Seed production in cluster clover (Trifolium glomeratum L.). 1. Flowering time, abortion, seed size, and hardseededness along branches

Seed production in cluster clover (Trifolium glomeratum L.) was investigated at the level of the individual inflorescence. Plants were grown in nursery rows under non-limiting conditions until 150 days after sowing, at which time irrigation was stopped to truncate the growing season. The number of florets, and the number, mass, and impermeability of seeds, were determined for each inflorescence along the main branches, and were related to the date and node of flowering. The interval between subsequent flowers increased (3·4 days at node 2 v. 7·3 days at node 7) and the rate of floret abortion increased (14% at node 1 v. 49% at node 7) with successive inflorescences along the branch, even while moisture conditions remained favourable. These effects are attributed to resource limitation during the latter stages of plant growth. On the other hand, seeds per floret and seed mass were unaffected by the node of flowering but declined rapidly once irrigation ceased. Hardseededness was not affected by either node of flowering or date of flowering, before or after the cessation of irrigation. Mean initial hardseededness from inflorescences produced after irrigation ceased was slightly higher than that from earlier formed inflorescences (99·6% v. 94·1%, P < 0·001). The results highlight the importance of reproductive timing in an environment with a limited growing season, since seed production fell rapidly once water conditions became unfavourable. They suggest that seed production in cluster clover is near to its potential while conditions remain favourable, although resource limitation may reduce the rate of flowering and increase floret abortion as the plant matures. Although unfavourable water conditions may sharply reduce production of viable seeds, in cluster clover the initial impermeability of those seeds is not affected, possibly a benefit of rapid seed formation.