After acclimation to elevated CO2, plants no longer appear to be TPU-limited
After the 30-h acclimation period, plants no longer showed the responses to elevated CO2 that indicate TPU limitation. The reduced or inverse response of A to CO2 was gone (Fig 2a). The expected CO2-dependent decline ofφII was absent after acclimation (Fig 2b). Elevated nonphotochemical quenching (NPQt) at high CO2, one of the effects that causes the decline inφII , was gone after acclimation (Fig 2c). TPU limitation is expected to decrease proton conductivity across the thylakoid membrane, causing an increase in PMF (measured as total electrochromic shift, ECSt ). These effects were not completely missing but they were decreased (Fig 2d,e). Based on the absence or decline of these physiological effects, we argue that the plants no longer experienced TPU limitation after acclimation, though not as a result of increased TPU capacity.