Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 3. Canopy architecture and gas exchange

The response of whole-canopy net CO2 exchange rate (CER) and canopy archite cture to CO2 enrichment and N stress during 1996 and 1997 for open-field-gr own wheat ecosystem (Triticum aestivum L. cv. Yecora Rojo) are described. E very Control (C) and FACE (F) CO2 treatment (defined as ambient and ambient +200 mu mol mol(-1), respectively) contained a Low- and High-N treatment. Low-N treatments constituted initial soil content amended with supplemental nitrogen applied at a rate of 70 kg N ha(-1) (1996) and 15 kg N ha(-1) (19 97), whereas High-N treatments were supplemented with 350 kg N ha(-1) (1996 and 1997). Elevated CO2 enhanced season-long carbon accumulation by 8% and 16% under Low-N and High-N, respectively. N-stress reduced season-long car bon accumulation 14% under ambient CO2, but by as much as 22% under CO2 enr ichment. Averaging both years, green plant area index (GPAI) peaked approxi mately 76 days after planting at 7.13 for FH, 6.00 for CH, 3.89 for FL, and 3.89 for CL treatments. Leaf tip angle distribution (LTA) indicated that L ow-N canopies were more erectophile than those of High-N canopies: 48 degre es for FH, 52 degrees for CH, and 58 degrees for both FL and CL treatments. Temporal trends in canopy greenness indicated a decrease in leaf chlorophy ll content from the flag to flag-2 leaves of 25% for FH, 28% for CH, 17% fo r CL, and 33% for FL during 1997. These results indicate that significant m odifications of canopy architecture occurs in response to both CO2 and N-st ress. Optimization of canopy architecture may serve as a mechanism to dimin ish CO2 and N-stress effects on CER.