Measurement of Photosynthetic Gas Exchange in Controlled Environments

Abstract

Early controlled-environment research used growth chambers primarily as “photoperiod rooms” to study effects of environment on long-term plant developmental processes (Downs and Hellmers, 1975). More recently, growth dynamics analysis has been used to measure photosynthetic productivity of crop canopies or individual plants in controlled environments over periods of only a few days (Knight and Mitchell, 1983). However, as the emphasis of controlled-environment research shifts toward definition of optimum growth conditions (Hoff et al., 1982), there will be increasing demand to measure instantaneous photosynthetic response within modified controlled environments. Net photosynthesis and respiration are measured most conveniently as CO2 consumption or evolution, respectively, by a given area or mass of plant tissue over a defined period. At ≈ 3 5 0 μmol·mol –1(μl·liter –1, vpm), CO2 is a trace gas compared to the major components of air (N2, O2, H2Ov). As such, CO2 is detectable to 1 μmol·mol –1 by thermal conductivity gas chromatography (TCD-GC) or infrared gas analysis. The infrared gas analyzer (IRGA) is amenable to continuous-flow monitoring of the CO2 level in air and will detect near-real-time changes in gas-exchange rate by plants as environmental conditions vary. Modem electronic detector systems permit a degree of miniaturization of IRGAs that allows their portability into growth chambers as well as to the field. Gas exchange by chamber-grown plants may be measured for individual attached leaves, entire shoots, or leaf canopies of multiple plants. Various plant enclosure techniques are used for each type of measurement. Because a limited number of off-the-shelf assimilation chambers are available on the commercial market, many researchers have developed their own cuvettes for specific applications. Assimilation chambers often are constructed from transparent acrylic (plexiglass) or polycarbonate (mylar) plastic. Because these plastics adsorb water vapor (H2Ov) and, consequently, CO2