Nitrogen requirements for maximum growth and photosynthesis of rice, Oryza sativa L. cv. Jarrah grown at 36 And 70 Pa CO2

The hypothesis that growth of rice (Oryza sativa L. cv. Jarrah) at elevated atmospheric CO2 partial pressure alters leaf nitrogen (N) concentrations required to support maximum dry mass production and photosynthetic rates during the period of rapid tiller initiation was tested by growing plants for 30 days in unstirred sand/hydroponic culture with N concentrations of 5, 20, 40, 60 and 100 mg N L–1. Maximum growth and photosynthetic potential was greater at 70 than 36 Pa CO2 at all N concentrations in the solution. Elevated CO2 reduced leaf N concentrations required to support 90% of maximum growth and photosynthetic rates (critical concentration) from 40 to 27 g kg–1 for growth and from 45 to 30 g kg–1 for photosynthesis. Morphological changes at elevated CO2 included increased tiller numbers and reduced leaf area ratio. The latter could be explained by lower plant N concentrations which occurred at high CO2 at each N concentration in the solution, primarily due to lower leaf blade and root N concentrations. Changes in tiller numbers at high CO2 were unrelated to leaf or plant N but were strongly correlated with leaf soluble carbohydrate concentrations. We conclude that elevated CO2 alters the nutritional physiology of rice during the rapid tillering phase in a way that increases the efficiency of N utilisation for growth and photosynthesis.