Variation in photosynthetic efficiency among maize cultivars and its implications for breeding strategy.

Maize is one of the most important crops worldwide. Exploring the factors affecting photosynthetic efficiency alongside with the genetic variability within cultivars is fundamental to breed for high-yield maize. Despite its importance, there is limited knowledge about the variation in steady-state and dynamic photosynthetic efficiency among maize cultivars. Here, we investigated the anatomical and physiological mechanisms that contribute to the variability in photosynthetic CO2 assimilation under both, steady-state and fluctuating light conditions, respectively, in 12 high-yielding, farm-preferred maize cultivars. Under steady-state conditions, the saturating CO2 assimilation rate (Asat) varied from 50.2 to 63.1 µmol CO2 m-2 s-1. This variation was tightly related to the area ratio of mesophyll cells to bundle sheath cells, pointing out the role of leaf anatomy in determining Asat of maize. During the initial 5 minutes of light induction, we recorded a considerable range of cumulative CO2 fixation from 9.02 to 13.1 mmol m-2, with the loss of potential CO2 uptake (Closs) varying from 23% to 47% among the cultivars. This variation in Closs was primarily attributed to the kinetics of gs, which were significantly correlated to the expression of the ethylene synthesis genes ACS1 and GAD1, rather than stomatal density. Taking together, we discovered a greater degree of variation in dynamic photosynthetic efficiency compared to steady-state efficiency among the studied maize cultivars. This finding highlights the potential of manipulating gs kinetics as a valuable breeding target to enhance photosynthetic efficiency and yield in maize.