Comparison of Water Potentials Measured by In Situ Psychrometry and Pressure Chamber in Morphologically Different Species

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Abstract
Leaf water potentials measured by in situ psychrometry were compared with leaf water potentials measured by the pressure chamber technique at various values of water potential in Helianthus annuus, H. nuttallii, Vigna unguiculata, Nerium oleander, Pistacia vera and Corylus avellana. In V. unguiculata, the leaf water potentials measured by the in situ psychrometer oscillated at the same periodicity as, and proportional to, the leaf conductance. In all species, potentials measured by in situ psychrometers operating in the psychrometic mode were linearly correlated with potentials measured with the pressure chamber. However, the in situ psychrometers underestimated the leaf water potential in the 2 Helianthus spp. at low water potentials and overestimated the water potential in P. vera, N. oleander and C., avellana. The underestimation in the 2 Helianthus spp. at low water potentials resulted from differences in water potential across the leaf. The overestimation in P. vera, N. oleander and C. avellana was considered to arise from low epidermal conductances in these species even after abrasion of the cuticle. Pressure-volume studies with Lycopersicon esculentum showed that less water was expressed fromdistal than proximal leaflets when the whole leaf was slowly pressurized. The implication of this for water relations characteristics obtained by pressure-volume techniques is discussed. In situ psychrometers are suitable for following dynamic changes in leaf water potential, but should be used with caution on leaves with low epidermal conductances.