Effects of supplemental ultraviolet-B radiation on the growth and physiology of field-grown soybean

Abstract

Soybeans [ Glycine max (L) Merr. cv Essex and Williams] were grown outdoors in field plots during 1983 and 1984 under ambient and 2 supplemental levels of biologically effective ultraviolet-B (UV-B) radiation (daily supplemental dose: 3.0 or 5.1 effective kJ/m 2). These correspond to a 16 and 25% ozone reduction during clear sky conditions on the summer solstice. In 1983, neither cultivar showed any significant differences in growth, CO 2 assimilation, transpiration, stomatal conductance nor leaf water potential under enhanced levels of UV-B irradiation. By contrast in 1984, during late vegetative and early reproductive stages of development, leaf area and CO 2 assimilation in Essex were significantly reduced while stomatal conductance was increased by UV-B irradiation. At a later reproductive stage (full pod), UV-B induced effects were no longer evident except for a decrease in stomatal conductance. Williams remained unaffected throughout the entire season. These intraspecific response differences between years may be related to the frequency of drought and overcast skies. In 1983, plants experienced drought for long periods up to the full bloom stage, whereas in 1984 drought was less frequent and occurred only after the full pod stage. The study illustrates that the effectiveness of enhanced levels of UV-B radiation is related to several factors, including microclimatic conditions prevailing during the growing season, cultivar, and the stage of plant development.