Nitrogen requirements for maximum growth and photosynthesis of rice, Oryzasativa 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 r equired to support maximum dry mass production and photosynthetic rates dur ing 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. Eleva ted CO2 reduced leaf N concentrations required to support 90% of maximum gr owth 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 w hich occurred at high CO2 at each N concentration in the solution, primaril y due to lower leaf blade and root N concentrations. Changes in tiller numb ers at high CO2 were unrelated to leaf or plant N but were strongly correla ted with leaf soluble carbohydrate concentrations. We conclude that elevate d CO2 alters the nutritional physiology of rice during the rapid tillering phase in a way that increases the efficiency of N utilisation for growth an d photosynthesis.