LIMITING FACTORS FOR SEED PRODUCTION AND PHENOTYPIC GENDER IN THE GYNODIOECIOUS SPECIES ECHIUM VULGARE (BORAGINACEAE)

In bermaphrodite individuals of Echium vulgare, seed production was resource rather than pollen limited in a sand dune area. Addition of pollen had no effect on seed production per plant whereas seed production per plant was increased by watering. The number of seeds per flower was not affected by adding pollen or watering the plants. The number of seeds per flower increased only marginally significantly with plant mass. Pollen number per flower was unrelated to plant mass. In hermaphrodite individuals standardized phenotypic femaleness ranged from 0.21 to 0.51 and all hermaphrodite individuals produced seeds indicating that there were no individuals acting as pure males. In the studied population 26% of the individuals were male-sterile. Male-sterility was also not related to plant mass. Plant mass explained therefore only a very small fraction of the within-population variation in phenotypic gender. In hermaphrodite individuals, number of flowers, number of seeds, and total seed mass all increased more than proportionally with plant mass. Male-steriles produced ca 42% more seeds per gram plant mass than bermaphrodites. Both hermaphrodite (0.91 seeds per flower) and male-sterile(1.12 seeds per flower) individuals produced far less than the maximum number of four seeds per flower. The low seed to flower ratio in male-sterile individuals suggests that selective embryo abortion may play an important role in the regulation of seed production.