CLOSED-CHAMBER EFFECTS ON LEAF TEMPERATURE, CANOPY PHOTOSYNTHESIS, AND EVAPOTRANSPIRATION

The portable, closed-chamber technique permits simultaneous measurement of crop canopy photosynthesis (carbon dioxide exchange rate, CER) and evapotranspiration rate (ET) under field conditions and is a valuable tool for evaluating new management practices to increase water-use efficiency. Because plants react quickly to environmental changes, and the technique may affect the plant environment during the brief (1 min) measurement period, further exploration of the technique was required. The objective was to measure the effect of a portable closed chamber (8.15-m3 volume, covering 2.67 m2 of soil surface ares) on leaf and air temperatures and to determine any subsequent effects on calculated CO2 and water vapor exchange rates. Measurements of CER and ET were made on field plots of irrigated and non-irrigated maize (Zea mays L.) shortly after anthesis. Leaf and air temperatures were measured at 2-s intervals inside and outside the chamber for 1 min prior, 1 min during, and 1 min after chamber closure. Leaf and air temperatures in the closed chamber increased 2 to 4-degrees-C in 1 minute. Changes in canopy CER and ET were not strongly correlated with mean leaf temperatures. The calculated CER and ET rates underestimated the rates extrapolated to time 0 by approximately 10%, partially attributed to the increase in leaf temperature. Therefore, chamber measurements should be made rapidly to minimize possible effects of temperature increases and other plant environment changes. While there are limitations, the chamber technique provides a practical method for measurements of field canopies.