Diel Shifts in Carboxylation Pathway and Metabolite Dynamics in the CAM Bromeliad Aechmea 'Maya' in Response to Elevated CO[sub]2

Background and Aims The deployment of temporally separated carboxylation pathways for net CO2 uptake in CAM plants provides plasticity and thus uncertainty on how species with this photosynthetic pathway will respond to life in a higher-CO2 world. The present study examined how long-term exposure to elevated CO2 influences the relative contributions that C3 and C4 carboxylation make to net carbon gain and to establish how this impacts on the availability of carbohydrates for export and growth and on water use efficiency over the day/night cycle. Methods Integrated measurements of leaf gas exchange and diel metabolite dynamics (e.g. malate, soluble sugars, starch) were made in leaves of the CAM bromeliad Aechmea 'Maya' after exposure to 700 µmol mol-1 CO2 for 5 months. Key Results There was a 60 % increase in 24-h carbon gain under elevated CO2 due to a stimulation of daytime C3 and C4 carboxylation in phases II and IV where water use efficiency was comparable with that measured at night. The extra CO2 taken up under elevated CO2 was largely accumulated as hexose sugars during phase IV and net daytime export of carbohydrate was abolished. Under elevated CO2 there was no stimulation of dark carboxylation and nocturnal export and respiration appeared to be the stronger sinks for carbohydrate. Conclusions Despite the increased size of the soluble sugar storage pool under elevated CO2 , there was no change in the net allocation of carbohydrates between provision of substrates for CAM and export/respiration in A. 'Maya'. The data imply the existence of discrete pools of carbohydrate that provide substrate for CAM or sugars for export/respiration. The 2-fold increase in water-use efficiency could be a major physiological advantage to growth under elevated CO2 in this CAM bromeliad.