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Softening of impermeable seeds of six Mediterranean annual legumes on the soil surface and buried beneath the soil surface

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Softening of the hard seeds of six species of Mediterranean annual legumes (five clovers and one medic) was measured over two years on the soil surface (Expt 1) and over six months buried at depths of 0, 2 and 6 cm in the soil (Expt 2). Reverse logistic equations were used to estimate three parameters: hardseededness after exposure to field conditions, proportion of seeds that softened during field exposure and time taken for half of the seeds to soften.

There were marked differences in the seed softening patterns of the six legumes. After two years on the soil surface (Expt 1), M. polymorpha L., had 49% of the original seed population still hard and was the hardest-seeded species. Of the clovers, T. spumosum L. (11%) and T. lappaceum L. (9%) were the hardest and T. subterraneum L. (0%) was the softest. More than 70% of the hard seeds of T. subterraneum L. softened in the first year compared with 37% of T. spumosum L. and M. polymorpha L.. Half of the seeds of T. subterraneum L. that softened in the first year did so within 51 days of exposure. In contrast, it took 163 days for T. lappaceum L. seeds to soften to the same stage, 150 days for T. clypeatum L. and 137 days for T. spumosum L..

Burial affected the softening of hard seeds of the six species differently. Five of the species softened least when buried at 6 cm, while the sixth (T. clypeatum L.) softened least on the soil surface. Softening of hard seeds of T. subterraneum L. was significantly less at 2 cm than it was at 0 cm. There was no difference in seeds of T. lappaceum L., T. glanduliferum L. and M. polymorpha L., while those T. spumosum L. and T. clypeatum L. softened more rapidly at 2 cm than they did at 0 cm. Time for half the seeds to soften was less affected by depth of burial than was the proportion of hard seeds softening. However, at 6 cm half of the seeds of T. subterraneum L. softened in 105 days after the experiment began compared with only 52 days on the soil surface. In contrast, the time for half of the seeds of T. spumosum L. to soften at 6 cm was 113 days compared with 132 days on the soil surface.

The results reveal that residual hardseededness and the seasonal pattern of softening vary between seasons and are affected by environment. Species that soften late in the autumn are most likely to resist germination from out-of-season rains, while seed banks of those that soften early, such as T. subterraneum L., are likely to be threatened in these conditions. The contrasting effects of seed burial indicate that the environmental requirements for seed softening are likely to vary between species. For example, T. spumosum L., which responds to shallow burial, behaves very like some accessions of Ornithopus compressus, in which softening has been shown to be inhibited by light. The softening of all but one species is slower when buried at 6 cm, which suggests that their requirement for heat may not be met when insulated from the extremes of temperature.
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Document Type: Research Article

Publication date: October 1, 2005

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