Atmospheric Ozone Depletion: Reduction of Photosynthesis and Growth of a Sensitive Higher Plant Exposed to Enhanced u.v.-B Radiation

Net photosynthesis and leaf growth were determined for Rumex patientia L. exposed to four levels of u.v.-B irradiation (280–315 nm). When weighted for biological effectiveness u.v.-BBE the lowest level of this radiation employed, was less than that calculated for normal cloudless days in April. This resulted in detectable depressions of photosynthesis and leaf growth. Reciprocity was demonstrated in the reduction of photosynthesis by u.v.-BBE. However, unlike photosynthesis, reduction of leaf expansion by u.v.-BBE did not show a reciprocal mode of action. Leaf expansion was reduced as a function of the level of u.v.-BBE and only during the initial days of exposure. A model was developed to predict reductions of net photosynthesis and leaf expansion rates under increased u.v.-BBE corresponding to any atmospheric ozone concentration reduction. After 35 d simulation, the calculated reductions in photosynthetic rates for a leaf initiated on June 1 were 6, 21, and 42 percent for u.v.-BBE corresponding to atmospheric ozone reductions of 5, 15, and 40 percent, respectively. Predicted reductions in photosynthetic capacity and leaf expansion rates for leaves active before June were less severe. This is due to the lower normal u.v.-BBE at this time of year because of greater angles from the zenith and higher atmospheric ozone concentrations. A test of the model showed a good correspondence between the predicted and measured reduction in photosynthesis of leaves exposed to supplemental u.v.-BBE irradiation under field conditions.