Different formulations of efficiency are possible depending on which outputs and inputs are considered. For instance, average quantum efficiency is the ratio between gross assimilation and either absorbed or incident light intensity. Large variability of measured quantum efficiency is reported in the literature between plants grown in different conditions and classified in different subtypes but the underpinnings are still unclear. One of the components of quantum efficiency is the efficiency of dark reactions, biochemical efficiency, which is generally expressed in reciprocal terms as ATP cost of gross assimilation (ATP/GA).
In  photosynthesis ATP/GA depends mainly on  and O concentration at the carboxylating sites of RuBisCO. When  concentration is high and O concentration is low photorespiration is suppressed and  assimilation is fast and efficient, with ATP/GA approaching the theoretical minimum of 3.
In  photosynthesis  concentration at the RuBisCO carboxylating sites is mainly the result of the operation of the  concentrating mechanisms, which cost circa an additional 2 ATP/GA but makes efficiency relatively insensitive of external  concentration in a broad range of conditions.
Biochemical efficiency depends mainly on the speed of  delivery to the bundle sheath, and will generally decrease under low light when PEP carboxylation rate decreases, lowering the ratio of /O concentration at the carboxylating sites of RuBisCO. The key parameter defining how much efficiency will decrease under low light is bundle sheath conductance. Plants with higher bundle sheath conductance will be facilitated in the exchange of metabolites between the mesophyll and bundle sheath and will be capable of high rates of assimilation under high light. However, they will also have high rates of  retro-diffusion from the bundle sheath (called leakage) which will increase photorespiration and decrease biochemical efficiency under dim light. This represents an inherent and inevitable trade off in the operation of  photosynthesis.  plants have an outstanding capacity to attune bundle sheath conductance. Interestingly, bundle sheath conductance is downregulated in plants grown under low light and in plants grown under high light subsequently transferred to low light as it occurs in crop canopies where older leaves are shaded by new growth.