AbstractA new modulated lamp system is described. This system has successfully provided an ultraviolet‐B (UV‐B) supplement in proportion to ambient UV‐B. The modulated system was used to simulate the UV‐B environment resulting from an annual mean reduction of 15 in the stratospheric ozone under UK field conditions, but taking account of seasonal variation in depletion. The effects of this enhanced level of UV‐B on the growth, physiology and yield of four cultivars of pea were assessed.Enhanced UV‐B resulted in small reductions in the number of stems and total stem length per plant (respectively 4.7 and 8.7). There were also significant decreases in the dry weight of peas (10.1), pods (10.3) and stems (7.8) per plant. UV‐B treatment had no effect on the number of peas per pod or average pea weight, but did significantly reduce (12.1) the number of pods per plant. This decrease in pod number was partly due to enhanced abscission of pods during the final month of plant growth. UV‐B treatment had no significant effect on chlorophyll fluorescence characteristics or CO2assimilation rate per unit leaf area. These results are consistent with previous controlled environment experiments, and suggest that reduction in yield may be due to direct effects of UV‐B on plant growth rather than a decrease in photosynthetic capacity p
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