FIRE-FOLLOWERS IN CHAPARRAL - NITROGENOUS COMPOUNDS TRIGGER SEED-GERMINATION

1 Application of nitrates (optimal concentration c. 10 mM) promoted se ed germination in the fire annuals Emmenanthe penduliflora and Phaceli a grandiflora and to a lesser extent in the fire-adapted shrub Salvia mellifera. Ammonium ions, although virtually inactive in E. penduliflo ra, were as effective as nitrate in both P. grandiflora and S. mellife ra. 2 Diurnal illuminations and a chilling pretreatment, though of lit tle effect by themselves, significantly enhanced the nitrate-mediated promotion of germination in the two fire annuals. 3 Nitrate was the pr incipal factor inducing germination in E. penduliflora (nitrate or amm onium in P. grandiflora) whereas in S. mellifera a light was the princ ipal agent. It was also shown in E. penduliflora that nitrates are not required during chilling, i.e. seed sensitisation by chilling and nit rate-mediated induction of germination are entirely different mechanis ms. 4 In the three species tested, the effect on seed germination caus ed by nitrogenous substances was nearly identical to that produced by an extract of charred wood, although chemical analysis of the extract showed that the combined concentrations of nitrates, ammonium ions and free amino acids could not account for the promotive action of charat e. However, the nitrate and ammonium concentrations required to induce germination are very close to the increased values encountered after a fire in the otherwise nitrogen-poor chaparral soil. Therefore, in ad dition to the possible effect of charred wood, the postfire germinatio n flush observed in chaparral may be induced by the increased levels o f available nitrogen as well.