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Seed ecophysiology and plant population ecology of Cynoglossum officinale L. and Tragopogon spp. Qi, Meiqin

Abstract

In order to understand persistence strategies of Cynoglossum officinale L., Tragopogon pratensis L., and T. dubius Scop. in the rangelands of British Columbia, ecophysiological characteristics of seed germination in C. officinale and Tragopogon spp. and dynamics of seed and plant populations in T.pratensis were investigated. A deep innate (primary) dormancy was found in C. officinale seeds, and the seed coat of this species played an important role in the regulation of seed germination by controlling 02availability to the embryo. Removal of the seed coat stimulated02 uptake which was due to both an increase in seed respiration and phenolic oxidation; seeds of C. officinale contained high levels of phenolic substances and seed extracts showed high polyphenol oxidase activity. The seed coat also prevented leaching of phenolic substances, which however, were not necessary for seed germination. Rosmarinic acid, the mostprominent phenolic substance present in the seeds of C.officinale, did not inhibit germination of decoated seeds at concentrations up to 3 mM. Analysis of the pattern of methanol-extractable phenolic substances showed no significant quantitative or qualitative correlation between changes inspecific phenolic compounds and seed germination induced by stratification or seed coat removal. These results suggest that phenolic substances in C. officinale seeds do not inhibit germination of this weed, but seed coat regulates germination by controlling 02 availability to the embryo. Ecophysiological characteristics of T. pratensis and T.dubius seed germination were studied under controlled conditions. Existence of secondary dormancy in these species was demonstrated by inducing dormancy by anaerobiosis treatment. This dormancy may play an important role in the persistence of Tragopogon spp. Secondary dormancy could be released by stratification and after-ripening in T. pratensis, but not by light. Seeds germinated over a range of temperatures with 15 C as the optimumin both weeds. A seed burial study showed that increased seeding depth had no effect on seed germination, but reduced seedling emergence. These results suggest that seeds in Tragopogon spp.can germinate in darkness and the reservoir of non-dormant seeds will become completely depleted under conditions conducive to germination regardless of burial depth. Patterns of plant mortality and reproduction in populations of T. pratensis and dynamics of its seed banks were investigated. Seedlings were marked in 1990 and 1991 and their fates monitored. High density-independent seedling mortality was observed in both cohorts. None of the plants flowered in the first or second growing season in the 1990 cohort. These results indicate that T.pratensis is not a biennial species. Size of age-specific plants varied significantly suggesting variations in growth rate among individuals. Flowering and seed production in non-age-specific populations were correlated with the root crown diameter (RCD).The variation of minimum RCD for plants that flowered in two years (0.2 and 0.6 cm) suggested that T. pratensis does not have a critical size requirement for flowering. The dynamics of seed banks were studied in space and time. Seed burial depth had a significant effect on seed dormancy after 2 to 3 months of burial. The seed populations were almost completely depleted after 9 to 10 months of burial. Since T. pratensis only has a transient seed bank, high seed production and long vegetative survival may play important roles in the persistence of this weed.

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