Common approach for determining respiration in the day (R_d), the compensation point within the cell in the absence of respiration (Γ_*), and the internal leaf CO₂ concentration at which photosynthesis is balanced by photorespiration (c_i*). The method of Laisk (1977) as modified by von Caemmerer et al. (1994) uses the intersection of the linear portion (low CO₂) of multiple photosynthetic CO₂ response curves, each measured at a different light intensity (low light is favoured for at least one, here Populus deltoides was measured at 500, 250, and 100 µmol m⁻² s⁻¹). The usual interpretation is that the intersection of the lines occurs at c_i* and R_d, and Γ_* is at a higher CO₂ concentration due to the effects of mesophyll conductance and respiration. This approach has several assumptions that are not always valid and may cause significant errors (von Caemmerer, 2013).