Epicuticular wax and its effect on canopy temperature and water use of Sorghum

Epicuticular wax is thought to enable sorghum [Sorghum bicolor (L.) Moench] plants to cope with drought. Increased reflectivity of solar radiation and reduced conductance of water vapor are mechanisms responsible for aiding plants in water conservation. Increased reflectivity should lead to a decrease in canopy temperature and water use, whereas decreased conductance should lead to an increase in canopy temperature because of a decrease in evaporative cooling. It is not clear how these competing effects exert control over water use in a crop such as sorghum. To better understand the role of epicuticular waxes on the energy balance of sorghum, experiments were conducted to determine the effects of waxes on field-scale energy fluxes using near-isogenic lines of grain sorghum having different levels of epicuticular wax loading. Waxes caused an overall 2% increase in albedo, and about 86% of the reflected energy between 400 and 1100 nm was from near-infrared wavelengths. This is at variance with recent reports that suggest that waxes can substantially increase the reflectivity of sorghum. Instead, our results indicated that the amount of reflected radiant energy due to waxes was small. When water was non-limiting, waxes caused a 22% decrease in canopy conductance compared to a 2% increase in albedo on average. Consequently, at the expense of higher canopy temperatures, waxes caused a 5% reduction in latent heat flux when water was not limiting. Without rain, water became limiting to plants with lower wax load sooner than it did for plants with higher wax load. The high wax load canopy then was on average 0.4 °C cooler, had 24% greater canopy conductance and 13% greater latent heat fluxes compared to the low wax one. Results suggested that the primary mechanism through which waxes affect the energy balance of sorghum is by means of reduced conductance of water vapor.