Elevated CO2 induces down-regulation of photosynthesis and alleviates the effect of water deficit in Ceiba pentandra (Malvaceae)

Abstract

The simultaneous effect of elevated CO₂ concentration and drought on trees is still under investigation in the Amazon. We evaluated the effect of CO₂ levels (400 and 800 ppm) and water regimes (50% and 100% soil field capacity) on photosynthetic traits, chlorophyll fluorescence, and total biomass accumulation in Ceiba pentandra. In well-watered plants, light-saturated photosynthesis (P_N-sat) increased in plants exposed to elevated CO₂, but both P_N-sat and stomatal conductance decreased in response to water deficit. The maximum carboxylation rate of Rubisco declined under elevated CO₂, which indicates down-regulation of photosynthesis at elevated CO₂. The F_v/F_m ratio was not affected by treatments. Notwithstanding, total plant biomass and leaf area were reduced by 34-37% under water deficit, but they were not affected by CO₂ levels. The P_N-sat values measured in well-irrigated plants at ambient CO₂ were similar to those observed in plants subjected to elevated CO₂ and water deficit (p = 0.26). We concluded that the effect of water deficit on P_N-sat was mitigated by elevated CO₂. These results suggest that the increase of atmospheric CO₂ concentrations associated to climate changes can at least partly offset the negative effect of drought in this multiuse and widely distributed species.

Keywords: Biomass accumulation; Water use efficiency; Photosynthesis acclimation.