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 Rumexpatientia L. exposed to four levels of u.v.-B irradiation (280–315nm). When weighted for biological effectiveness u.v.-B_BE thelowest level of this radiation employed, was less than thatcalculated for normal cloudless days in April. This resultedin detectable depressions of photosynthesis and leaf growth.Reciprocity was demonstrated in the reduction of photosynthesisby u.v.-B_BE. However, unlike photosynthesis, reduction of leafexpansion by u.v.-B_BE did not show a reciprocal mode of action.Leaf expansion was reduced as a function of the level of u.v.-B_BEand only during the initial days of exposure. A model was developed to predict reductions of net photosynthesisand leaf expansion rates under increased u.v.-B_BE correspondingto any atmospheric ozone concentration reduction. After 35 dsimulation, the calculated reductions in photosynthetic ratesfor a leaf initiated on June 1 were 6, 21, and 42 percent foru.v.-B_BE corresponding to atmospheric ozone reductions of 5,15, and 40 percent, respectively. Predicted reductions in photosyntheticcapacity and leaf expansion rates for leaves active before Junewere less severe. This is due to the lower normal u.v.-B_BE atthis time of year because of greater angles from the zenithand higher atmospheric ozone concentrations. A test of the modelshowed a good correspondence between the predicted and measuredreduction in photosynthesis of leaves exposed to supplementalu.v.-B_BE irradiation under field conditions.