Should a Tomato Grower Buy CO2 Enrichment or Better Cooling?
A polyhouse tomato grower near Nadia wants to buy a CO2 enrichment system. The seller claims that raising CO2 from 400 to 800 µmol mol−1 will 'double photosynthesis'. The grower's agronomist measures net photosynthesis (µmol CO2 m−2 s−1) with an infrared gas analyser under two light levels: at PPFD 250 µmol m−2 s−1 (the current 50% shade net), values are 5.8 at 400 ppm and 6.3 at 800 ppm; at PPFD 1200 µmol m−2 s−1, values are 13.5 at 400 ppm and 24.0 at 800 ppm. Dark respiration is 1.1 µmol CO2 m−2 s−1. The house is ventilated poorly, and summer leaf temperature reaches 38°C when the shade net is removed; under those conditions net photosynthesis falls to about 9 µmol m−2 s−1 even at 800 ppm. The grower has budget for either better ventilation and cooling or CO2 enrichment, not both, and wants a decision before the next planting.
1. Using the measured values, decide whether light or CO2 is the primary limiting factor at the grower's current PPFD. Support your answer with a calculation.
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Compare the CO2 response at the two light levels. At PPFD 250, doubling CO2 from 400 to 800 raises net photosynthesis from 5.8 to 6.3, an increase of 0.5/5.8 = 8.6%. At PPFD 1200, the same doubling raises it from 13.5 to 24.0, an increase of 10.5/13.5 = 78%. The CO2 response is therefore strongly light-dependent: at low light the photosynthetic apparatus is already limited by the rate at which light-driven electron transport can supply ATP and NADPH, so extra CO2 cannot be used efficiently. Under the current shade net, light is the primary limiting factor and CO2 is close to saturation for that light level.
The correct interpretation is co-limitation rather than a single limiting factor. CO2 becomes strongly limiting only when PPFD is high enough. Buying CO2 enrichment while keeping 50% shade would give roughly a 9% gain in leaf photosynthesis, unlikely to repay the capital and gas costs. Raising light first, by removing or thinning the shade and improving house transmission, would both increase photosynthesis directly and make any later CO2 investment far more effective.
2. At PPFD 1200 and 800 ppm, calculate gross photosynthesis and the fraction of gross carbon gain lost to dark respiration. Why is net photosynthesis the relevant measure for fruit yield?
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Net photosynthesis = gross photosynthesis − respiration, so gross ≈ 24.0 + 1.1 = 25.1 µmol CO2 m−2 s−1. The respiratory loss is 1.1/25.1 = 4.4% of gross carbon fixation. That seems small, but it is a continuous loss over 24 hours from all respiring tissues, not only leaves, and it rises with temperature.
Net photosynthesis is the relevant measure because it is the carbon that remains available for growth, storage and fruit filling after respiratory carbon loss. Gross photosynthesis includes carbon fixed and immediately respired and therefore overstates the plant's carbon budget. Two cautions are needed. Dark respiration measured at night is not identical to mitochondrial respiration in the light, and photorespiration is not included in the dark measurement. Also, instantaneous leaf net photosynthesis is only a proxy for yield: cumulative canopy net carbon gain over the season and the fraction partitioned to fruit determine the harvest. A treatment that raises leaf photosynthesis but delays fruit set or increases vegetative growth may not raise yield.
3. After the shade net is removed, leaf temperature reaches 38°C and net photosynthesis at 800 ppm falls to about 9 µmol m−2 s−1. Explain the likely causes and choose between ventilation/cooling and CO2 enrichment.
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The fall from 24.0 to about 9 µmol m−2 s−1 has two possible components. Stomatal limitation: at 38°C the leaf-to-air vapour pressure deficit is high, so stomata close partly, reducing the supply of CO2 to the mesophyll. Non-stomatal limitation: Rubisco activase and components of the thylakoid membrane are heat-labile, photorespiration increases relative to carboxylation, and mitochondrial respiration rises, so even a high internal CO2 concentration cannot restore the earlier rate. The fact that 800 ppm CO2 does not rescue photosynthesis suggests that non-stomatal limitation, or severe stomatal closure, is dominant. Measuring stomatal conductance and intercellular CO2 would distinguish these, but the management decision does not need to wait for that.
Adding more CO2 to a leaf whose stomata are closed, or whose Rubisco is heat-damaged, is ineffective. Ventilation and cooling reduce leaf temperature and vapour pressure deficit, restore stomatal conductance, protect the photosynthetic apparatus, and also reduce heat-induced pollen sterility. The grower should invest in ventilation/cooling first and consider CO2 only after light and temperature are optimised. That sequence follows directly from the measured interaction between light, temperature and CO2.