AN EXPLANATION FOR SIZE-DEPENDENT REPRODUCTIVE ALLOCATION IN PLANTAGO-MAJOR

This study examined whether variation in reproductive allocation with size could be explained by differences in the trade-off between reprod uction and growth. Seeds from 32 half-sibling families were collected from sites differing in mowing frequency and availability of light, nu trients, and water. Six seedlings from each family were grown in contr olled environments and photoperiod manipulations were used to control reproduction. Mass of vegetative and reproductive plants of the same f amily were compared to assess the trade-off between reproduction and g rowth. Families collected from habitats with a tall canopy were larger and experienced a greater decrease in growth with reproduction than f amilies from habitats where mowing maintained a short canopy. Reproduc tive output showed no relationship with size, while reproductive alloc ation (capsule mass to vegetative mass) decreased with size. This decr ease may be a direct consequence of the increase in reproductive cost with size. The increase in cost can be partially explained by increase d allocation to reproductive support structures; large individuals pro duce capsules on more elongate scapes and have a lower capsule to scap e ratio. Differences in size and morphology among habitats are presuma bly the result of selection to avoid damage in mown sites and to avoid shade and ensure pollination in sites with a tall canopy and reduced wind movement.