CO2 Enrichment in a Greenhouse: Does More Carbon Dioxide Always Pay?
Nadia grows tomatoes in a polytunnel in Malta. In January, inside CO2 is 410 µmol mol−1, temperature 22 °C, and maximum light at midday is 600 µmol m−2 s−1. Her measured net photosynthesis is 18 µmol CO2 m−2 s−1. When she raises CO2 to 1,000 µmol mol−1 at the same light and temperature, net photosynthesis rises to 26. At 1,200 µmol m−2 s−1 light, net photosynthesis is 22 at 410 and 38 at 1,000 µmol mol−1 CO2. Enrichment costs €1,200 per month and leakage means 35% of injected CO2 escapes. At 1,500 µmol mol−1 CO2, stomatal conductance falls by 40%, and at 30 °C dark respiration increases by 45% compared with 22 °C. Nadia wants to know whether to enrich all winter, only on bright days, or not at all.
1. Explain why the response to CO2 is greater at 1,200 than at 600 µmol m−2 s−1 light.
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At 600 µmol m−2 s−1 light, the light reactions supply ATP and NADPH at a modest rate, so CO2 fixation is limited by RuBP regeneration and electron transport rather than by CO2. Increasing CO2 from 410 to 1,000 raises net photosynthesis by 8 units (18→26), a 44% increase. At 1,200 µmol m−2 s−1, more ATP and NADPH are available; Rubisco becomes the limiting step and CO2 competes with O2, so raising CO2 suppresses photorespiration and gives a 16-unit increase (22→38), a 73% increase. The higher relative response at high light shows that CO2 and light are co-limiting; adding CO2 is most useful when light is already high.
2. Evaluate the three options using the data, including the effects of temperature and stomata.
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Enriching all winter is hard to justify: on dark days the CO2 response is small, and 35% leakage wastes gas. Bright days give the larger return. At 30 °C, dark respiration rises 45%, so the net gain is reduced; enrichment should be combined with ventilation or shading to keep temperature near 22–25 °C. Above about 1,000–1,200 µmol mol−1, stomatal conductance falls 40%, which limits CO2 entry and may also reduce transpiration and nutrient transport; 1,500 µmol mol−1 is unlikely to pay. Recommended: enrich to about 800–1,000 µmol mol−1 on bright days, monitor temperature and stomatal behaviour, and stop enrichment when light is below the point where the response is negligible.
3. Why might net photosynthesis not increase proportionally with CO2, and what other measurements would help diagnose the limitation?
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Net photosynthesis may plateau because of triose-phosphate utilisation or sink limitation (the leaf cannot export or use more sugar), because Rubisco activity or electron transport capacity is saturated, or because stomatal closure limits CO2 supply. Measure intercellular CO2 (Ci), chlorophyll fluorescence (Fv/Fm and electron transport rate), leaf temperature, and carbohydrate accumulation; a rising Ci with no increase in assimilation suggests sink limitation rather than stomatal limitation.