Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2008
Publication Date: 1/8/2009
Citation: Baker, J.T., Van Pelt, R.S., Gitz, D.C., Payton, P.R., Lascano, R.J., McMichael, B.L. 2009. Canopy gas exchange measurements of cotton in an open system. Agronomy Journal. 101(1):52-59. Interpretive Summary: Plants grow by using sunlight to capture carbon dioxide out of the air and fixing this into simple sugars in a process called photosynthesis. At the same time, plants loose water to the atmosphere in a process called transpiration. Both of these processes are good indicators of how various environmental factors, such as drought, influence plant growth and crop yield. We developed a portable transparent chamber that can be placed over plants in the field and used to measure the rates of both photosynthesis and transpiration. We tested this chambers ability to measure transpiration by comparing this transpiration rate with water loss from potted plants measured with a simple weighing scale. We found good agreement between transpiration measured by the chamber and the scale indicating that this chamber system will provide a valuable tool for future research on crop water use and water use efficiency.
Technical Abstract: A portable, CETA (Canopy Evapo-Transpiration and Assimilation) chamber system was built and evaluated in 2006. This chamber system is an open or flow-through system that, once field deployed, can operate unattended for extended periods. The CETA chamber consisted of an aluminum framework, 1 m x 0.75 m in cross-section and 1 m tall covered with transparent film. 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 airflow rate to control daytime chamber air temperature to within 0.5 ºC of ambient air temperature using a feedback control algorithm. The use of the variable speed fan to limit heat build up during the day was found to be feasible provided there is sufficient leaf area and soil water for latent energy removal from the system. To validate the mass balance equations used to calculate E, the CETA chamber was placed over sealed soil potted cotton plants resting atop a weighing scale. A similar weighing scale, external to the chamber, was used to measure E of cotton plants outside the chamber in order to quantify potential “chamber effects”. A wide range of crop canopy leaf areas and soil water content were created with greenhouse-grown plants for both of these comparisons. Data analysis indicated agreement between CETA E measurements and the internal weighing scale, as well as comparison between the internal and external weighing scales across wide ranges of soil water contents and canopy leaf area. Bias estimates of E for CETA vs. internal scale and the internal vs. external scale were -6.0 and 4.6 g (H2O) h-1. We conclude that the CETA chamber can provide sufficiently accurate estimation of E for many field applications such as comparison of canopy gas exchanges and water use efficiencies among different cultivars.