Failed CO2 Enrichment in a Polyhouse Tomato Crop
A commercial tomato grower in Chitwan installs CO2 enrichment in a polyhouse, raising daytime CO2 from 400 to 1,000 micromol/mol. Tomato is C3. Measurements at midday: air temperature 34 degrees C, relative humidity 35%, leaf-to-air VPD 3.4 kPa, stomatal conductance 0.12 mol m-2 s-1, leaf water potential -1.8 MPa, net photosynthesis 17 micromol m-2 s-1 (versus 18 at 400 ppm), and substantial midday wilting despite soil moisture at field capacity. The grower expected a 30-50% yield increase. He proposes increasing CO2 further to 1,500 ppm and adding more fertilizer. Another farmer growing maize (C4) nearby reports no benefit from CO2 enrichment. The greenhouse has no cooling or fogging system, and the irrigation schedule is unchanged. The grower asks whether to invest in a fogging system, more CO2, or shade netting.
1. Explain why CO2 enrichment did not increase net photosynthesis. Distinguish stomatal from non-stomatal limitation.
Read the worked answer
At 400 ppm, photosynthesis was 18; at 1,000 ppm, 17, so there was no gain. If CO2 were the limiting factor, adding it should raise photosynthesis. The measurements show stomatal conductance 0.12 mol m-2 s-1, VPD 3.4 kPa, leaf water potential -1.8 MPa, and wilting. High VPD draws water from the leaf; the plant closes stomata to limit transpiration. Closed stomata create a stomatal limitation: CO2 cannot diffuse to the chloroplasts, so the intercellular CO2 concentration does not rise enough to stimulate Rubisco, despite high ambient CO2. There is also non-stomatal limitation: at 34 degrees C, Rubisco activase and membrane stability are impaired, photorespiration is high in a C3 plant, and sink capacity may be limiting. The combination means the extra CO2 is wasted. More CO2 would not fix the water stress; it could even worsen it by encouraging stomatal closure.
2. Compare the expected response of tomato and maize to elevated CO2, with reasoning.
Read the worked answer
Tomato is C3: the first carboxylation product is 3-phosphoglycerate, and Rubisco also oxygenates RuBP (photorespiration). Under warm, high-light conditions, photorespiration can consume a large share of fixed carbon. Raising CO2 suppresses photorespiration and increases the CO2 concentration at Rubisco, so C3 plants often gain 20-40% growth when water and nutrients are adequate. Maize is C4: it uses PEP carboxylase to concentrate CO2 in bundle-sheath cells, so Rubisco already sees a high CO2/O2 ratio and photorespiration is minimal. C4 photosynthesis is therefore nearly CO2-saturated at current levels, and the nearby farmer's observation of little benefit is expected. C4 plants may benefit indirectly through improved water use because they can close stomata more. In this greenhouse, the tomato's potential gain is blocked by stomatal closure, not by lack of C4 machinery.
3. Recommend one intervention and predict its effect. Explain why adding more fertilizer would not help.
Read the worked answer
Recommend fogging/cooling to lower VPD and air temperature. If VPD falls from 3.4 to about 1.5-2.0 kPa and leaf temperature drops, stomata can reopen; conductance should rise, intercellular CO2 should increase, and the 1,000 ppm CO2 can then stimulate photosynthesis. Shade netting would reduce heat but also reduce light, possibly lowering photosynthesis; it may be a partial option. More CO2 is futile while stomata are closed. More fertilizer would not help because the limitation is water/vapour pressure and carbon supply, not nutrients; excess fertilizer raises soil salinity and can worsen water uptake, causing further wilting. Monitor leaf water potential and conductance to confirm.