Electronic Supplemental Material
Supplemental Methods
We used a portable gas exchange system (LI 6400, Li-Cor Biosciences, Lincoln, NE) to measure changes in leaf photosynthesis with albedo. For each soybean isoline, we made four light response curves by varying Photosynthetic Photon Flux Density (PPFD) between 0 and 1000 µmol m-2 s-1under stable temperature and CO2 concentrations (20 °C, 370 ppm).
We used a two-stream radiation transfer model to convert leaf level hemispherical reflectance and transmittance to approximate surface albedo (Dickinson, 1983).
Supplemental Results
Effect of albedo changes on leaf level photosynthesis
There was no significant difference (P > 0.05) in maximum photosynthetic capacity (PPFD > 1000 µmol m-2 s-1) between isolines differing in trichome density (sparse and dense), orientation (appressed, curly, erect), or color (tawny and grey). Photosynthetic capacity was not significantly lower (P0.05) in soybeans with trichomes than in glabrous varieties. Varieties had no significant differences in the quantum yield between light levels of 0-100 µmol m-2 s-1 (P0.05).
Scaling albedo from leaf to canopy scale
We converted leaf-level reflectance and transmittance to surface albedo using a two-stream radiation transfer model. Leaves with trichomes reflected 0.017 more incident radiation in the NIR than leaves without trichomes (-0.023*0.66 - 0.004*0.33) (we assume about 66% of energy with wavelengths greater than 700nm is between bands 700-1400nm, and 33% is in wavelengths greater than 1400nm), and did not reflect more in the visible. Leaves with trichomes transmitted 0.041 less incident radiation in the NIR than leaves without trichomes (0.34*0.66 + 0.55*.33) and 0.033 less in the visible. This leads to a noontime surface albedo increase of 0.025 in the NIR and 0.007 in the visible assuming an LAI of 4. If we assume an equal partitioning of energy in the visible and the NIR, then surface albedo would increase by ~0.016 by exchanging soybeans with trichomes for soybeans without trichomes.
Supplemental discussion
Can trichomes increase crop albedo?
Soybean albedo decreased in isolines with trichomes versus glabrous varieties. This result was unexpected. Trichomes increased reflectance but decreased transmittance by a greater amount leading to a net increase in absorption. Although our results are contrary to our expectations, we demonstrate that by changing isolines, surface albedo can be changed by ~0.016. We also tested soybean isolines with different trichome orientation, color, and density to find the permutation that would maximize albedo. We found few significant differences between the different trichome isolines and none of the significant differences in albedo were greater than 0.01 (Table 1).
Isolines of soybeans with modified trichomes may have benefits aside from increased albedo, such as providing a mechanical barrier against insect infestation (Johnson, 1975). Comparisons of soybeans with dense trichomes to normal soybeans demonstrate that trichome modification does not necessarily lead to yield declines (Zhang et al., 1992) or reducing CO2 uptake (Baldocchi et al., 1983).
Trichomes in certain plants, such as Encelia farinosa, can greatly increase leaf albedo (Ehleringer and Bjorkman, 1978) due to air-filled trichomes that efficiently scatter photons. These air filled trichomes substantially decreased absorption, especially in the visible wavelengths. However, there was a net increase in photosynthesis in Encelia farinosa because the decrease in photosynthesis due to reduced absorption in the visible was offset by a reduction in temperature stress on photosynthesis due to cooler leaves in the hot desert environment. Soybean trichomes are less reflective than Encelia trichomes, possibly because wild soybeans were originally found in southern China (Xu et al., 2002) in a humid climate where they would be unlikely to need to develop brighter leaves to avoid water stress.
The genetic control of trichome development in Arabidopsis is well understood (Larkin et al., 2003; Larkin et al., 1996; Schwab et al., 2000). Future efforts to increase crop albedo through modifying plant morphology could use this genetic understanding of trichomes to research whether Encelia type trichomes could be added to commercial crops.
Modeling differences with the CAM 4.0
The model simulations were run using the older version of the NCAR model (CLM 3.0), but there is a newer version now available (CLM 4.0). The largest difference between the newer and older version with relevance to this study is that crop NIR reflectance was reduced from 0.58 (as used in the current manuscript) to 0.35 because simulated cropland albedos were found to be too high compared to MODIS albedo data. A CLM4.0 technical note describing the new optical properties and a submitted paper can be found at http://www.cesm.ucar.edu/models/cesm1.0/clm/index.shtml. However, our results are unlikely to be greatly affected by this change because we were measuring the potential impact of a change in albedo not of absolute albedo itself.
Supplemental Figures
SM Figure 1 – A comparison of changes in crop leaf albedo to changes in annually averaged total grid cell albedo. Grey dots are agricultural pixels <30˚ N and S and black dots are agricultural pixels >30˚ N and S. Grid cell leaf albedo varies based on percentage of pixel coved with agriculture and changes in leaf level reflectance among three simulations (increasing leaf reflectance by 0.05, 0.1 and 0.15).
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