Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 2. Net assimilation and stomatal conductance of leaves

Atmospheric CO2 concentration continues to rise. It is important, therefore, to determine what acclimatory changes will occur within the photosynthetic apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a future high-CO2 world at ample and limited soil N contents. Wheat was grown in an open field exposed to the CO2 concentration of ambient air [370 mu mol (CO2) mol(-1); Control] and air enriched to similar to 200 mu mol (CO2) mol(-1) above ambient using a Free-Air CO2 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 CO2 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 CO2 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 CO2 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-CO2 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 CO2 on carbon gain in wheat can be maintained if nutrients such as nitrogen are in ample supply.