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| The seed-germination of the six phylogenetically-related Caragana species that thrive in arid to humid regions was investigated in a series of controlled temperature and water regimes. C. korshinskii (from an arid area), C. intermedia and C. microphylla (from semi-arid areas) had rapid seed germination, a high final seed germination percentage (GP), a large decrease in GP with an increase in temperature, but germinated at water/osmotic potentials as low as -1.2 MPa. However, C. arborescens and C. biosi (from semi-humid areas) had slow seed germination, a low final GP, which did not change with increase in temperature, and had no seed germination at water/osmotic potentials below -1.0 MPa. Seeds of C. stipatata (from a humid area) did not germinate until the seed coat was cracked after cold (4 C) had been imposed for 18 months. Using light microscopy, gaps were observed between the macrosclereids of the seed coat of species from the arid, semi-arid, and one species from the semi-humid areas (C. arborescens), but not the second species from semi-humid area (C. biosi). Gaps were not observed between the brachysclereids of the species from the humid area. The results showed that seed germination behaviour was not strongly conserved phylogenetically in the Caragana genus, but was strongly related to their seed coat structure and the environment in which they evolved. We conclude that the observed seed germination responses play an important role in determining the distribution of these species in the different environments of China. |
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| Keywords:Plant ecology; environmental gradients; environmental evolution; macrosclereids; seed coat structure; phylogenetic constraints |
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