Effects of free-air carbon dioxide enrichment on PAR absorption and conversion efficiency by cotton

Anticipated changes in global climate and atmospheric CO2 concentrations have very important, albeit poorly understood consequences for production agriculture. Effects of these changes on plants have usually been examined in controlled-environment enclosures, glass-houses, or open-top field chambers. Beginning in 1989, an innovative experimental free-air CO2 enrichment (FACE) facility was operated in central Arizona to evaluate crop response to increased CO2 levels within a large, open-field production environment. Cotton (Gossypium hirsutum L.) was grown for three consecutive seasons under well-watered conditions and exposed to either ambient (control, about 370 μmol mol−1) or elevated (FACE, 550 μmol mol−1) CO2 concentrations. Deficit irrigation regimes supplying 75% (beginning in July 1990) or 67% (beginning in mid-May 1991) of the crop's evapotranspiration requirement were included as additional treatment variables. Plant growth was monitored by periodic sampling. Canopy reflectances in visible (blue, 0.45-0.52 μm; green, 0.50-0.59 μm; red, 0.61-0.68 μm) and near-infrared (NIR; 0.79-0.89 μm) wavebands were measured frequently with an Exotech radiometer and related to absorbed photosynthetically active radiation (PAR; 0.4-0.7 μm) measured with a line quantum sensor. Dry biomass of plants in the FACE treatment was significantly (P < 0.05) greater than control values during each year of the study. The FACE plant canopy also absorbed significantly more PAR than controls during the early and middle portion of the 1990 and 1991 seasons. Light use efficiency (LUE, biomass produced per unit absorbed PAR) was significantly higher in FACE plots during each year. In the well-watered irrigation treatment, the 3 year mean LUE was 1.97 g MJ−1 for FACE and 1.56 g MJ−1 for controls. The deficit irrigation treatment in 1991 produced significantly smaller plants, which absorbed less PAR and had lower LUE than plants in the well-watered treatment (P < 0.05). No interaction was observed between CO2 and irrigation treatments. FACE research under realistic field conditions revealed positive consequences of increased CO2 on cotton plant biomass, PAR absorption, and LUE. It also demonstrated the effectiveness of this new technology for examining community-level plant responses to possible changes in global environment.

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