Discussion

Our results with fava bean (Vicia faba) indicated little change in plant structure or biomass under an experimental global dimming environment where ambient PAR was reduced approximately 13%. We also found minimal additional differences with UV-B exclusion in the same environment. However, we found significant changes in leaf optical properties related to UV transmittance in both the reduced PAR and UV-B treatments.

The growth response to UV-B exclusion in this experiment was less than expected for a temperate-latitude crop grown at a high-elevation tropical location with high solar UV. Biologically effective UV radiation increases substantially from high to low latitudes, and there is also an increase with elevation (Caldwell et al., 1980). Because of its elevation and latitude, the study site on Mauna Kea, Hawaii, experiences some of the highest effective UV doses on the planet (Bodhaine et al., 1996). Thus, we were somewhat surprised that attenuation of these high UV fluxes had minimal effect on fava bean growth and morphology. Some small morphological changes in fava bean, without accompanying changes in above-ground mass, have been reported with UV-B exclusion in the Netherlands (Visser et al., 1997) and in a greenhouse UV-B supplementation experiment (Barnes et al., 1990a). However, under field UV-B supplementation at mid-latitudes, this variety of fava bean showed changes in leaf optical properties, but no changes in photosynthesis (Flint et al., 1985).

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