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

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 P_r. In addition, the establishment within the lifetime of a flash of a photochromic system between the red absorbing form of phytochrome (P_r) 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 P_r 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 P_r?ΣI₇₀₀ toward P_r. Finally, the saturation level produced by a series of laser flashes depends, additionally, on absorption by P_fr.