DISCUSSION
This experiment shows how nitrogen application enhances plant height and dry matter production in A. annua. Nitrogen fertilization appears to increase the allocation of dry matter more to the stem than to the leaf. However, this enhancement in biomass plateaus with the two highest N treatment applications (N5 and N6) and there was no significant increase in stem height or plant biomass above the N4, 106 mg L21, treatment. Measurement of leaf nitrogen concentration and total leaf nitrogen content (mg per plant) showed, with the exception of the highest N application (N6), that leaf N concentration was directly proportional to the concentration of the applied solution. That is to say, there was no evidence that uptake or demand had been saturated at the higher nitrogen treatments (N4–N6) and that this provided an explanation for a limitation in further growth enhancement at the highest N treatments. Enhanced dry matter production induced by elevated plant nitrogen is extremely well documented and probably driven by increases in photosynthetic efficiency.