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.