A theoretical basis for the leaf equivalence point temperature

Several recent crop stress and yield models depend on the difference between air and leaf temperature. When this difference is zero, the ‘equivalence point temperature’ is defined, if the crop is well-watered under sunny conditions. A simple energy budget equation is presented, which shows that the theoretical equivalence point temperature is quantitatively related to the absorbed radiation load, the vapor pressure deficit, the surface and aerodynamic resistance to water vapor transport, and the emissivity of the leaf. This implies that the theoretical equivalence point varies diurnally, rising in the morning, reaching a relative maximum near midday and falling in the afternoon and evening. At certain times and under certain conditions, the theoretical equivalence point may be physically impossible. The widely reported value of the equivalence point temperature, 33°C, is not supported by theory or empirical evidence.