UV-B Biodosimetry in Turfgrass Canopies G. Y. Yuen,* C. C. Jochum, L. J. Giesler, M. D. Shulski, E. A. Walter-Shea, K. G. Hubbard, and G. L. Horst ABSTRACT and Lindow, 1995). Radiation in the UV spectrum ac- counts for less than 9% of the total incoming solar Phylloplane microorganisms are affected by ultraviolet (UV) radia- energy, but it has deleterious effects on all biological tion penetrating into plant canopies, but data as to the relationships between microorganism activity and canopy UV levels is lacking. systems. Wavelengths shorter than 280 nm (UV-C) are Current instrumentation and modeling systems are inadequate to effectively absorbed by the earth’s atmosphere, and analyze canopy radiation environments at scales relevant to microor- therefore, only UV-A (320-400 nm) and UV-B (280- ganisms. A biological dosimeter system was developed for measuring 320 nm) are considered to be biologically important at UV-B in turfgrass and other compact canopies. Cell suspensions of the earth’s surface (Young et al., 1993). Although there a DNA repair-deficient strain of Escherichia coli (CSRO6) were is a greater amount of UV-A than UV-B (roughly 10:1) enclosed in packets (0.2-mL volume) of UV-transmissible polyethyl- and both can cause cell death and mutation, UV-B ene. After the packets were exposed to sunlight, numbers of surviving causes more direct DNA damage and thus is the more bacteria were determined. The log percent survival was found to be harmful of the two. linearly related to accumulative UV-B dosage, as measured with a The response of microorganisms to UV varies among broad-band UV-B radiometer, but was not related to UV-A dosage. In one experiment, the performance of the biodosimeter system was taxa. Most bacteria and fungi isolated from leaves are compared with that of a miniature UV-B radiometer mounted in soil- chromogenic, suggesting that pigmentation may be a level tracks in eight plots of tall fescue (Festuca arundinacea Schreb.) protective adaptation to UV (Ayers et al., 1996; Sundin that varied in leaf area index (LAI). The two methods yielded similar and Jacobs, 1999). Genes that confer UV tolerance in mean transmittance values that decreased with increasing LAI, closely some phylloplane bacteria have been identified (Sundin fitting Beer’s law. A similar relationship was found in a second experi- et al., 1996; Willis et al., 1988). Foliar pathogens and ment, in which biodosimeter packets were placed at the base of undis- phylloplane saprophytes that also have an endophytic turbed tall fescue canopies. The packets also revealed considerable existence can survive unfavorable phylloplane condi- variation in transmittance possibly because of localized shading and tions such as UV through escape, i.e., gain entry into sun flecks in the natural canopies. In a third experiment, direct and the plant tissues (Beattie and Lindow, 1995; Wilson diffuse UV-B at different heights within a tall fescue canopy was measured by packets attached to narrow, flat wooden sticks simulating et al., 1999). Our understanding of the role of UV in grass leaves. This method has potential as a tool to capture the variabil- regulating microorganism numbers and distributions on ity in UV levels related to nonuniformity in canopy structure, depth the phylloplane, however, is based primarily on labora- in a canopy, and leaf orientation. tory research. The response to UV by phylloplane mi- crobes, including those used for biological control (Gaug- ler et al., 1992; Inglis et al., 1995; Ignoffo and Garcia, M icroorganisms (bacteria, fungi, and nematodes) 1978), was described in numerous reports, but few exam- are being developed and used for the control of ined microorganism populations on leaves in relation insect pests, plant pathogens, and weeds. Although most to natural UV irradiation. In a study by Newsham et of the effective biological control agents were selected al. (1997), numbers of phylloplane fungi on adaxial and in part for their ability to colonize leaf surfaces, they abaxial leaf surfaces varied with changes in levels of are often introduced to foreign plant environments and natural UV radiation and natural radiation supple- subjected to unfavorable microenvironmental condi- mented by lamps. In another study, Sundin and Jacobs tions to which they are not adapted. This reduces the (1999) found high numbers of pigmented, and thus UV applied agent’s population size, restricts its distribution tolerant, bacteria on leaves in the field and higher bacte- on leaf surfaces, and alters its metabolic activity, ulti- rial populations on abaxial surfaces than on adaxial sur- mately causing reduced biocontrol efficacy (Andrews, faces. They suggested that UV tolerance and avoidance 1992; Weller, 1988). Ultraviolet radiation is thought to strategies were necessary for surviving solar radiation. be an environmental factor that limits the survival and Although UV fluxes above canopies were measured in growth of microorganisms on the phylloplane (Beattie both studies, UV irradiance on plant surfaces was not assessed. In our own studies on bacterial biocontrol G.Y. Yuen, Dep. of Plant Pathology, Univ. of Nebraska, Lincoln, NE agents of turfgrass pathogens, the relative ability of bac- 68583-0722; C.C. Jochum, Dep. of Plant Pathology, Univ. of Nebraska, terial strains to colonize field turfgrass was related to Lincoln, NE 68583-0722; L.J. Giesler, Dep. of Plant Pathology, Univ. their tolerance of UV-A and UV-B irradiation demon- of Nebraska, Lincoln, NE 68583-0722; M.D. Shulski, Dep. of Soil, strated in the laboratory (Giesler, 1998). The bacteria, Water and Climate, Univ. of Minnesota, St. Paul, MN 55108; E.A. when applied to field turf, typically established higher Walter-Shea, School of Natural Resource Sciences, Univ. of Nebraska, Lincoln, NE 68583-0728; K.G. Hubbard, School of Natural Resource numbers in the lower canopy region as opposed to the Sciences, Univ. of Nebraska, Lincoln, NE 68583-0728; G.L. Horst, upper canopy, and this differential was negated when Dept of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE the turf was shaded (Giesler et al., 2000). These col- 68583-0724. Journal number 13386 in the Univ. of Nebraska Agricul- onization patterns could not be explained solely by mea- tural Research Division journal series. Received 16 May 2001. *Corre- sponding author (gyuen1@unl.edu). Abbreviations: CFU, colony-forming units; DOY, day of year; LAI, leaf area index; UV, ultraviolet. Published in Crop Sci. 42:859–868 (2002). 859