Seedlings of Triticum aestivum L. cv. Maris Huntsman were grown for 7 days in a controlled environment chamber (16 hour photoperiod: PAR - photosynthetically active radiation), in the presence and absence of ultraviolet-B (UV-B: 280-320nm) radiation (+30% increase on ambient). UV-B resulted in a 17% reduction in leaf length due to changes in both the rate and duration of cell division and elongation. Measurements of the spatial distribution of cell division and elongation within the primary leaf were used to determine the temporal distribution of cells (i.e. cell age). The cell age gradient allows for the comparison of direct, and indirect UV-B responses, which result from the altered growth. Direct effects of UV-B included a reduction in chloroplast and mitochondrial transverse area, and an increase in chloroplast number, which suggests that UV-B affects organelle division. The developmental changes in protein content and amino acid free pools were increased as a direct result of UV-B treatment. In contrast, increases in chlorophyll content were due to an indirect effect of UV-B via altered growth. UV-B had no effect on the developmental changes in photosynthetic capacity and efficiency, and carbohydrate status of the primary leaf The primary leaf of wheat has provided a model system in which to examine the effects of UV-B on leaf development. This study highlights the need to consider cell age when determining the response of plants to UV-B.