Exploring intra-specific variation in photosynthesis of maize and sorghum

Enhancing crop yield through improved photosynthesis is a key for feeding the global population and providing feedstock for a sustainable green economy. However, effectively linking photosynthetic performance and biomass production in C4 species requires an integrative approach at plant canopy. This study aimed to characterize photosynthesis along the canopy of five maize (BM3069-PRO2, AG8701-PRO4, K7500-VIP3, DKB355-PRO3 and B2401-PWU) and sorghum (DKB560, Enforcer, IAC 7021, Brandelisa and Santa Elisa) cultivars, focusing on leaf gas exchange and chlorophyll fluorescence evaluations in three canopy strata: top, middle, and bottom. Photosynthetic responses to increasing intercellular CO2 concentration and light (A–Ci and A−PAR curves, respectively) were performed and key photosynthetic traits estimated. We found a significant variability in the maximum photosynthetic rates across the canopies. Modern maize cultivars exhibited high and uniform CO2 assimilation in the top and middle canopy leaves, demonstrating physiological adjustments for increasing canopy homogeneity in terms of photosynthesis. Such adjustments included high maximum quantum efficiency of CO2 assimilation (φ), stomatal conductance, carboxylation rates of phosphoenol pyruvate carboxylase (PEPC) and Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf nitrogen concentration (LNC) along the canopy. In contrast, sorghum cultivars showed high variability among canopy strata, with DKB560 and Brandelisa standing out for their enhanced CO2 uptake and (φ) throughout the plant canopy. Our findings highlight maize as an efficient C4 crop, characterized by a high photosynthetic capacity and relative uniform photosynthesis across the canopy. This is attributed to reduced stomatal limitation and higher stomatal conductance, carboxylation of Rubisco (Vcmax) and LNC at the top and middle canopy, along with higher (φ) at the middle and bottom layers. Overall, physiological adjustments such as nitrogen redistribution to upper canopy leaves and the optimization of light-use efficiency in lower layers are key for enhancing canopy-level photosynthesis in maize and sorghum cultivars.