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

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Abstract
The response of glasshouse crops to the nitrogen oxide pollutants which may be generated during enrichment with O2 has been studied in controlled environments. Lettuce (Lactuca sativa L. cv. Ambassador) was grown in air containing either low CO2, (380μmol mol−1), high CO2, (1200 μmol mol −1), or high CO2 plus oxides of nitrogen? (NOx)‐ Carbondioxide enrichment increased the rate of emergence and expansion of leaves and the growth of Young plants. Addition of NOx (2μmol mol−1 NO and c. 0.5 μmol mol−1 NO2) to CO2‐enriched air significantly reduced the yield, compared with the ‘clean, high CO2 treatment, without producing visible symptoms of toxicity. Fumigation of single plants in high CO2, with NOx rapidly inhibited photosynthesis per unit leaf area. This did not appear to be due to a reduction in stomatal conductance. Removal of NOx 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 CO2 and NOx on the photosynthetic capacity of single leaves when measured in NOx‐free air. The decrease in photosynthetic rate as the fourth leaf aged occurred earlier in plants grown in CO2‐enriched air than in those from the low CO2 treatment. Leaves which developed in the CO2‐enriched air containing NOx did not suffer any long‐term damage to photosynthetic activity in comparison with those of the ‘clean’ high CO2. In mature leaves the principal long‐term effect of enrichment (with or without NOx was to reduce the rate of photosynthesis in saturating CO2. In contrast, there was less effect on the rate of Photosynthesis in low CO2. The absence of a long‐term effect of NOx, on the photosynthetic capacity suggested that photosynthesis by the lettuce crop will be inhibited only during the transient periods of NOx accumulation in the glasshouse.