Field crop responses to ultraviolet-B radiation: a review

This paper provides an overview of existing literature on the ultraviolet-B (UV-B) radiation effects on field crops. Earlier reviews on field crop responses to UV-B considered few physiological processes or crops. For this review, we easily located about 129 studies on 35 crop species published since 1975. Here, we report the effects of UV-B radiation on visual symptoms, leaf ultrastructure and anatomy, photosynthetic pigments, UV-B absorbing compounds, photosynthesis, growth and development, yield, genotypic differences, and finally, interactions of UV-B with abiotic and biotic factors of crop plants. Experiments conducted in glasshouses, in closed and open top chambers, and under field conditions, with varying source (solar or artificial) and intensity of photosynthetically active radiation (PAR, 50-1800 mumol m(-2) s(-1)) and UV-B (0-50 kJ m(-2) per day) are included. It is easy to conduct experiments that purport to evaluate the effects of projected UV-B intensities on crop species by providing supplemental irradiance with lamps or by reducing UV-B with filters; however, it is very difficult to simulate UV-B irradiance spectral changes that are likely to occur in nature. Collated results for each process are presented as percent change from control along with the experimental conditions in tabular format. Many of the studies showed dramatic effects of UV-B radiation, but under conditions with supplemental UV-B irradiance that was higher than would ever occur outside experimental conditions or in which the longer wavelengths in the PAR and UV-A, which moderate UV-B effects, were greatly reduced. Only 25 of the studies reviewed used experimental conditions and supplemental UV-B irradiance that approached realism. However, unrealistic the experimental conditions might be, an increase in understanding of basic plant physiology was gained from most of the studies. Visual symptoms consisting of chlorotic or necrotic patches on leaves exposed to UV-B were not unique. Both vegetative and reproductive morphology were altered by UV-B radiation. Leaf anatomy was altered due to changes in thickness of epidermal, palisade, and mesophyll layers. Enhanced UV-B generally decreased chlorophyll content (10-70%), whereas it increased UV-B absorbing compounds (10-300%) in many crops. Decrease in photosynthesis (3-90%), particularly at higher UV-B doses, was due to both direct (effect on photosystem) and indirect (decrease in pigments and leaf area) effects. The decreases in chlorophyll pigments and photosynthesis resulted in lower biomass and yield of most crop plants. Genotypes of crop species exhibited variability in leaf wax layer thickness, loss of chlorophyll, and increase in phenolics as mechanisms of tolerance to enhanced UV-B radiation resulting in changes in biomass/yield. Results from the few studies on interaction of UV-B with other abiotic and biotic factors did not lead to useful conclusions. Studies are needed to quantify the effects of UV-B radiation on crops in order to develop dose response functions that can facilitate development of dynamic simulation models for use in UV-B and other environmental impact assessments. (C) 2003 Elsevier B.V. All rights reserved.