Performance of the Silver-foil Psychrometer for Measuring Leaf Water Potential in situ
1976
Hoffman, Glenn J. | Hall, Anthony E.
Continuous in situ measurements of leaf water potential are often needed in studies of the soil-plant-atmosphere continuum. The development of the silver-foil thermocouple psychrometer has made these measurements possible. The objective of this investigation was to evaluate the performance of the silver-foil psychrometer under various steady-state and dynamic conditions. Leaf water potential measurements were made in situ on pinto bean (Phaseolus vulgaris L.) in a controlled-environment plant chamber where temperature, relative humidity, CO₂ concentration, radiation, and photoperiod were precisely controlled. Typically, the psychrometer will reach equilibrium with an intact leaf 2 hours after attachment. Consistent leaf water potentials are attainable when the leaf temperature varies by as much as 0.25 C/min. This thermal stability is as good as that of any multijunction psychrometer described to date. At radiation levels below 50 Wm⁻², leaf water potential readings can be made on unshaded leaves; above this level, the leaf must be shaded to prevent thermal gradients across the leaf and psychrometer. The silver-foil psychrometer responded to rapid oscillations in leaf water potential. During oscillations, leaf water potential preceded leaf conductance and transpiration by one-third of the oscillation period. Net photosynthesis oscillated in phase with leaf conductance, indicating that net photosynthesis is limited by the internal CO₂ concentration, even at very low irradiances.
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