Maize and rice are the two most economically important grass crops and utilize distinct forms of photosynthesis to fix carbon: C4 and C3 respectively. Relative to C3 photosynthesis, C4 photosynthesis reduces photorespiration and affords higher water and nitrogen use efficiencies under hot arid conditions. To define key innovations in C4 photosynthesis, we profiled metabolites and gene expression along a developing leaf gradient. A novel statistical method was implemented to compare transcriptomes from these two species along a unified leaf developmental gradient and define candidate cis-regulatory elements and transcription factors driving photosynthetic gene expression. We also present comparative primary and secondary metabolic profiles along the gradients that provide new insight into nitrogen and carbon metabolism in C3 and C4 grasses. These resources, including community viewers to access and mine these datasets, will enable the elucidation and engineering of C4 photosynthetic networks to improve the photosynthetic capacity of C3 and C4 grasses. Overall design: [Maize] Nine day old third leaves of maize were cut into fifteen 1 cm segments; samples were pooled from an average of seven plants per biological replicate and six biological replicates in total were collected on different dates. [Rice] 14 day old third leaves of rice were cut into eleven 2 cm segments, samples were pooled from an average of 15 plants per biological replicate and four replicates in total were collected.