Problems with porometry: measuring stomatal conductances of potentially transpiring plants

Porometer measurements of the stomatal conductances (C_s) of potentially transpiring water hyacinth plants at Phoenix, Arizona in October of 1984, May–June of 1985, and September of 1986 indicate that C_s steadily drops as the vapor pressure deficit (VPD) of the air in the measuring system's cuvette or leaf chamber rises. Utilizing this relationship to calculate the foliage-air temperature differential (T_F−T_A) response of these leaves to leaf-chamber air VPD, as per the basic equations of standard heat and water vapor transport theory; we obtain a leaf-chamber “non-water-stressed baseline” that is consistent with leaf-chamber measurements of T_F−T_A vs. air VPD. Free-air T_F−T_A vs. air VPD data, on the other hand, produce a relationship that is similarly consistent with a plant stomatal conductance which is invariant with respect to the air VPD. Hence, we conclude that the very act of stomatal conductance measurement alters a potentially transpiring plant's evaporative water loss rate in such a way that, for very high air VPD conditions, the directly measured C_s value (although correct for the leaf in the cuvette or leaf chamber) may be much reduced from that characteristic of comparable non-chamber-encumbered plants in the free air. We then demonstrate that this instrument-induced reduction in directly measured C_s values is a unique function of the leaf-chamber IJ index, evaluated with respect to the plant's free-air non-water-stressed baseline. Similar results obtained by others for cotton suggest that this phenomenon may be quite general, and that the C_s vs. air VPD interaction, believed by many to be widely operative throughout the plant kingdom, may not really exist in actual field situations.