Vacuum infiltration of inorganic nitrogen ions affected the photosynthetic rate and CO2 compensation point of Themeda triandra and Zea mays in a similar way. However these two plants differed markedly in their photorespiratory mechanisms in response to inorganic nitrogen. Increased nitrogen levels in the in vitro reaction mixture stimulated T. triandra RuBP oxygenase activity (500%), but the similar enzyme from Z. mays showed no response to nitrogen. Using metabolic inhibitors it appears that glycolate and glyoxylate may serve as substrates for photorespiratory CO2, producing formate. T. triandra leaf formate levels increased considerably with infiltration of increased inorganic nitrogen levels. This was not the case in Z. mays. In T. triandra an alternative photorespiratory pathway appears to be operating under high nitrogen. This pathway would utilise inorganic nitrogen more efficiently with a greater loss of photorespiratory CO2, resulting in decreased productivity. Z. mays did not exhibit any of these responses to nitrogen, suggesting that the observed responses of T. triandra to nitrogen could in part account for its sensitivity to nitrogen inhibition of growth.
Keywords: botany; co2; infiltration; inhibition; inhibitors; inorganic nitrogen; leaves; nitrogen; nitrogen levels; photosynthetic rate; plant physiology; productivity; south africa; themeda triandra; zea mays