12 1 Richard H. Grant , Kent G. Apostol , and Hans F. Schmitz
1 Department of Agronomy, Purdue University, West Lafayette, IN 47907-2054, USA E-mail: [email protected] E-mail: [email protected] 2 Department of Biological Sciences, Bethel University, St. Paul, MN 55112, USA E-mail: [email protected]
Abstract Solar ultraviolet-B (UV-B) radiation affects the growth and physiology of different cultivars of soybean (Glycine max (L.) Merr.) in different ways. Consequently chronic exposure studies of various cultivars have indicated the full range of biomass productivity sensitivities to UV exposure: from highly sensitive and detrimental to tolerant and advantageous. Since most current and future exposures to UV radiation in mid-latitudes come as alternating periods of intense UV followed by periods of low UV, soybean sensitivity to UV should be studied as transient events. The effects of transient UV exposure on stomatal conductance (gs), UV-B absorbing compounds, pigment concentrations, photosynthesis (A), transpiration (E), and photosystem II efficiency on two soybean cultivars were evaluated. Soybean cultivars "Essex" and "Williams 82" were studied in a number of experiments under greenhouse conditions and in one study that was conducted in the field. Essex and Williams 82 cultivars responded differently to transient UV-B effects. In Essex, UV exposure initially resulted in increased gs and E, which over time became a decreased gs and E. Ultraviolet exposure initially resulted in decreased gs, which over time resulted in no change or increase in gs in Williams 28 after the first day. The UV exposure of Williams 82 plants had no statistically-supported effect on E. UV exposures and changes in gs did not affect A for either cultivar. The concentrations of carotenoids and chlorophyll a were not influenced by UV exposure in Essex, but increased significantly in Williams 82 in a short term exposure. UV-B absorbing compounds were substantially higher in Essex compared to Williams 82. After the initial responses to UV exposure, both Williams and Essex had reduced responses to additional UV exposures. This suggests that the overall long-term plant response to UV exposure is triggered by high levels of short term UV exposure.
Keywords soybean, transpiration, plant physiology, short-term exposure
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