A Portable System for Measuring the Photosynthesis and Transpiration of Graminaceous Leaves

Abstract

A portable system has been developed for measuring rates of photosynthesis and transpiration of Graminaceous leaves; it comprises a transparent chamber, into which the leaves are sealed, and a gas supply. The chamber is made from poly 4-methylpent-1-ene (PMP), chosen for its transparency and small water absorption. The leaf is sealed into the chamber by an inflatable rubber seal. The chamber contains a humidity sensor, photocell, thermistor, and a fan which is positioned to give efficient gas-mixing and rapid thermal equilibration. The chamber is surrounded by a Perspex tube to absorb part of the incident infrared radiation. Another fan blows ambient air between the chamber and the Perspex tube to counteract solar heating of the chamber wall. The gas supply to the chamber comes from a gas cylinder via a dilutor. The dilutor contains four small orifices arranged in parallel. Each orifice is mounted in the inlet of a two-way gas valve: one way goes to the leaf chamber directly, the other via a CO₂ absorber. The total gas flow-rate is independent of valve position, and the CO₂ concentration entering the chamber can be varied from zero to the cylinder concentration by switching the valves. The system has been tested both in the laboratory and in the field, with satisfactory results.