UV-B biodosimetry in turfgrass canopies

Phylloplane microorganisms are affected by ultraviolet (UV) radiation penetrating into plant canopies, but data as to the relationships between microorganism activity and canopy UV levels is lacking. Current instrumentation and modeling systems are inadequate to analyze canopy radiation environments at scales relevant to microorganisms. A biological dosimeter system was developed for measuring UV-B in turfgrass and other compact canopies. Cell suspensions of a DNA repair-deficient strain of Escherichia coli (CSRO6) were enclosed in packets (0.2-mL volume) of UV-transmissible polyethylene. After the packets were exposed to sunlight, numbers of surviving bacteria were determined. The log percent survival was found to be linearly related to accumulative UV-B dosage, as measured with a broad-band UV-B radiometer, but was not related to UV-A dosage. In one experiment, the performance of the biodosimeter system was compared with that of a miniature UV-B radiometer mounted in soil-level tracks in eight plots of tall fescue (Festuca arundinacea Schreb.) that varied in leaf area index (LAI). The two methods yielded similar mean transmittance values that decreased with increasing LAI, closely fitting Beer's law. A similar relationship was found in a second experiment, in which biodosimeter packets were placed at the base of undisturbed tall fescue canopies. The packets also revealed considerable variation in transmittance possibly because of localized shading and sun flecks in the natural canopies. In a third experiment, direct and diffuse UV-B at different heights within a tall fescue canopy was measured by packets attached to narrow, flat wooden sticks simulating grass leaves. This method has potential as a tool to capture the variability in UV levels related to nonuniformity in canopy structure, depth in a canopy, and leaf orientation.