title: Acclimation response of spring wheat in a free-air CO(2) enrichment (FACE) atmosphere with variable soil nitrogen regimes. 2. Net assimilation and stomatal conductance of leaves.
abstract: Atmospheric CO(2) concentration continues to rise. It is important, therefore, to determine what acclimatory c hanges will occur within the photosynthetic apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a future high-CO(2) world at ample and limited soil N contents. Wheat was grown in an open field exposed to the CO(2) concentration of ambient air [370 mumol (CO(2)) mol(-1); Control] and air enriched to approximately 200 mumol (CO(2)) mol(-1) above ambient using a Free-Air CO(2) Enrichment (FACE) apparatus (main plot). A High (35 g m(-2)) or Low (7 and 1.5 g m(-2) for 1996 and 1997, respectfully) level of N was applied to each half of the main CO(2) treatment plots (split-plot). Under High-N, FACE reduced stomatal conductance (g (s)) by 30% at mid-morning (2 h prior to solar noon), 36% at midday (solar noon) and 27% at mid-afternoon (2.5 h after solar noon), whereas under Low-N, g (s) was reduced by as much as 31% at mid-morning, 44% at midday and 28% at mid-afternoon compared with Control. But, no significant CO(2) x N interaction effects occurred. Across seasons and growth stages, daily accumulation of carbon (A') was 27% greater in FACE than Control. High-N increased A' by 18% compared with Low-N. In contrast to results for g (s), however, significant CO(2) x N interaction effects occurred because FACE increased A' by 30% at High-N, but by only 23% at Low-N. FACE enhanced the seasonal accumulation of carbon (A'') by 29% during 1996 (moderate N-stress), but by only 21% during 1997 (severe N-stress). These results support the premise that in a future high-CO(2) world an acclimatory (down-regulation) response in the photosynthetic apparatus of field-grown wheat is anticipated. They also demonstrate, however, that the stimulatory effect of a rise in atmospheric CO(2) on carbon gain in wheat can be maintained if nutrients such as nitrogen are in ample supply.
authors.affiliation: Agricultural Research Service, US Department of Agriculture, US Water Conservation Laboratory, Phoenix, AZ, 85040, USA, gwall@uswcl.ars.ag.gov.
authors.forename: G W
authors.initials: GW
authors.lastname: Wall
authors.forename: N R
authors.initials: NR
authors.lastname: Adam
authors.forename: T J
authors.initials: TJ
authors.lastname: Brooks
authors.forename: B A
authors.initials: BA
authors.lastname: Kimball
authors.forename: P J
authors.initials: PJ
authors.lastname: Pinter
authors.forename: R L
authors.initials: RL
authors.lastname: Lamorte
authors.forename: F J
authors.initials: FJ
authors.lastname: Adamsen
authors.forename: D J
authors.initials: DJ
authors.lastname: Hunsaker
authors.forename: G
authors.initials: G
authors.lastname: Wechsung
authors.forename: F
authors.initials: F
authors.lastname: Wechsung
authors.forename: S
authors.initials: S
authors.lastname: Grossman-Clarke
authors.forename: S W
authors.initials: SW
authors.lastname: Leavitt
authors.forename: A D
authors.initials: AD
authors.lastname: Matthias
authors.forename: A N
authors.initials: AN
authors.lastname: Webber