KNO, concentration (for LF). The non‐photochemical events leading to germination seem to be triggered by a similar action mechanism for both GA, and KNO3.

Sown on water, seeds of Kalanchoëbiossfetdiana Poelln. cv. Feuerblute are absolutely light‐requiring and show full red/far‐red reversibility. In seeds, sown on 2 ×10‐3 M gibberellic acid, red/far‐red reversibility disappears and both short red and far‐red irradiations induce germination. Gibberellic acid alone does not induce germination, but it increases the physiological activity of Pfr to the extent, that the low Pfr level obtained by far‐red irradiation becomes very effective. The synergism between gibberellic acid and far‐red light appears after a two‐day incubation; period. The nature of this lag phase was examined by measuring both germination and uptake of labelled gibberellic acid in intact seeds and seeds with a punctured seed coat. The lag phase was shown to be independent of the uptake kinetics of gibberellic acid and allows development to a specific stage, necessary for germination after phytochrome‐phototransformation. The kinetics of the uptake of gibberellic acid by intact seeds and embryos of intact seeds are different. In intact seeds most of the gibberellic acid is retained in the seed coat; only a small fraction actually penetrates to the embryo where it can exert its physiological activity.

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...

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