ERRORS IN FIELD MEASUREMENT OF LEAF DIFFUSIVE CONDUCTANCE ASSOCIATED WITH LEAF TEMPERATURE

Meyer, W.S., Reicosky, D.C. and Schaefer, N.L., 1985. Errors in field measurement of leaf diffusive conductance associated with leaf temperature. Agric. For. Meterol., 36: 55--64. The ability of leaf diffusive conductance (CL) measurements to discriminate between different treatments in field studies has generally been disappointing. Since values of C L are sensitive to leaf temperature and temperature gradients between the leaf and air, inaccurate temperature measurements will produce substantial errors. This work compared two methods of leaf temperature measurement from which values of C L were calculated and subsequently compared. Experiments using well-watered field- and glasshouse-grown cotton (Gossypium hirsutum L.) where diffusion porometer readings and leaf temperatures were measured indicated that the porometer thermistor reading differed markedly from leaf temperature as assessed with an infrared thermometer. Under high air vapor pressure deficit (AVPD) conditions (5.2 kPa), leaf temperature (T L) to ambient air temperature • o . (TA) differences were as large as --8 C. Porometer thermistor temperature was strongly influenced by T A and to a lesser extent by T L. Values of C L calculated from porometer measurements of T L can be up to 96% less than true C L values. This is the result of both incorrect T L measurement and a failure to account for the effects of the leaf to porometer-cup temperature differential. Until these two sources of error are addressed, values of C L obtained from diffusion porometer measurements in the field must be regarded as only qualitative.