Are flowers physiological sinks or faucets? Costs and correlates of water use by flowers of Polemonium viscosum

Water loss through inflorescences may place extreme demands on plant water status in arid environments. Here we examine how corolla size, a trait know n to influence pollination success, affects the water cost of flowering in the alpine skypilot, Polemonium viscosum. In a potometry experiment, water uptake rates of inflorescences were monitored during bud expansion and anth esis. Corolla volume of fully expanded flowers predicted water uptake durin g bud expansion (R-2 = 0.61, P = 0.0375) and corolla surface area predicted water uptake during anthesis (R-2 = 0.59, P = 0.044). To probe mechanisms underlying the relationship between corolla size and water uptake, cell dim ensions and densities were measured in several regions of fully expanded co rollas. Corolla length was positively correlated with cell length in the mi ddle of the corolla tube and cell diameter in the corolla lobe (Pearson's r from 0.26-0.33, n = 86, P < 0.05). Cell density was negatively correlated with cell dimensions in the upper corolla tube and lobe (Pearson's r from - 0.39 to -0.42, P less than or equal to 0.0015). These findings suggest that more water may be required to maintain turgor in large corollas in part be cause their tissues have lower cell wall densities. The carbon cost of wate r use by flowers was assessed in krummholz and tundra habitats for P. visco sum flowering, respectively, during dry and wet portions of the growing sea son. For plants in full flower, average leaf water potentials were signific antly more negative (P = 0.0079) at mid-day in the krummholz (June) than in the tundra (July), but were similar before dawn (P = 0.631). Photosyntheti c rate at the time of flowering declined significantly with increasing coro lla size in the krummholz (P = 0.0376), but was unrelated to corolla size o n the tundra (P > 0.72). Plants losing water through large corollas may clo se leaf stomata to maintain turgor. If photosynthesis limits growth in this perennial species, then the water cost of producing large flowers should e xacerbate the cost of reproduction under dry conditions. Such factors could select for flowers with smaller corollas in the krummholz, countering poll inator-mediated selection and helping maintain genetic variation in corolla size components of P. viscosum.