The effects of oxides of nitrogen and carbon dioxide enrichment on photosynthesis and growth of lettuce (Lactuca sativa L.)

summary The response of glasshouse crops to the nitrogen oxide pollutants which may be generated during enrichment with O 2 has been studied in controlled environments. Lettuce ( Lactuca sativa L. cv. Ambassador) was grown in air containing either low CO 2 , (380μmol mol −1 ), high CO 2 , (1200 μmol mol −1 ), or high CO 2 plus oxides of nitrogen? (NO x )‐ Carbondioxide enrichment increased the rate of emergence and expansion of leaves and the growth of Young plants. Addition of NO x (2μmol mol −1 NO and c. 0.5 μmol mol −1 NO 2 ) to CO 2 ‐enriched air significantly reduced the yield, compared with the ‘clean, high CO 2 treatment, without producing visible symptoms of toxicity. Fumigation of single plants in high CO 2 , with NO x rapidly inhibited photosynthesis per unit leaf area. This did not appear to be due to a reduction in stomatal conductance. Removal of NO x from the atmosphere caused a rapid and complete recovery in the rate of photosynthesis. Studies were made of the effects of growing Plants for long periods in atmospheres containing high CO 2 and NO x on the photosynthetic capacity of single leaves when measured in NO x ‐free air. The decrease in photosynthetic rate as the fourth leaf aged occurred earlier in plants grown in CO 2 ‐enriched air than in those from the low CO 2 treatment. Leaves which developed in the CO 2 ‐enriched air containing NO x did not suffer any long‐term damage to photosynthetic activity in comparison with those of the ‘clean’ high CO 2 . In mature leaves the principal long‐term effect of enrichment (with or without NO x was to reduce the rate of photosynthesis in saturating CO 2 . In contrast, there was less effect on the rate of Photosynthesis in low CO 2 . The absence of a long‐term effect of NO x , on the photosynthetic capacity suggested that photosynthesis by the lettuce crop will be inhibited only during the transient periods of NO x accumulation in the glasshouse.