1 Smoke-stimulated germination in the post-fire flora of California chaparral does not appear to be triggered by nitrate. Application of freshly prepared unbuffered KNO³ solutions (pH c. 6.2) failed to enhance germination of five populations of Emmenanthe penduliflora or one Phacelia grandiflora population, regardless of light or stratification conditions. 2 KNO³ buffered at acidic pH (or unbuffered solutions equilibrated with atmospheric CO²) did induce germination, but KNO³ solutions at pH 7 failed to induce germination. Induction of germination is therefore not due to the nitrate ion per se. but rather to high [H+], although buffered controls gave weak germination at low pH, suggesting a role for H+ plus nitrate. However, other anions such as sulphate were equally as effective as nitrate at breaking dormancy. 3 The germination response to KNO³ was affected by the type of filter paper used and this may be linked to differences in pH. 4 NO², at concentrations present in biomass smoke, was highly effective at inducing germination. and other oxidizing agents also induced germination. 5 Several growth regulators, including nitrite and gibberellin, were stimulatory only at acidic pH, but KCN was stimulatory across a broad pH range. 6 Germination decreased at smoke exposures longer than a few minutes. Also, smoked water samples effective at breaking dormancy were acidic and were less effective when buffered to pH > 7. 7 Physical scarification of the seed coat induced germination but the effect was not due to penetration of a water barrier, or to enhanced oxygen uptake or to wound responses such as C02 or ethylene production. 8 Different effects of the gibberellin inhibitor CCC (chlorocholine chloride) suggested that the mechanisms of scarification-induced and smoke-induced germination may differ. 9 We conclude that either oxidizing gases in smoke and/or acids generated on burnt sites play a role in germination of post-fire annuals in chaparral. © Blackwell Science Ltd. Abstract reproduced by permission.