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Stress Measurement Using Foliage Temperatures¹

The total (stomatal and aerodynamic) conductance for water vapor of a crop (g_v) reflects the rate of crop production. Determination of g_v should therefore be valuable in predicting crop yield. With an energy balance approach to crop stress assessment, foliage-air temperature differences (T_f — T_a) may provide a means of determining g_v. This study was conducted to model the response of T_f — T_a to environmental parameters and conductances, and then, in a field experiment with kidney beans (Phaseolus vulgaris L.) grown under two radiation regimes, to determine g_v using measured T_f — T_a values, and to relate this to crop yields. Kidney beans (‘Red Kidney’) were grown on deep Yolo clay loam (Typic Xerorthent) at Davis, Calif., under normal irradiance and with irradiance reduced by 50%. Different stress levels (different g_v) were established by varying time and frequency of irrigation. Foliage temperatures (corrected for emissivity and sky radiation), net radiation and wet and dry bulb temperatures were recorded daily at to 1 h after solar noon through most of the growing season. Seed yields were measured at the end of the season. The energy balance model and field data showed that T_f — T_a is sensitive to air saturation deficit (0.16°C mb^–1) and to net radiation (0.5°C/100 W m^–2). Using an energy balance scheme, calculations of g_v were made from field data for the seed filling period. Relative yields estimated as
, with
determined from well-watered plots, agreed well with relative seed yields (r = 0.75). Major T_f — T_a responses to saturation deficit and net radiation showed that T_f-based crop stress estimates will not relate well to crop production unless these environmental parameters are accounted for, or unless they vary little between times of measurement. By using the energy balance, T_f — T_a measurements can be related explicitly to g_v and, thereby, to crop production.

Key Words: Infrared thermometry • Crop diffusion conductance • Phaseolus vulgaris L. • Energy balance

² Former graduate research assistant (currently research scientist, Dep. of Land Resource Science, Univ. of Guelph, Guelph, Ontario, Canada, N1G 2W1) and associate professor of biometeorology, Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616.

Received for publication April 7, 1982.