Ultraviolet-B and photosynthetically active radiation environment of inclined leaf surfaces in a maize canopy and implications for modeling

Abstract

The potential increase in ultraviolet-B (UVB) flux density and potential decrease in productivity of agricultural crops due to stratospheric ozone loss requires knowledge of the characteristics of UVB flux density above and within crops. Measurements of the photosynthetically active (PAR) and UVB radiation on inclined surfaces above and within a maize ( Zea mays L.) canopy were made in 1990. Sunlit surfaces within the canopy had similar UVB and PAR relative flux density levels to that above the canopy. Shaded surfaces within the canopy had very different relative flux density in the UVB and PAR as a result of differences in sky view, diffuse fraction, and orientation. These differences can be expected to result in canopy transmittance estimation errors of the order of 0.1 in the UVB and 0.05 in the PAR when one assumes an invariant diffuse radiation of leaves with differing aspect. The diffuse fraction of global radiant flux density influenced the radiation regimes present in the canopy. Two distinct radiation regimes were found for surfaces throughout the canopy in the relatively low diffuse fraction PAR waveband; a sunlit regime and a deep shade regime. Three distinct radiation regimes were evident for the relatively high diffuse fraction UVB waveband for surfaces at a height with cumulative leaf area index (LAI) of 1; a sunlit regime, a deep shade regime, and a light shade regime. At greater cumulative LAI, this additional light shade regime in the UVB was absent. Therefore, the bi-modal approach of modeling sunlit or shaded radiation regimes is adequate to estimate the photosynthetically active photon flux density throughout the canopy as well as to estimate the UVB radiant flux density when the cumulative canopy LAI was 2. At lower LAI, the UVB radiant flux density should be modeled using either a three-dimensional model or a model partitioning the gap probability into large and small gaps in the canopy. The necessity for two shade sub-models to model the UVB radiation environment apparently depends on the size distribution of gaps in the canopy.