Canopy Gas Exchange Measurements of Cotton in an Open System

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.