Mechanisms Of Smoke-Elicited Seed Germination Responses and Possible Candidate Active Compounds In Smoke

Abstract

Smoke derived from burning plants has been well recognized as a seed germination promoter in a variety of plant species in many ecosystems. Previous studies revealed that smoke from legumes resulted in different germination responses compared to that from graminoids. However, the mechanisms for such differences remain unclear. This research was conducted to understand how different smoke types interact with germination conditions to improve germination, to identify bioassay species among fire-ephemerals that respond positively to alfalfa smoke, and to identify germination stimulants in alfalfa smoke. Salad Bowl lettuce (Lactuca sativa) seeds treated with serial dilutions of smoke solutions from eight plant species and incubated at 25/25 °C or 25/15 °C in 12 h light/12 h darkness or 24 h darkness for three days. Seeds of six fire-ephemeral species from Africa and wild mustard (Sinapis arvensis L.) were primed in serial dilutions of alfalfa or/and wheat smoke solutions or distilled water for 24 h at 20 °C in darkness for one week and subsequently incubated at species-specific germination conditions. A lettuce seed bioassay, conducted at 25/15 °C in darkness, was used to identify active fractions and compounds derived from alfalfa smoke solution. Fractions were prepared by reversed phase separations (resin and C18 RP-CC), and compounds present were determined by HPLC, GC-MS and NMR and comparison of spectra with library spectra of known compounds. Lettuce seed germination percentage was improved by diluted smoke solutions from all plants at 25/15 °C in darkness, but by only two graminoid-based smoke types (wheat and E. curvula) at a constant 25 °C. Similarly, the germination rate of lettuce seeds treated with alfalfa or wheat smoke solution was enhanced mainly at 25/15 °C under darkness. Therefore, the efficiency of compounds in smoke solutions in replacing the effect of light appears to be greater at 25/15 °C than at 25 °C. Resin fractions, EtOAc separated neutral fraction, and C18 fractions of alfalfa smoke solution enhanced germination in a lettuce bioassay, but KAR1 was not present as a major germination stimulant in alfalfa smoke based on fractions. Phenolic compounds such as hydroquinone and catechol were more evident as possible active germination promoters in alfalfa smoke, to which six fire-ephemerals responded positively. Germination of wild mustard seeds increased with increasing smoke solution concentration. A bioassay based on mustard seed germination has the potential to guide the identification of active compounds isolated by fractionation from complex mixtures. Non-KAR1 active compounds in alfalfa smoke may have broad applicability in weed control in agroecosystems.