Environmental effects on seed development and hardseededness in Stylosanthes hamata cv. verano. I. Temperature

The level of hardseededness in Stylosanthes hamata cv. Verano grown in the open at St Lucia was positively and linearly related to temperature during seed formation; this relationship was stronger in 1976 than in 1977 when warmer conditions occurred. In two other experiments Verano plants were grown under the same conditions until early flowering and then exposed to differing temperature regimens. Seed production was maximal at 31/24°C (day/night), and cool temperatures (20/16°C or 18°C constant) had negative effects on inflorescence differentiation, rate of floret blooming and seed setting. Hardseededness developed most strongly at the high temperatures (35/28, 31/24°C, or 27 and 24°C constant), and this was also allied with lower seed moisture contents at harvest and in varying storage conditions. These findings may explain the variation in hardseededness of seed lots of differing provenance, which also may bear on the climatic adaptation of this short-lived plant. A range of mechanisms promote continued seed dormancy and its progressive release. Verano exhibited a transitory and weakly developed embryo dormancy at maturity, strong pod and testa inhibition of germination which decreased after 120 days of storage, and hardseededness continuing after 180 days of storage; the last was greater in the upper than in the lower articulation or seed pod