Case 3: Two yellow tanks in a hydroponic greenhouse
A grower supplies tomato plants with nutrient solution in three tanks in the same greenhouse. Every tank receives the same solution except for one mineral, which is left out:
- Tank A: no nitrate
- Tank B: no magnesium
- Tank C: complete solution
After five weeks the measurements are:
| Tank | Height | Older leaves | Chlorophyll meter (a.u.) | Dry mass |
|---|---|---|---|---|
| A | 40 cm | uniformly pale yellow, thin stems | 12 | 8 g |
| B | 62 cm | yellow between the veins, veins still green | 18 | 14 g |
| C | 65 cm | dark green | 34 | 18 g |
The solutions are aerated continuously and the pH is held at 6.0. The grower says: 'Both A and B just need more light, because yellow leaves mean the plant is not photosynthesising.'
1. Identify which mineral is missing from which tank and justify your answer using the data and the symptoms.
Read the worked answer
Tank A = nitrate deficiency. The plants are much shorter (40 cm vs 65 cm), have low dry mass (8 g vs 18 g) and uniformly pale older leaves with thin stems. Nitrate supplies nitrogen for amino acids, proteins and nucleic acids, so without it growth and protein synthesis are limited - stunting and general pallor are the expected result.
Tank B = magnesium deficiency. Height and dry mass are only slightly reduced, but chlorophyll is distinctly lower (18 vs 34) and the older leaves are yellow between the veins while the veins stay green (interveinal chlorosis). Magnesium is the central atom of the chlorophyll molecule, so its shortage hits chlorophyll most directly.
The comparison is a controlled one: all three tanks share the same greenhouse, the same solution apart from one mineral, the same aeration and pH. Because tank C is green and healthy under the same light, the grower's 'more light' explanation cannot account for A and B, and because the two deficient tanks show different symptom patterns, a single environmental cause cannot explain both.
2. Explain why both deficiencies show first in the oldest leaves, and why the grower's suggestion about light is not supported.
Read the worked answer
Both nitrate and magnesium are mobile in the plant. When supply from the roots falls short, the plant remobilises these minerals from older leaves through the phloem and moves them to the growing points, where they are needed most. The older leaves are therefore the first to become deficient and the first to yellow, while young leaves stay green longest. By contrast, an immobile mineral such as calcium produces symptoms in the youngest leaves first - a useful way to tell deficiencies apart.
Light is not the explanation:
- All three tanks stand in the same greenhouse, so light intensity and day length are effectively the same for A, B and C. Tank C is dark green under that light, so light is adequate.
- More light would not produce interveinal chlorosis with green veins, nor the difference in dry mass between A and B.
- Yellow leaves do photosynthesise less, but that is a consequence of low chlorophyll, not a cause of the deficiency.
A testable follow-up would be to add the single missing mineral to half of a deficient tank and compare new growth after two weeks, with the other half as a control.
3. The grower proposes doubling the nitrate concentration in tank A's solution. Evaluate this proposal.
Read the worked answer
Doubling nitrate would probably increase growth up to a point, because nitrogen is the mineral limiting tank A, but the proposal has clear limits:
- Nutrient balance matters, not just amount. Adding nitrate without correcting the rest does not fix a magnesium deficiency, and an excess of one ion can interfere with uptake of others (for example, high nitrate can worsen magnesium or potassium uptake). The solution should be rebalanced, not simply enriched.
- Uptake is active and needs oxygen. Mineral ions are taken up by active transport, which depends on respiration in the root cells. That is why the solutions are aerated and why waterlogging or poor aeration reduces uptake even when ions are plentiful.
- pH and concentration interact. At pH 6.0 the ions are available; raising the concentration far above the recommended level can raise the solute concentration of the soil solution, making water uptake by osmosis harder and scorching leaf margins.
- Cost and pollution. Nitrate not taken up by the plants is washed out in the drainage water, wasting money and causing eutrophication of nearby watercourses - algal blooms, then oxygen depletion.
Best practice: restore the recommended complete solution, compare new growth against tank C, and monitor run-off nitrate.