The Effects of CO2Enrichment and Nitrogen Oxides on some Calvin Cycle Enzymes and Nitrite Reductase in Glasshouse Lettuce

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Abstract
Glasshouse lettuce (cvs Pascal and Talent) was grown during late autumn and early winter in an atmosphere polluted with nitrogen oxides (NOx) generated from direct-fired natural gas burners used for CO2 enrichment and warm air heating (high CO2 + NOx treatment). Concentrations of 0.3–0.4 vpm NOx were detected during the daytime when near 3-fold CO2 enrichment (1 000 vpm) was practised without heating. In cold weather, the CO2 and NOx levels were dependent on the amount of heating required to maintain minimum temperatures of 5 °C (night) and 7 °C (day). Concentrations of between 2000–5000 vpm CO2 and 1–2.5 vpm NOx were recorded at night during an intensely cold period in early January just prior to sampling for leaf enzymes. The plants were compared with those grown in unpolluted atmospheres with either a natural (340 vpm) or an enriched level (1000 vpm) of CO2. Pascal grown in elevated CO2 had less activity per g fresh weight of RuBPc (E.C. 4.1.1.39), 3PGA phosphokinase(E.C. 2.7.2.3) and NADP-G3P dehydrogenase (E.C. 1.2.1.13) than plants grown in a normal ambient CO2 atmosphere. The cytoplasmic enzyme PEPc (E.C. 4.1.1.31) was not significantly affected by the pure CO2 enrichment. With high CO2 + NOx the activities of the Calvin cycle enzymes were restored to values close to those present in non-enriched plants, while the activity of PEPc was increased. The activity of nitrite reductase (NiR) (E.C. 1.7.7.1) was increased in Pascal and Talent by high CO2+NOx. Immunoblotting techniques were used to show that the increase in activity of this enzyme was accompanied by an increase in the steady-state concentration of the protein. Only one molecular form of NiR was detected by immunoblotting, and it would appear that the ‘induction’ of NiR activity resulted from increased net enzyme synthesis rather than activation of pre-existing enzyme. At the time of sampling no visible damage by high CO2+NOx was evident and the lack of symptoms may have been associated with the enhanced levels of nitrite reductase in these cultivars.

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