Invisible or White, but Always Bright

The optical properties of plant canopies control the Earth’s energy balance, and the vegetation net primary productivity, water use, and ability to withstand heat stress. Plant canopies reflect about 25% of the incoming solar radiation and re-emit some of the absorbed energy as thermal, invisible radiation. The foliage albedo –the fractional solar radiation reflected back to the atmosphere– and emissivity –the fractional emission in relation to a perfect emitter- control the radiation balance. Higher albedo would in theory reduce the canopy surface temperature (a net cooling effect) and reduce the demand of water for transpiration, ameliorating heat and water stress. The degree of cooling via thermal radiation is limited because the atmosphere is also an absorber and emitter in the thermal range, except in the atmospheric window, a narrow waveband that allows thermal radiation to escape from Earth. Here we propose to cool down plant canopies by (1) using nanoparticles specifically designed with optical properties that allow high albedo and temperature-dependent emissivity, and (2) by engineering plant canopies emissivity in the atmospheric window by altering plant hairs (trichomes) to reflect solar radiation and emit thermal radiation with high efficiency, properties recently discovered in the hair layer of the Saharan Silver ants. Our model plant is wheat. We propose field and greenhouse experiments to alter the temperature of leaves and spikes subjected to different coatings and environmental conditions, to study their optical properties when dusted on the plants, and to understand the wheat yield and water use response to these changes. The benefits are reduced heat stress and higher productivity, of critical importance in high input production systems and in semiarid environments encompassing subsistence agriculture. This project can provide transformational engineering to regulate the temperature of canopies, the temperature of the planet, canopy water use, and heat stress in wheat and other crops.