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 photosyntheti c apparatus of wheat (Triticum aestivum L. cv. Yecora Rojo) grown in a futu re 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 pl ot). A High (35 g m(-2)) or Low (7 and 1.5 g m(-2) for 1996 and 1997, respe ctfully) level of N was applied to each half of the main CO2 treatment plot s (split-plot). Under High-N, FACE reduced stomatal conductance (g(s)) by 3 0% 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) w as reduced by as much as 31% at mid-morning, 44% at midday and 28% at mid-a fternoon compared with Control. But, no significant CO2 x N interaction eff ects occurred. Across seasons and growth stages, daily accumulation of carb on (A') was 27% greater in FACE than Control. High-N increased A' by 18% co mpared 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 ca rbon (A'') by 29% during 1996 (moderate N-stress), but by only 21% during 1 997 (severe N-stress). These results support the premise that in a future h igh-CO2 world an acclimatory (down-regulation) response in the photosynthet ic apparatus of field-grown wheat is anticipated. They also demonstrate, ho wever, that the stimulatory effect of a rise in atmospheric CO2 on carbon g ain in wheat can be maintained if nutrients such as nitrogen are in ample s upply.