EFFECTS OF FREE-AIR CARBON-DIOXIDE ENRICHMENT ON PAR ABSORPTION AND CONVERSION EFFICIENCY BY COTTON

Anticipated changes in global climate and atmospheric CO2 concentratio ns have very important, albeit poorly understood consequences for prod uction agriculture. Effects of these changes on plants have usually be en examined in controlled-environment enclosures, glass-houses, or ope n-top field chambers. Beginning in 1989, an innovative experimental fr ee-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 grow n for three consecutive seasons under exposed to either ambient (contr ol, about 370 mumol mol-1) or elevated (FACE, 550 mumol mol-1) CO2 con centrations. Deficit irrigation regimes supplying 75% (beginning in Ju ly 1990) or 67% (beginning in mid-May 1991) of the crop's evapotranspi ration requirement were included as additional treatment variables. Pl ant growth was monitored by periodic sampling. Canopy reflectances in visible (blue, 0.45-0.52 mum; green, 0.05-0.59 mum; red, 0.61-0.68 mum ) and near-infrared (NIR; 0.79-0.89 mum) wavebands were measured frequ ently with an Exotech radiometer and related to absorbed photosyntheti cally active radiation (PAR; 0.4-0.7 mum) 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 contr ols 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-wate red 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 199 1 produced significantly smaller plants, which absorbed less PAR and h ad lower LUE than plants in the well-watered treatment (P < 0.05). No interaction was observed between CO2 and irrigation treatments. FACE r esearch under realistic field conditions revealed positive consequence s of increased CO2 on cotton plant biomass, PAR absorption, and LUE. I t also demonstrated the effectiveness of this new technology for exami ning community-level plant responses to possible changes in global env ironment.