The maintenance of water balance in flowers: Physiological mechanisms and ecological implications
For over 200 years, pollinators have been thought to be the predominant forces shaping floral form and function. Indeed, in many cases pollinators are responsible for large and rapid changes in floral morphology. However, flowers are also subject to abiotic constraints associated with their resource and maintenance costs. Flowers often reside in the same microhabitats and face similar biophysical demands as leaves. While leaves have been optimized for water transport, flowers, which must also remain turgid throughout anthesis, have additional constraints associated with their reproductive functions, yet we know remarkably little about floral hydraulic constraints. Using a novel flower trait dataset, we asked whether the biophysical constraints of maintaining floral water balance have influenced floral ecology and evolution. We surveyed ~150 species from across the angiosperm phylogeny for traits associated with flower water balance and, for a subset of species, the maximum hydraulic conductance of flowers. We used these two datasets to quantify the hydraulic structure-function relationships for flowers, to determine whether traits associated with flower water balance have undergone coordinated evolution, whether floral hydraulic traits have been under selection, and whether flowers display a diversity of hydraulic strategies.
Across a wide range of habitats in the California Floristic Province, certain traits associated with floral water balance were significantly correlated with climate variables. Flowers from hotter, drier habitats had traits associated with limiting water loss from flowers, which slows the rate of desiccation. Because larger flowers had slower rates of desiccation per unit area, there were no significant relationships between flower size and climate. These results strongly suggest that floral hydraulic traits have been under selection and may be an important set of traits involved in habitat filtering. Vegetative traits are important in plant establishment, but hydraulic traits of flowers may influence reproductive success and whether populations can persist. Additionally, hydraulic strategies of flowers seem to be defined by a tradeoff between maintaining a high hydraulic conductance and high hydraulic capacitance. A shift toward slower desiccation rates (driven lower epidermal conductances to water vapor) and greater reliance on hydraulic capacitance among the monocots and eudicots seems to be associated with novel structural traits that limit water loss and increase water storage. These results suggest that the development of complex floral structures involved significant shifts in physiological traits that are important over ecological and evolutionary timescales.