Fig 1.
Average absorption (red line) of leaves from tomato, pepper, basil and broccoli.
Variation among species is due to differences in leaf reflectance. The broccoli leaf had slightly higher reflectance of PAR than the other species. All plants were grown in a greenhouse.
Fig 2.
Radiance spectrum from four radiation sources (black line) and average leaf absorbance (red line).
Electric lights (HPS and LED) output most of their radiation in the photosynthetic regions. Sunlight has significant NIR radiation, but this is poorly absorbed by leaves.
Fig 3.
Leaf temperature response to air temperature.
Vapor pressure deficit was held constant. Calculation is based on the average absoprtion of the four species measured. Differences among species were biologically insignificant.
Table 1.
Incident radiation, fraction absorbed, and total absorbed radiation for each source.
The absorbed radiation was normalized to a PPF of 1000 µmoles per m2 per s for each radiation source. This does not result in exactly equal PAR (in watts per m2) because of spectral differences among radiation sources. The total absorbed radiation for each source is shown in bold. Leaf temperature was held constant at 25°C. Net longwave exchange with lower leaves or surfaces was assumed to be zero.
Fig 4.
Calculated effects of environmental conditions on the difference between leaf temperature and air temperature under four radiation scenarios.
Fig 5.
Calculated effects of PPF on the difference between leaf temperature and air temperature under four radiation scenarios in near worst-case conditions of water stress and low wind.
Calculation is based on the average absoprtion of the four species measured. Differences among species was biologically insignificant.