Stress Measurement Using Foliage Temperatures 1

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 ½ 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 g v /g max , with g v max 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.