Germination and seed coat histology of physically dormant Desmanthus illinoensis seeds

Illinois bundleflower (Desmanthus illinoensis) is a legume used in meadow and prairie restoration seed mixes in the United States of America. An important constituent of grasslands, there is little information on the natural mechanisms of seed germination, the structure of the seed coat or the locations of impermeability within the seed coat. Germination and histological studies were conducted to exoamine the effect of various environmental factors on dormancy of Illinois bundleflower and to elucidate structures of the 'hard' seed coat. Following mechanical scarification and incubation between blotters moistened with distilled water, final germination percentage (FGP) was 86-100% at three temperature regimes (11/23, 20/30 or 32 degrees C); however, FGP declined and mean days to 50% germination (D-(lambda) over bar, an inverse measure of germination rate) increased at 11/23 degrees C and -0.4 MPa. Brief exposure of the funiculus and nearby lens to fire resulted in permeable seeds and increased FGP to 74% and decreased D-(x) over bar to 2.1 days compared with control seeds (FGP = 9%; D-(x) over bar = 5.8 days). Impermeability was caused by 'caps' on outer palisade tangential walls, by inner palisade adjacent hour-glass cells within natural U-shaped fissures called the pleurogram, and by palisade radial/transverse walls within epidermal fractures. Physical dormancy was broken following brief exposure of the lens to fire that resulted in lens palisade lifting and palisade obliterations, which rendered seeds permeable. Dye tracking using 1% Azure II stain indicated that the lifted lens was the primary site of imbibition. We conclude that high temperatures resulting from fire is probably a significant environmental cue of physical dormancy break for seeds of Illinois bundleflower.