Fire-Followers in Chaparral: Nitrogenous Compounds Trigger Seed Germination

1 Application of nitrates (optimal concentration c. 10mM) promoted seed germination in the fire annuals Emmenanthe penduliflora and Phacelia grandiflora and to a lesser extent in the fire-adapted shrub Salvia mellifera. Ammonium ions, although virtually inactive in E. penduliflora, were as effective as nitrate in both P. grandiflora and S. mellifera. 2 Diurnal illuminations and a chilling pretreatment, though of little effect by themselves, significantly enhanced the nitrate-mediated promotion of germination in the two fire annuals. 3 Nitrate was the principal factor inducing germination in E. penduliflora (nitrate or ammonium in P. grandiflora) whereas in S. mellifera light was the principal agent. It was also shown in E. penduliflora that nitrates are not required during chilling, i.e. seed sensitisation by chilling and nitrate-mediated induction of germination are entirely different mechanisms. 4 In the three species tested, the effect on seed germination caused 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 charate. 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 addition to the possible effect of charred wood, the postfire germination flush observed in chaparral may be induced by the increased levels of available nitrogen as well.