Does being honest pay off? The relationship between attractiveness and floral resource mediated by legume nutrition

Abstract

The outcome of mutualisms can vary depending on the quantity and quality of services and resources exchanged between species. In pollination, pollinators are attracted by flower signals associated with resources such as nectar and pollen. When the signal and the resource are correlated, we consider the system honest to pollinators due to the phenotypic coupling of both. In species with poricidal flowers, greater floral honesty is expected, with larger flowers containing more pollen grains. Many of these flowers belong to the Fabaceae family, associated with nitrogen-fixing bacteria in the plant root system, reducing nitrogen limitation in the soil. The overlap of two mutualisms in the same plant can generate synergistic effects, increasing available organic nitrogen and favoring floral attractiveness and the quality of resources available to pollinators. In this study, we will investigate how the plant-bacteria interaction may affect pollination by determining differences in attractiveness traits and flower resources among Chamaecrista species in the campo rupestre phytophysiognomy. We formulated three questions: I- How does floral attractiveness relate to the quantity and quality of floral resources among species? More attractive plant species are expected to have a greater quantity and quality of floral resources. II- How are floral attractiveness and resources related to bee visitation among species? Plant species that honestly signal floral resources are expected to have a higher rate of pollinator visitation. III- How does the association of nitrogen-fixing bacteria with plants relate to floral attractiveness and the quantity and quality of resources among species? Plant species that have a greater interaction with rhizobia are expected to have higher floral attractiveness and a greater quantity and quality of resources. To answer these questions, we will evaluate floral traits, flower visitors, and root nodulation in the root system of 20 Chamaecrista species. Initially, we will characterize the hue, saturation, and contrast of flower colors through reflectance measurements of petals and anthers of three flowers and three leaves per plant in at least five plants per species. In the same flowers, we will estimate the flower size, including individual organ measurements and symmetry patterns in standardized photographs of fresh flowers. The dry mass of the flower and its verticils will also be estimated. In the field, we will estimate floral advertisement at least three times during the reproductive period of each plant. The opening time, flower lifespan, and closure of flowers will be estimated through observations between 5:00 and 17:00 on three flowers per plant, in five plants per species. Stigma receptivity will be assessed throughout the flower lifespan using hydrogen peroxide. The amount of pollen in the androecium will be estimated using a Neubauer chamber, and pollen quality will be estimated using the colorimetric method. In the field, we will sample bee visitation rates through focal observations. Pollination experiments will be conducted to assess plant dependence on bees and pollen limitation, comparing three treatments: (i) flowers exposed to pollinators without manipulation, (ii) bagged flowers that exclude visitors, and (iii) flowers supplemented with pollen. Finally, we will sample the root system of plants and their nodules. The nodule to root biomass ratio will serve as a proxy for the intensity of the interaction, and soil samples will be obtained for physical and chemical analysis of nutrients. The dataset will allow us to test the association between the mutualism with nitrogen-fixing bacteria in the roots with floral attractiveness, its resources, and bee visitation on flowers.