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AGROCLIMATOLOGY

Canopy Gas Exchange Measurements of Cotton in an Open System

^a USDA-ARS, Plant Stress and Water Conservation Laboratory, 302 West I-20, Big Spring, TX, 79720
^b USDA-ARS, Plant Stress and Water Conservation Laboratory, 3810 4th Street, Lubbock, TX 79415

^* Corresponding author (Jeff.Baker{at}ars.usda.gov).

A portable, open transparent chamber system for measuring canopy gas exchanges was developed and tested. Differentials between incoming and outgoing atmospheric H₂O and CO₂ concentrations were used to calculate canopy transpiration (E) and net assimilation (A) at 10-s intervals using solenoid valve actuated sample lines connected to an infrared gas analyzer. A programmable data logger controlled fan speed and air flow rate to control daytime chamber air temperature to within 0.5°C of ambient air temperature. To validate the mass balance equations used to calculate E, the chamber was positioned over sealed soil potted cotton (Gossypium hirsutum L.) plants which were placed on a weighing scale. A second scale was used to measure E of cotton plants outside the chamber to quantify potential chamber effects. A wide range of crop canopy leaf areas and soil water contents were created with greenhouse-grown plants for these comparisons. Data analysis indicated agreement between chamber E measurements and the internal weighing scale (R² = 0.93), as well as comparison between the internal and external scales (R² = 0.88) across wide ranges of soil water contents and canopy leaf area. Transpiration ranged from near zero at night to 900 g (H₂O) h^–1 during the day. Bias estimates of E for chamber vs. internal scale and the internal vs. external scale were –6.0 and 4.6 g (H₂O) h^–1. With minor chamber effect, the chamber accurately estimates E for many field applications such as comparison of canopy gas exchanges and water use efficiencies among irrigation treatments.

Abbreviations: A, canopy net assimilation • BREB, Eddy Correlation and Bowen Ratio Energy Balance • CETA, Canopy EvapoTranspiration and Assimilation chamber • E, canopy transpiration • ET, evapotranspiration • IRGA, infrared gas analyzer • NEE, net ecosystem exchange • PAR, photosynthetically active radiation • SPAR, Soil-Plant-Atmosphere-Research chamber

¹ Mention of this or other proprietary products is for the convenience of the readers only, and does not constitute endorsement or preferential treatment of these products by USDA-ARS.

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Received for publication July 3, 2008.