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High concentrations of carbon dioxide decrease critical nitrate and total nitrogen concentrations in wheat

P.J. Hocking and C.P. Meyer

CSIRO Division of Plant Industry, GPO Box 1600, Canberra ACT 2601
CSIRO Division of Atmospheric Research, Mail Bag No. 1, Mordialloc VIC 3195

Atmospheric carbon dioxide (CO2) levels may double by the middle of next century, and there is evidence high CO, increases nitrogen (N)-use efficiency (2) and lowers N concentrations in leaves of C3 plants(3). Critical concentrations of total N and NO3-N are used for wheat to decide if the crop has adequate N and to predict grain yield (1), but it is not known if critical values are affected by high CO2. We report a study of the effects of high CO2 on critical concentrations of total N and NO3-N in stem bases and leaves of wheat.


Wheat cv. Avocet was grown in sand culture in identical glasshouses at two levels of CO2 (340 cm3/cm3) for control plants and 1500 cm3/cm3 for enriched plants. Within each glasshouse, plants received five N supplies: 7, 35, 84,168 and 350 mg NO3-N/L, added to a minus-N nutrient solution. Plants were harvested at Feekes Stages (4) 1.5, 5 and 10.3, separated into stem bases and leaves, dried, and analysed for total N and NO3-N.

Results and discussion

At ear emergence (FS 10.3), CO2-enriched plants were taller, had more tillers, and had twice the dry matter production of the controls (3). Critical N supply for 90% of maximum growth was fairly similar for enriched and control plants (115 vs. 135 mg/L), and this suggests that approximately the same rates of N fertiliser as at present will need to be applied under high CO, to achieve maximum dry matter production (and presumably yield). Critical concentrations of NO3-N and total N in stem bases and leaves of enriched plants were clearly lower than those of control plants at the three growth stages (Table 1). Consequently, critical values obtained for NO3-N or total N under high CO2 could indicate that plants were N-deficient by present standards, whereas their N status may be satisfactory for maximum growth. We conclude that it may be necessary to calibrate critical N concentrations at a given growth stage to account for increasing levels of atmospheric CO2.

Table 1. Effect of CO2 enrichment on critical concentrations (mg/g dry wt) of NO3-N and total N in wheat. C, control plants; E, CO2-enriched plants. (All C vs. E differences significant (P<0.01) except where marked ns).


Elliott, D.E., Reuter, D.J., Growden, B., Schultz, J.E., Muhlhan, P.H., Gouzos, J. and Heanes, D.L. 1987. J. Plant Nutr. 10, 1761-1770.

Goudriaan, J. and de Ruiter, H.E. 1983. Neth. J. Agric. Sci. 31, 157-169. Hocking, P.J. and Meyer, C.P. 1991. Aust. J. Plant Physiol. 18, 339-356. Large, E.C. 1954. Plant Pathol. 3, 128-129.

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