Article Title - Analysis of environmental effects on leaf temperature under sunlight, High Pressure Sodium and Light Emitting Diodes

The use of LED technology is commonly assumed to result in significantly cooler leaf 1 temperatures than high pressure sodium technology. To evaluate the magnitude of this 2 effect, we measured radiation incident to and absorbed by a leaf under four radiation 3 sources: clear sky sunlight in the field, sunlight in a glass greenhouse, and indoor plants 4 under either high pressure sodium or light emitting diodes. We then applied a common 5 mechanistic energy-balance model to compare leaf to air temperature difference among 6 the radiation sources and environments. At equal photosynthetic photon flux, our 7 results indicate that the effect of plant water status and leaf evaporative cooling is much 8 larger than the effect of radiation source. If plants are not water stressed, leaves in all 9 four radiation sources were typically within 2°C of air temperature. Under clear sky 10 conditions, cool sky temperatures mean that leaves in the field are always cooler than 11 greenhouse or indoor plants-when photosynthetic photon flux, stomatal conductance, 12 wind speed, vapor pressure deficit, and leaf size are equivalent. As water stress increases 13 and cooling via transpiration decreases, leaf temperatures can increase well above air 14 temperature. In a near-worst case scenario of water stress and low wind, our model 15 indicates that leaves would increase 6°, 8°, 10°, and 12°C above air temperature under 16 field, LED, greenhouse, and HPS scenarios, respectively. Because LED fixtures emit 17 much of their heat through convection rather than radiative cooling, they result in 18 slightly cooler leaf temperatures than leaves in greenhouses and under HPS fixtures, but 19 the effect of LED technology on leaf temperature is smaller than is often assumed. 20