Water-use strategies in flowers from a Neotropical savanna under contrasting environmental conditions during flowering

Flower production and maintenance require high amounts of water, which ultimately may compromise pollinator attractiveness when water is limited. Water-use and -conservation strategies in leaves from hot and dry ecosystems are well documented globally, yet little is known about mechanisms of water allocation in flowers, particularly in tropical savanna ecosystems. We evaluated physiological and structural traits related to corolla water status in two Kielmeyera (Calophyllaceae) species that differ in flowering phenology and flower size: larger-flowered K. regalis blooms during the rainy summer and smaller-flowered K. coriacea blooms during the dry winter. To test the hypothesis that water demand in corollas increases with increasing vapor pressure deficit (VPD), we analyzed interspecific differences in corolla stomatal conductance and density, water content, and fresh and dry mass per unit area. We also performed hand-pollination and pollinator-exclusion experiments to determine variation in floral longevity. Corolla transpiration rates were two-fold higher during the dry season (157 vs 95 g·H2O·m-2·h-1 for K. coriacea and K. regalis, respectively), and significantly increased with VPD in both species. Stomatal density was 25-fold higher in K. coriacea, and corolla fresh and dry mass per unit of area were 47% and 21% higher, respectively, in K. coriacea, mainly due to thick pectin-rich cell walls. The high pectin content increases water content in corollas of K. coriacea. Regardless of pollination treatment, flowers lasted one day in K. coriacea and three in K. regalis. Our study suggests structure-function relationships of floral traits with flowering season, and that K. coriacea displays small and short-lived corollas with high water content to buffer the high evaporative water demand during the dry period in a seasonal savanna.