The influence of ultraviolet-B (UV-B) radiation (280-320 nanometers) on the morphology
of 12 common dicot and monocot crop or weed species was examined to determine whether
any common responses could be found that might, in tum, be useful in predicting possible
changes in competitive balance under solar UV-B enhancement. Under glasshouse conditions,
UV-B exposure (simulating a 20% reduction in stratospheric ozone at Logan, Utah) was found
to reduce leaf blade and intemode lengths and increase leaf and axillary shoot production in
several species. Overall, the directions of these trends were similar in the majority of species
that exhibited a significant response. These morphological changes occurred without any significant
reduction in total shoot dry matter production. There was no clear distinction in the
response of crops and weeds, though monocots were found to be generally more responsive
than dicots. Previous work in dense canopies has shown that the photomorphogenetic effects
of UV-B alter leaf placement and thereby influence competition for light. Our results suggest
that, under these conditions, changes in competitive balance resulting from increased UV-B
might be expected more frequently when monocots are involved in mixtures, rather than
mixtures of only dicots