Do larger plants produce more and better seeds and seedlings? Testing the hypothesis in a globose cactus, Wigginsia sessiliflora.

In iteroparous plants, an increase in allocation to reproduction is expected with increasing plant size. The aim of this study was to analyze how plant diameter is related to total produced seed mass, seed number, mean seed mass, percentage and mean germination time (MGT), and seedling size and shape in Wigginsia sessiliflora (Cactaceae), a slow-growing globose species from central Argentina. Plant diameter was measured in a population of n185 individuals, and fruits were collected. We counted all seeds to obtain seed number, and weighed them to get seed mass and total seed mass. Seeds were germinated and data of germination percentage and MGT were collected. We also measured the size (height and width) and shape of seedlings (height/width relationship). Fifty four percent of the plants did not produce fruits. Plant diameter was unimodally related to fruit number, total seed mass, and seed number, i.e. intermediate-sized plants presented the highest values. Height and shape of seedlings were positively related to plant diameter. Surprisingly, plant diameter was related in a unimodal way to reproductive outputs. As plants grow, surface-volume ratio decreases, meaning that respiratory losses would be higher than the increase in photosynthetic capacity; therefore, fewer resources may be available for reproduction.