Seasonal Changes in Water Relations, Photosynthesis and Leaf Anatomy of Two Species Growing along a Natural Co2 Gradient

In order to gain knowledge on the physiological and anatomical responses of plants to long-term growth under elevated CO2 concentrations, measurements of water status, leaf gas exchange and leaf anatomical characteristics were made during the rainy and the dry season in plants of Brownea coccinea and Spatiphylum cannifolium growing along a natural CO2 gradient, from 35000μmol·mol (supra-atmospheric CO2 concentration, SC) to 435μmol·mol (ambient CO2, AC) in the riparian forest of the Santa Ana river, Sucre State, Venezuela. Water potential (ψ) was higher during the rainy than the dry season in both species. Growth under SC did not cause changes in ψ of B. coccinea, whereas it decreased it in S. cannifolium; both drought and growth under SC decreased leaf sap osmotic potential (ψπ) in both species, helping maintain turgor pressure in B. coccinea but not in S. cannifolium. Elevated CO2 increased photosynthetic rate (A) and leaf conductance (gs) of B. coccinea under drought but no effect was found in S. cannifolium. Leaves of both species growing under SC were 10% thicker than under AC, owing to a proportional increase in the thickness of the mesophyll tissues; additionally, during the dry season a higher deposition of epicuticular waxes took place on leaves of S. cannifolium growing under SC. Changes in photosynthetic rate, leaf conductance and leaf anatomy suggest acclimation to growth under elevated CO2. SEASONAL CHANGES IN WATER RELATIONS, PHOTOSYNTHESIS