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.