Nitrogen defíciency caused pronounced reductions in the photosynthetic capacity and differential losses in chlorophyll, cytochrome / and Mg2'''>specific ATPase amounts or activities in suspension cultured cells of Chenopodium rubrum L. This reduced outfit of the photosynthetic machinery and limited protein tumover capacity are possible reasons for our observation that nitrogen deficiency exacerbates the hannful effects of high irradiance on photosystem 2 photochemical efficiency. The effect of nitrogen defíciency on photoinhibition increased over a broad range of photon flux densities and it was detectable in both the short-term and long-tenn experiments. Differences in the effects of the nitrogen regime and irradiance on several growth parameters were demonstrated. The main effect of nitrogen defíciency was a reduction of protein synthesis and cell division, whereas the irradiance chiefly affected the accumulation of carbon in the cell suspensions. Synergistic effects of nitrogen regime and irradiance could also be demonstrated for betalain accumulation which was the greatest under high irradiance and expressed nitrogen defíciency.