CO2 enrichment in a tomato glasshouse: interpreting limiting factors
Tomato plants in a glasshouse are grown at 25 °C. Net carbon dioxide uptake is measured at two light intensities, low light (PPFD 200 µmol m−2 s−1) and high light (PPFD 1200 µmol m−2 s−1), and at two CO2 concentrations, 400 ppm and 1000 ppm. At low light, net uptake is 6.2 µmol m−2 s−1 at 400 ppm and 6.5 µmol m−2 s−1 at 1000 ppm. At high light, net uptake is 14.1 µmol m−2 s−1 at 400 ppm and 22.8 µmol m−2 s−1 at 1000 ppm. The grower is considering raising CO2 to 1500 ppm. At 1500 ppm and high light, net uptake is 23.4 µmol m−2 s−1. On a hot afternoon, when leaf temperature reaches 32 °C, many stomata close. The glasshouse CO2 supply is expensive and the gas-tightness of the structure is limited.
1. Explain why increasing CO2 from 400 to 1000 ppm has little effect at low light but a large effect at high light.
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At low light, net uptake increases only from 6.2 to 6.5 µmol m−2 s−1, an increase of about 4.8%. Light is the limiting factor. The light-dependent reactions produce ATP and NADPH at a low rate, so the Calvin cycle cannot fix much more CO2 even when more CO2 is available. Rubisco activity and RuBP regeneration are limited by the supply of ATP and NADPH, not by CO2 concentration. At high light, the increase is from 14.1 to 22.8 µmol m−2 s−1, about 61.7%. The light reactions now supply more ATP and NADPH, so CO2 becomes the limiting factor. A higher CO2 concentration increases the rate of carboxylation by rubisco and reduces competition from oxygen at the active site, which lowers photorespiration in this C3 crop. The different responses show that the effect of enriching CO2 depends on which other factor is limiting.
2. Calculate the percentage increase from 1000 to 1500 ppm at high light and advise the grower whether 1500 ppm is worthwhile.
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From 1000 to 1500 ppm, net uptake rises from 22.8 to 23.4 µmol m−2 s−1. The increase is (23.4 − 22.8) ÷ 22.8 × 100 = 2.6%. This is very small compared with the 61.7% gain from 400 to 1000 ppm. The response is showing diminishing returns because another factor, probably rubisco capacity or electron transport, is becoming limiting. Raising CO2 from 1000 to 1500 ppm means supplying 50% more CO2 for a 2.6% increase in net photosynthesis, which is unlikely to repay the cost of gas and the loss through a leaky glasshouse. In addition, at 32 °C stomata close, so the extra CO2 cannot enter the leaf efficiently. The grower should maintain about 1000 ppm and invest in cooling, ventilation and humidity control to keep stomata open. A cost–benefit analysis using fruit yield, not just short-term CO2 uptake, is needed before choosing 1500 ppm.
3. Explain why net CO2 uptake is used rather than gross photosynthesis, and suggest one variable that should be controlled in this comparison.
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Net CO2 uptake is the difference between the CO2 fixed by photosynthesis and the CO2 released by respiration. It represents the actual carbon gain of the plant and is therefore directly relevant to growth and yield. Gross photosynthesis would measure only fixation and would not account for respiratory losses, which increase with temperature. For the comparison to be valid, leaf temperature should be controlled or measured, because the two light treatments and the high CO2 treatment may alter leaf temperature and stomatal opening. Humidity, leaf age, leaf area and the time of day should also be standardised. If stomata close at 32 °C, the measured net uptake will fall even if the photosynthetic machinery is capable of more, so the comparison would confound CO2 effect with stomatal limitation.