Root-associated fungi increase male fitness, while high simulated herbivory decreases indirect defenses in Croton lachnostachyus plants

Abstract

Plants interact with a diverse array of organisms below and above ground; some interactions with ants allow plants to be protected against herbivorous insects, influencing their growth or reproduction. In addition, indirect plant defenses such as those mediated by extrafloral nectaries (EFNs) could be affected by plant root symbionts. However, it is not clear how the suppression of root symbionts might affect extrafloral nectar (EFN) production and plant reproductive output. We made an experimental study with a shrub species with EFNs. Firstly, we tested if root-associated fungi (i.e., comparing plants with and without fungicide) increased the production of pollen (male function) and EFN (volume, nectar concentration, and total sugar content) in Croton lachnostachyus. Subsequently, we implemented a second experiment on the same plants, adding different levels of simulated herbivory (none, low, high) to assess the combined effects of root-associated fungi and herbivory. While we found high levels of mycorrhizal colonization, we found no signs of pathogenic fungi and negligible values of dark septate endophytes in roots so we attributed our results mostly to arbuscular mycorrhizal fungi (AMF). The first experiment showed that plants without the fungicide treatment increased pollen production and secreted a lower mean volume of EFN with higher concentration of dissolved soluble solids. In the second experiment, high levels of simulated herbivory showed a diminution on EFN variables; also, we detected a lower shoot dry mass on plants with low levels of herbivory and no interactions with AMF. Our results suggest complex ecological responses related to the root-associated fungal community and simulated herbivory.