| dc.description.abstract | Epicuticular waxes are hypothesized to enable plants to cope with drought. There is evidence that waxes alter the energy balance of plants through increase in reflectivity of solar radiation and through decrease in conductance of water vapor from the leaf to the atmosphere. Under radiation load from the sun, increase in reflectivity should lead to a decrease in leaf and canopy temperature, whereas decrease in conductance should lead to increase in leaf and canopy temperature because of decrease in evaporative cooling. It is not clear how these competing effects exert control over water use in a crop such as sorghum [Sorghum bicolor (L.) Moench], which is known to resist drought. Experiments were conducted to determine the effects of waxes on spectral reflectivity, stomatal conductance, and energy balance of near-isogenic lines of grain sorghum having different levels of leaf epicuticular wax. Energy balances under field conditions were determined with the Bowen ratio method. At the leaf level, waxes increased reflectivity of solar radiation, but decreased transmissivity, and, as a result, small differences in absorptivity were observed between waxy and bloomless leaves.
Waxes had a negligible effect on the emissivity of longwave radiation. At the canopy level, waxes reduced net radiation of canopies by 3 to 5% compared to that of a non-waxy canopy. An overall 2% increase in albedo was the main driver for those differences, and about 86% of the reflected energy originated from near-infrared wavelengths. Rainfall was an important factor modulating the responses of bloomless plants. When water was non-limiting, waxes caused a relative decrease in conductance that was greater than the relative increase in reflectivity. Consequently, at the expense of higher canopy temperatures, waxes caused a 5% reduction in latent heat flux. Relative differences in energy partitioning between the phenotypes changed as a drying cycle progressed. These results suggest that epicuticular waxes enabled plants to have a better control over transpiration. This study helped elucidate the biophysical mechanisms through which epicuticular waxes influence the water and energy relations of sorghum. This information may aid plant scientists in selecting phenotypes that are better suited to cope with water deficits | en |
