Survival strategy of Watsonia fourcadei (Iridaceae); allocation of resources to sexual reproduction and vegetative survival of the parent plant

Abstract

Geophytes are abundant in the Cape fynbos vegetation and many of the Cape geophytes, such as Watsonia species, flower mostly only in the first year after a fire. This study investigates the question if these plants have a resource allocation strategy which limits sexual reproduction to the post-fire environment and that they favor resource allocation to the storage organ for the survival of the parent plant at a cost to sexual reproduction. To test the hypothesis that the Cape geophyte Watsonia fourcadei represses sexual reproduction in favor of the survival of the parent plant, three levels of defoliation (20, 50 and 100%) were applied during a phenological stage when resources were least in the parent corms. Resource allocation to different plant parts was investigated when plants were in the seeding phase. Dry mass and nutrient (N, P. K, Ca, Mg and Ca) content were used as indicators of the resource allocation pattern. Dry mass and nutrient allocation patterns were similar, with significantly higher percentages of resources allocated to vegetative survival of the parent plant than to sexual reproductive organs (inflorescence and seeds) after complete defoliation, than after 20 and 50% defoliation. The W. fourcadei plants were able to accumulate adequate resources in the post-fire growth season to support sexual reproduction after up to 50% defoliation, albeit to a lesser extent than in control plants, but sexual reproduction was poorly supported after complete defoliation. Results indicate that W. fourcadei plants must be able to accumulate at least 3.5 g of dry mass in the new developing corm before resources are allocated to sexual reproductive plant parts. Replenishment of nutrient content after defoliation was not a limiting factor for resources allocation to sexual reproduction, except in the case of phosphate. W. fourcadei plants acted as a functional unit and resources allocation to sexual reproduction was controlled and depended on the availability of resources. None of the current resource allocation models are applicable to W. fourcadei and factors that could have selected for the apparent ‘K’ survival strategy in this and other Cape geophyte species are discussed.