Effect of boiling water, seed coat structure and provenance on the germination of Acacia melanoxylon seeds

Acacia melanoxylon (Mimosoideae or Mimosaceae) is a high quality timber tree with an extensive natural distribution in Australia and a wide genetic and phenotypic diversity. Seeds from three widely differing provenances in Tasmania were tested to determine whether they had different responses to various dormancy-breaking treatments. All provenances had limited germination (< 11%) if seeds were untreated and between 85% and 91% germination after 40 days if the seeds were nicked. For all provenances short (<= 60 s) exposure to boiling water gave high germination percentages. These values were generally lower, although usually not significantly so, than the germination percentages following nicking. Germination percentages decreased with increasing time of exposure to boiling water, although one provenance had a significantly greater tolerance to one of the longer (20 min) treatments. Nicked seeds germinated quickly and uniformly, whereas those subjected to the boiling-water treatments germinated after a longer period and more gradually. In untreated seeds, the lens was a low, elliptically shaped dome (similar to 110-135 mu m wide, 140-190 mu m long). In more than 99% of the seeds examined, the structure of the lens was markedly altered after a 10-s exposure to boiling water. A wide diversity of altered lens structure was found, from a circular hole between the macrosclereids, to a short fissure where the macrosclereids did not separate to their bases. Nicked seeds had a 200-375 times greater area for water uptake than a fully disrupted lens and this was probably the principal reason why the nicked seeds germinated sooner and more rapidly.