Rapidly Determining Sorghum Canopy Photosynthetic Rates with a Mobile Field Chamber1

Abstract Progress in characterizing the photosynthesis of plant communities under field conditions demands simple, rapid methods for determining CO 2 exchange rates in the field. With conventional flow‐through gas exchange systems, difficulties are faced because of the need for an electrical power source to operate the analyzer in the field and the lack of mobility of this multiple component system. Recently, a technique was developed for precisely measuring CO 2 concentrations of small gas samples collected in medical syringes. We describe the design and operation of an assimilation chamber that adapts this technique into a rapid and mobile method for canopy photosynthesis measurements on grain sorghum [ Sorghum bicolor (L.) Moench). The chamber and all accessories can be hand‐carried by two people. Two to 3 min are required to sample the photosynthetic rate of a 0.86 m length of crop row at any given location. The chamber is constructed of acrylic plastic with aluminum reinforced corners. A unique ventilation system was designed using an exhaust blower powered by a lightweight 12 v battery. The differential in CO 2 concentration between air samples drawn at 0 and 60 s provided the basis for canopy apparent photosynthesis determinations. Diurnal measurements of the canopy photosynthesis of sorghum under different soil moisture regimes on two dates are reported. The convenience and mobility features of the method suggest numerous applications in field investigations where conventional CO 2 exchange systems are impractical.