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Evaluation of a Dual-Radioisotope Instrument for Measuring Leaf Conductance and Photosynthesis¹

Modifications in both the design and operation of the dual-radioisotope instrument are described. The accuracy and reliability of the modified instrument were then evaluated in both laboratory and field tests. Leaf conductance and net CO₂ exchange rates determined with this instrument and by gas exchange methods in the laboratory were similar for both corn (Zea mays L.) and wheat (Tritictim aestivum L.). A field test of the instrument on barley (Hordeum vulgare L.) subjected to two levels of salinity gave coefficients of variation which indicated that errors attributable to the instrument and/or the plant material were less than 35% for leaf conductance and 16% for photosynthesis. The instrument provides rapid, simultaneous measurements of leaf conductance and photosynthesis under field conditions.

Key Words: Porometer • Tritium • Carbon-14 • Leaf CO₂ exchange rate • Stomatal diffusion resistance

² Former research assistant and research leader, respectively, at the U.S. Salinity Laboratory; and associate professor in the Dep. of Botany and Plant Science, Univ. of California, Riverside. (Present address of senior author is Institute Nacional de Investigaciones Agrarias, Moncada (Valencia), Spain.

Received for publication September 25, 1981.