Limiting factors and investment in a tomato glasshouse
A commercial tomato grower in a controlled glasshouse measures net CO2 uptake of a leaf at 25 °C. At 400 µmol mol−1 CO2 and 800 µmol m−2 s−1 light, net uptake is 18 µmol CO2 m−2 s−1. Raising CO2 to 800 µmol mol−1 at the same light increases net uptake to 26. Raising light to 1200 µmol m−2 s−1 at 400 µmol mol−1 CO2 increases net uptake to 19. At 800 µmol mol−1 CO2 and 1200 µmol m−2 s−1 light, net uptake is 28. When leaf temperature is raised to 35 °C at 800 µmol mol−1 CO2 and 1200 µmol m−2 s−1, net uptake falls to 22. Dark respiration at 25 °C is 3 µmol CO2 m−2 s−1 and at 35 °C is 7 µmol CO2 m−2 s−1. The grower can either install CO2 enrichment (cost £12,000) or supplementary lighting (cost £20,000). The glasshouse is already at 400 µmol mol−1 CO2 and 800 µmol m−2 s−1 for 12 hours per day.
1. Calculate gross photosynthesis at 25 °C and at 35 °C under 800 µmol mol−1 CO2 and 1200 µmol m−2 s−1 light. Explain why net photosynthesis falls more than gross photosynthesis.
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Net photosynthesis = gross photosynthesis - respiration, so gross = net + respiration.
At 25 °C, 800 µmol mol−1 CO2 and 1200 µmol m−2 s−1 light:
- gross = 28 + 3 = 31 µmol CO2 m−2 s−1.
At 35 °C, same CO2 and light:
- gross = 22 + 7 = 29 µmol CO2 m−2 s−1.
The gross rate falls by only 2 µmol m−2 s−1, but net rate falls by 6, from 28 to 22. The extra fall is because respiration rises from 3 to 7, an increase of 4. So the temperature effect on net photosynthesis is mostly increased respiration, with a smaller fall in gross photosynthesis.
2. Identify the limiting factor at each measured condition. Using the marginal gains and costs, recommend whether the grower should invest in CO2 enrichment or supplementary lighting.
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Compare the marginal gains:
- At 400 CO2, raising light from 800 to 1200 gives 19 - 18 = +1 µmol m−2 s−1. Light is close to saturation, so CO2 is the main limiting factor.
- At 800 light, raising CO2 from 400 to 800 gives 26 - 18 = +8.
- At 1200 light, raising CO2 gives 28 - 19 = +9.
- At 800 CO2, raising light gives 28 - 26 = +2.
CO2 enrichment gives the larger gain at the current light level. Cost per unit gain is about £12,000 / 8 = £1,500 per µmol m−2 s−1 for CO2, compared with £20,000 / 1 = £20,000 for lighting at 400 CO2. Lighting becomes more useful after CO2 is raised, but even then it gives only +2 for £20,000. The recommendation is to install CO2 enrichment first, then re-measure; supplementary lighting is hard to justify unless natural light falls well below 800 for much of the day. The 12-hour photoperiod and any overheating must also be considered.
3. Explain why net photosynthesis is lower at 35 °C and why simply heating the glasshouse would be counterproductive.
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At 25 °C: gross 31, respiration 3, net 28. At 35 °C: gross 29, respiration 7, net 22. Heating therefore lowers net photosynthesis because respiration increases more than gross photosynthesis falls. The small fall in gross suggests that 35 °C is beginning to exceed the optimum for photosynthetic enzymes, and photorespiration may also increase relative to photosynthesis at higher temperature. Stomata may partly close to conserve water, reducing CO2 supply.
The grower should not simply heat the glasshouse. If 35 °C occurs on hot days, ventilation, shading or evaporative cooling to keep the leaf near 25 °C would be more effective. Heating would increase respiration and could reduce yield.