Photosynthesis in C4-type grasses such as maize involves the interaction of two cell types (bundle sheath (BS) and mesophyll (M)) which both contain cell-specific photosynthetic enzymes. Malate dehydrogenase, phosphoenolpyruvate carboxylase and pyruvate phosphate dikinase are located in the M cells and malic enzyme and ribulose bisphosphate carboxylase are found in the BS cells. We have studied photosynthetic development in leaves of the temperature-sensitive greening mutant argentia (ar). We have determined that with the exception of malate dehydrogenase, levels of C4 enzymes are lower in ar leaves than in normal leaves. Malate dehydrogenase accumulates identically in both. Using in situ immunolocalization techniques with normal and ar leaves, we have observed a developmental pattern of C4 protein accumulation. In normal leaves protein was detected first in cells surrounding the median vein, then in cells surrounding other major veins, and finally in cells surrounding minor veins. In ar leaves, C4 enzymes accumulate in the correct cell type and in this same order but their appearance is delayed. Furthermore, BS cell development is delayed with respect to M cell development. The observed pattern of photosynthetic development reflects an earlier manifested pattern of vascular development yet the timing of vascular differentiation in ar mutants appears to be normal.