Seed mass variation in the perennial herb Asphodelus albus: sources of variation and position effect.

Patterns of seed mass variation in the perennial herb Asphodelus albus (Liliaceae) were studied in one population over 3 years (1988-1990) and in three populations during 1989. Plant size, phenology and several components of plant fecundity showed no effect on mean seed mass per plant. Mean seed mass varied among populations and among plants within populations. Significant variation was also found among years and among plants within year, but most of the variation was accounted for by the within-plant component. Withinfruit variation may be as important as between fruits within plant. Fruit position within the plant influenced seed mass, being heavier the seeds at the bottom of the stalk. However, the plants markedly differed in the proportion of the variation accounted for by the position effect. The correlation between seed number per fruit and seed mass showed an interaction with fruit position. Seeds from small broods were heavier than those from large ones, but only in the lower part of the stalk. Decline in seed mass towards the top of the stalk may be attributed to seasonal reduction in resource availability. In addition, the change in the relationship between seed number and mass might be due to changes in the "resolution" of the parent-offspring conflict, also related to nutrient availability.