Agriculture, Ecosystems and Environment 79 (2000) 53–60 Leaf nitrogen concentration of wheat subjected to elevated [CO 2 ] and either water or N deficits T.R. Sinclair a,∗ , P.J. Pinter Jr. b , B.A. Kimball b , F.J. Adamsen b , R.L. LaMorte b , G.W. Wall b , D.J. Hunsaker b , N. Adam b , T.J. Brooks b , R.L. Garcia d , T. Thompson c , S. Leavitt c , A. Matthias c a USDA-ARS, Agronomy Physiology & Genetics Laboratory, University of Florida, Gainesville, FL 32611 0965, USA b USDA-ARS, U.S. Water Conservation Laboratory, Phoenix, AZ, USA c University of Arizona, Tucson, AZ, USA d LI-COR, Inc., Lincoln, NB, USA Received 7 January 1999; received in revised form 21 September 1999; accepted 27 October 1999 Abstract Leaf N concentration is important because it is associated with the CO 2 assimilatory capacity of crops, and in grasslands, it is an important determinant of forage nutritive value. Consequently, the productivity of both domestic and native animals in future global environments may be closely linked to possible changes in leaf N concentration of grasses. Since grasslands are frequently subjected to water-deficit or N-deficit conditions, it is important to investigate the interactive responses between elevated [CO 2 ] and these stress conditions. Therefore, this 4-year research program was undertaken with wheat (Triticum aes- tivum L.) as a model system for forage grasses, to document the potential changes in leaf N concentration in response to global environment changes. Wheat crops grown under field conditions near Phoenix, AZ, USA, were subjected to elevated [CO 2 ] and either water-deficit or N-deficit treatments using large Free Air Carbon dioxide Enrichment (FACE) arrays. Surprisingly, the elevated [CO 2 ] treatment under optimum conditions resulted in little change in leaf N concentration. Therefore, no change in the nutritive value of forage from highly managed pastures would be expected. Further, water-deficit treatment had little influence on leaf N concentration. To some extent, the lack of response to the water-deficit treatment resulted because severe deficits did not develop until late in the growing seasons. Only on one date late in the season was the water-deficit treatment found to result in decreased leaf N concentration. The low N treatment in combination with elevated [CO 2 ], however, had a large influence on leaf N concentration. Low levels of applied N resulted in decreased leaf N concentration under both [CO 2 ] treatments, but the lowest levels of leaf N concentration were obtained under elevated [CO 2 ] through much of the growing season. These results point to a potential problem with grasslands in that the nutritive value of the forage consumed by animals will be decreased under future global environment changes. © Published by Elsevier Science B.V. Keywords: CO 2 enrichment; Forage quality; Leaf nitrogen; Nitrogen deficit; Water deficit ∗ Corresponding author. Tel.: +1-352-392-6180; fax: +1-352-392-6139. E-mail address: trsincl@gnv.ifas.ufl.edu (T.R. Sinclair) 1. Introduction In consideration of the impact on plants of global environment changes, including atmospheric CO 2 in- creases, alterations in leaf N concentration may be 0167-8809/00/$ – see front matter © Published by Elsevier Science B.V. PII:S0167-8809(99)00146-2