INDUCTION OF SEED GERMINATION IN Lactuca sativa L. BY NANOSECOND DYE LASER FLASHES*^,†

Abstract

Abstract— A 15 ns, tunable dye laser was used to induce germination of the photoblastic seeds of Lactuca sativa. One red laser flash in the range from 620 to 690 nm was sufficient to increase germination significantly above the dark level. Repeated flashes, however, were necessary to saturate the physiological response. The wavelength dependence for induction of germination differed for single and repetitive flashes. After saturating far‐red irradiation, the effect of single‐flash induction was a function primarily of the absorption spectrum of Pr. In addition, the establishment within the lifetime of a flash of a photochromic system between the red absorbing form of phytochrome (Pr) and the sum of photoreversible intermediate forms (ΣI700) contributes to this wavelength dependence at high fluence rates. This photochromic system is assumed to be shifted significantly toward Pr by wavelengths 660 nm. Similarly, a strong double‐flash effect, which is seen as an increase in effectiveness when a given total fluence is provided by two consecutive flashes rather than by one flash only, is restricted to those wavelengths that considerably shift the photochromic system Pr⇔ΣI700 toward Pr. Finally, the saturation level produced by a series of laser flashes depends, additionally, on absorption by Pfr.

INDUCTION OF Trichoderma SPORULATION BY NANOSECOND LASER PULSES: EVIDENCE AGAINST CRYPTOCHROME CYCLING

“…Under the assumption that saturation of the response results from saturation of photoconversion, loss of photoproduct in favor of metastable photochemical intermediates should lower the effectiveness of a saturating short flash (Hartmann and Cohnen-Unser, 1972). This prediction fits some phytochrome data (Scheuerlein and Braslavsky, 1985). The absence of such an effect in Trichoderma suggests that the decay of the intermediates to active photoproduct(s) is strongly favored.…”

THE KINETICS OF THE EARLY STAGES OF THE PHYTOCHROME PHOTOTRANSFORMATION Pr→ Pfr. A COMPARATIVE STUDY OF SMALL (60 kDalton) and NATIVE (124 kDalton) PHYTOCHROMES FROM OAT

“…The data of this work are, in fact, compatible with both (i) and (ii), but they need be considered together with other findings relating to this aspect. In vivo results suggest that the photochemical behaviour of the transients formed within a nanosecond laser pulse duration differs with excitation wavelengths in the range from hex' 2 690 to 610 nm (Scheuerlein and Braslavsky, 1985). The occurrence of IblO, furthermore, has already been tentatively postulated in a recent optoacoustic investigation (Jabben et al, 1984).…”

Progress in the Molecular Analysis of Phytochrome

“…Thus it is clear that the amount of Pf, formed upon flash excitation of P, is not only wavelength dependent but also depends upon the fluence rate and pulse width of the laser flash. A direct demonstration of this effect was described by Scheuerlein et al (1985) who showed that the Pf,/P,,, ratio for large Avena phytochrome in vitro is fluence rate dependent at wavelengths where intermediates appreciably absorb light. That photochromism of intermediates on the P, to Pf, pathway also occurs in vivo was documented by Scheuerlein and Braslavsky (1985) using a double flash analysis for lettuce seed germination.…”

“…A direct demonstration of this effect was described by Scheuerlein et al (1985) who showed that the Pf,/P,,, ratio for large Avena phytochrome in vitro is fluence rate dependent at wavelengths where intermediates appreciably absorb light. That photochromism of intermediates on the P, to Pf, pathway also occurs in vivo was documented by Scheuerlein and Braslavsky (1985) using a double flash analysis for lettuce seed germination. Similarly, the production of stable intermediates in dry seeds for several plant species was reported to account for the far red light inhibition of seed germination (Bartley and Frankland, 1984).…”