Smoke-induced seed germination in California chaparral

Ecology, Oct, 1998 by Jon E. Keeley, C.J. Foteringham

Gases in smoke were tested individually by exposing dry seeds (and filter paper) to commercially prepared gases in a glass chamber over a time course of from 0.5 to 1440 min. Concentrations were: carbon dioxide (7.7 x [10.sup.3] or 1.5 x [10.sup.6] mg/[m.sup.3]), carbon monoxide (9.7 x [10.sup.3] mg/[m.sup.3]), nitrous oxide (100 mg/[m.sup.3]), nitrogen dioxide (790 or 7.7 X [10.sup.3] mg/[m.sup.3]), ethylene (98 mg/[m.sup.3]), and methane (55 mg/[m.sup.3]), with the balance gas as [N.sub.2]. Since the precise concentration and combination varies with fuel type, moisture, and combustion conditions, predicting their levels requires a chemically complex model (Ohlemiller et al. 1987, Lobert and Warnatz 1993); however, the levels used here were within the published ranges for biomass smoke.

To evaluate the effect of storage conditions on subsequent germination, a selection of species were placed in nylon bags and buried in soil outdoors in the autumn. After one year they were excavated, air-dried, given smoke treatments, and compared with seeds that had been stored over the same period in jars in the lab.

Seed coat characteristics

Physical scarification was performed by cutting with a scalpel, or puncturing with a pointed probe, through the coat until the underlying endosperm was exposed. Seeds were immediately wetted and incubation begun.

Uptake of dyes was determined by soaking either untreated or smoked seeds in eosin (1.6 mol/[m.sup.3], molecular mass = 624). Every day for a week, seeds were removed and blotted, and hand-cut thin sections were examined under 25 x magnification. Seeds also were treated with lucifer yellow carbo-hydrazide (1.6 mol/[m.sup.3], molecular mass = 522), an apoplastic fluorescent tracer (Owens et al. 1991) (where "apoplastic" refers to that portion of the plant outside cellular protoplasts) and examined under a fluorescent scope at 25 x. Using this lucifer yellow method, sites of dye impermeability within seeds are distinct and readily distinguished by their strong fluorescent emission. Since phenols and other cellular components may fluoresce, seeds soaked in water were compared for presence of autofluorescence.

Imbibition curves were determined by weighing dry seeds and re-weighing tissue-blotted seeds that had been emersed in water for periods of from 1 h to 1 wk.

Light micrographs of 1.5-[[micro]meter] transverse sections were sliced from seeds that had been fixed in paraformaldehyde-glutaraldehyde (pH 7.3), postfixed in 1% osmium tetroxide, stained in uraryl acetate, dehydrated in an alcohol series, and embedded in Spurr's resin.

RESULTS

Smoke and heat shock

Smoke induced a highly significant (P [less than] 0.001) increase in germination for 22 (Table 1) of the 34 species tested. The effect of heat shock, charred wood, and smoke on seeds resulted in several patterns that hold for all 22 smoke-stimulated species: (1) heat shock had no stimulatory effect, (2) charred wood also induced germination, and (3) smoke-stimulated germination was inhibited when coupled with either 105 [degrees] C or 115 [degrees] C treatment. In addition, some species stimulated by 5-min smoke exposure were inhibited by 8-min exposure; this was a lethal effect, as evidenced by the fact that, unlike controls, ungerminated seeds rotted.