The Greenhouse That Ran Hot
A student grows tomato seedlings in a sealed 3 m × 2 m × 2.5 m greenhouse. On day 1, the air inside contains 0.04% CO₂ and the plants photosynthesize at a measurable rate. By noon the internal temperature reaches 38 °C while outside it is 24 °C; the vents are stuck closed. The student notices that net photosynthesis, measured as oxygen released, rises in the morning but then falls sharply after 11 a.m. even though light intensity keeps increasing until 1 p.m. Soil moisture drops from 30% to 12% over the same period. The student wants to know which factor limits the plants and whether adding CO₂ would help.
1. Identify the most likely limiting factor for net photosynthesis after 11 a.m. and justify your choice using the evidence in the setup.
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The most likely limiting factor is temperature combined with water stress, not light or CO₂. Light keeps rising until 1 p.m., yet oxygen release falls after 11 a.m., so light is not limiting. The sealed greenhouse means CO₂ is being consumed, but the sharp drop coincides with the temperature reaching 38 °C and soil moisture falling to 12%. At high temperature, stomata close to conserve water, which simultaneously cuts CO₂ intake and reduces oxygen release. The decline is therefore best explained by stomatal closure driven by heat and drought, with the temperature also reducing enzyme efficiency in the Calvin cycle. A good answer distinguishes correlation from cause and notes that the sealed chamber makes CO₂ depletion a plausible secondary factor that should be measured, not assumed.
2. The student proposes pumping CO₂ into the greenhouse to raise photosynthesis. Evaluate whether this intervention is likely to restore the morning rate, and explain your reasoning.
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Adding CO₂ is unlikely to restore the morning rate under current conditions. If stomata are closed because of heat and water stress, extra CO₂ in the air cannot enter the leaf, so the CO₂ concentration outside the stomata is irrelevant. The intervention would only help if CO₂ were the primary limiting factor, which the evidence does not support because the decline tracks temperature and moisture rather than light or measured CO₂ drawdown. A stronger plan is to open the vents to lower temperature and irrigate to restore soil moisture, then measure oxygen release again. If photosynthesis recovers without added CO₂, that confirms stomatal limitation. The student should treat the CO₂ proposal as a hypothesis to test after controlling temperature and water, not as a first fix.