The Wilting Tomato in a Sealed Pot
A tomato plant is grown in a sealed transparent chamber with a fixed air supply. Over 24 hours the chamber CO₂ concentration is monitored: it falls steadily during the light period and rises during the dark period. The plant's leaves are then coated on their upper surface with a thin transparent grease that blocks gas exchange there but leaves the lower surface untouched. After coating, the plant wilts within a day and its CO₂ uptake in light drops sharply, even though the soil is moist and the roots are undamaged. A student concludes: 'The grease blocked photosynthesis, so the plant starved - that is why it wilted.'
1. Explain the daily CO₂ pattern in the uncoated chamber in terms of the plant's metabolism.
Read the worked answer
The chamber CO₂ rhythm reflects the balance of photosynthesis and respiration:
- Light period: photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) exceeds respiration, so net CO₂ is removed from the air → concentration falls.
- Dark period: photosynthesis stops; respiration (aerobic) continues, releasing CO₂ → concentration rises.
The net direction at any moment equals (photosynthetic rate − respiratory rate). The fact that it falls in light and rises in dark confirms the plant is both photosynthesising and respiring, and that the chamber is a closed system where the plant measurably shifts the gas balance.
2. Evaluate the student's explanation for the wilting. What is the more likely mechanism, and how does leaf anatomy explain it?
Read the worked answer
The student is partly right about the effect on photosynthesis but wrong about the cause of wilting.
- Coating the upper surface blocks gas exchange there, reducing CO₂ entry and thus photosynthesis - consistent with the drop in CO₂ uptake.
- But wilting within a day is a water-relations problem, not starvation (starvation is slow).
- In most dicot leaves (tomato), stomata are concentrated on the lower (abaxial) surface. The grease on the upper surface mainly blocks the cuticle/upper epidermis, but the key issue is that the coating also reduces transpirational and gas-exchange balance and, crucially, the experiment shows the plant cannot regulate water loss/gas exchange normally.
Most likely mechanism: the grease disrupts the leaf's gas-exchange surfaces, so transpiration and CO₂ uptake are decoupled from stomatal control; the plant loses the ability to balance water loss with supply, and with continued transpiration demand the shoot wilts even though soil is moist - a shoot-level water-relations failure, not root damage.
The precise anatomy point: stomata are mainly abaxial in tomato, so the lower surface is the main gas-exchange route; the experiment's design (coating the upper surface) tests the wrong surface for the student's claim and confounds the interpretation.
3. Redesign the experiment so that the effect of blocking the upper versus lower leaf surface can be separated, and state what result would support the stomatal explanation.
Read the worked answer
Use four matched plants in identical sealed chambers with CO₂ monitoring:
- Control - no coating.
- Upper surface coated (as before).
- Lower surface coated.
- Both surfaces coated.
Measure, over 24 h: CO₂ concentration change in light and dark, and wilting time.
Predictions supporting the stomatal explanation:
- If stomata are mainly on the lower surface, coating the lower surface should reduce CO₂ uptake and transpiration more than coating the upper surface.
- Coating both should nearly abolish photosynthesis and cause the fastest wilt.
- The control should show the normal light-fall/dark-rise CO₂ rhythm.
Interpretation rule: compare light-minus-dark CO₂ change across treatments to quantify net photosynthesis, and record wilting time to separate water-relations effects from carbon-gain effects. A transparent, gas-permeable-but-water-blocking film could be added to distinguish transpiration from CO₂ limitation.