Growing or reproducing? Assessing the existence of a trade-off in the globose cactus Gymnocalycium monvillei

Resource allocation in plants is a fundamental aspect of life history theory. In Cactaceae, the specific trade-off between sexual reproduction and vegetative growth has still not been studied. The aim of this work was to assess if there is a trade-off between growth and reproduction, and to analyse whether both growth and allocation to reproduction depend on size of the individual. In this study, we used Gymnocalycium monvillei, a globose cactus endemic to the mountains of central Argentina, as a model species. Specifically, we analysed the relationship of growth (percentage increase in diameter) and size of individuals (diameter) to seed production, seed mass, germination, and mean germination time. To relativize the effect of size on seed production, two variables were calculated: the ratio of seed production to plant size (RSPS), and the ratio of total seed mass to plant size (RSMS). We found that both seed production and total seed mass were significantly related to cactus size. However, growth was not related to seed mass or to seed production, even when they were relativized. Germination and mean germination time were not related to plant size or growth. In the studied species, a slow-growing globose cactus, we did not find a trade-off between growth and reproduction.