Both the California chaparral species, Emmenanthe penduliflora Benth. (Hydrophyllaceae), and a tobacco native to the Great Basin Desert of south-western Utah, Nicotiana attenuata Torr. ex Wats. (Solanaceae), germinate in response to component(s) of wood smoke. Nitrogen oxides (NO and NO2), in amounts produced by a fire, have been proposed to be germination signals for E. penduliflora. We examined the germination response of dormant seeds of E. penduliflora and N. attenuata to aqueous solutions of smoke adjusted to different pHs, and two NO donors [sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP)]. The smoke solutions, at pH 4 or 5, induced the maximum germination response. Aqueous solutions of SNP and SNAP, releasing NOx as high as 42 μM, had no effect on germination. Additionally, NO2– could not be detected in aqueous smoke extracts derived from combusted cellulose or wood. Therefore, unidentified cellulose combustion factors, rather than NOx, are likely to be the ecologically relevant germination signals.

The native tobacco, Nicotiana attenuata, synchronizes its germination with the immediate post-fire environment with a combination of germination stimulants found in wood smoke and inhibitors from the unburned litter of the dominant vegetation. The inhibitors override the stimulants and prevent seeds from germinating maladaptively in unburned habitats adjacent to burns. To understand the physiological basis of this environmental control of germination, we tested several previously isolated signals, phytohormones and their respective biosynthesis inhibitors. The germination inhibitors methyl jasmonate (MeJA, a constituent of sagebrush litter), bornane-2,5-dione (BD, a constituent of juniper litter extract, JLE) and JLE did not alter abscisic acid (ABA) content of imbibed seeds. Treatment with the ABA biosynthesis inhibitor, fluridone, inhibited the dormancy-inducing effects of BD, JLE and MeJA, but surprisingly did not affect endogenous ABA levels in treated seeds. However, ABA leached from litter of the species, which dominate the plant community before fires, plays an important role in germination control. We conclude that N. attenuata seeds, which can lie dormant in the soil for 150 years between fires, time their germination with the post-fire environment by responding to smoke, ABA and four terpenes (BD, 1,8-cineole, β-thujaplicin and camphor) leaching from the litter of the dominant vegetation.

Exposure to smoke is required for the germination of seeds from dormant genotypes of Nicotiana attenuata, a post-fire annual of the Great Basin Desert. Germination can be elicited by GA1,3,4,7 treatments and inhibited by the GA biosynthesis inhibitor, paclobutrazol (PAC), abscisic acid (ABA) and terpenes leached from unburned litter of the plant’s natural habitat. We analysed the endogenous GA and ABA dynamics during the 22 h after imbibition, when smoke-treated dormant seeds commit to germination. Extractable GA1+3 pools decreased in all seeds, but the decrease was more dramatic within 2 h of smoke exposure, which was followed by an increase between hours 2 and 4. Extractable ABA pools increased shortly after imbibition and remained stable in control, water-treated seeds, but decreased sharply in smoke-treated seeds. PAC completely inhibited smoke-induced germination when seeds were treated for 12 h after smoke exposure, consistent with the requirement of de novo GA synthesis for germination. Smoke treatment in the dark did not result in germination, whereas GA3 treatment did, a result consistent with phytochrome-mediated GA biosynthesis. Smoke exposure dramatically increased the sensitivity of seeds to exogenous GA3 treatments in both the light and dark, and light exposure increased this sensitivity an additional tenfold. Taken together, these results suggest that while germination requires endogenous GA synthesis, the effects of smoke treatment increase GA sensitivity, which is correlated with a decrease in endogenous ABA pools.