Five Hydroponic Tanks and a Diagnosis from Leaf Symptoms
A school raises tomato seedlings in five identical hydroponic tanks under the same light, pH 6.0 and temperature, with roots aerated. Tank 1 receives full nutrient solution. Tanks 2 to 5 each receive the same solution with one element omitted - nitrogen, magnesium, iron and potassium respectively. After four weeks the students record: Tank 2 - older leaves uniformly pale yellow, stunted growth, thin stem, and a crude protein test on the leaves gives a very low value. Tank 3 - older leaves yellow between the veins while the veins stay green. Tank 4 - the youngest leaves are yellow, almost white, with green veins, while older leaves look normal. Tank 5 - margins of older leaves are brown and scorched, and the plants wilt readily at midday. A visiting teacher asks them to name each deficiency from the symptoms, to explain why the symptoms appear on different leaves, and whether adding the missing element to Tank 4 now will turn the already-yellow leaves green.
1. Identify the element missing from each of tanks 2 to 5 and justify each identification from the recorded symptoms.
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Each symptom pattern is a fingerprint, so the diagnosis must rest on position plus pattern plus any associated chemical evidence.
- Tank 2 - nitrogen. Uniform, non-veinal yellowing of the older leaves, stunted growth and a thin stem fit nitrogen, which is needed for amino acids, proteins, nucleic acids and chlorophyll. The low leaf protein value is independent chemical support: without nitrogen, protein synthesis collapses. Anthocyanin or purple stems may also appear.
- Tank 3 - magnesium. Yellowing between the veins with the veins remaining green, again on older leaves, is the classic interveinal chlorosis of magnesium, which is the central atom of the chlorophyll molecule. Without it chlorophyll cannot be built, so the tissue palest in chlorophyll loses colour first.
- Tank 4 - iron. Chlorosis of the youngest leaves with veins still green is the signature of an immobile element; iron is required for chlorophyll synthesis even though it is not part of chlorophyll, and it is not retranslocated in the phloem.
- Tank 5 - potassium. Marginal browning and scorching of older leaves, beginning at the tips and edges, together with easy midday wilting, reflects potassium's role in stomatal regulation, turgor and enzyme activation.
The students should also note what is missing: no tank shows good growth, so the control tank is essential for comparison.
2. Why do nitrogen, magnesium and potassium deficiencies appear first in older leaves while iron deficiency appears in young leaves?
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The deciding factor is phloem mobility. Plants do not discard an element when a new leaf demands it; they withdraw it from older tissue and reload it into the phloem to supply expanding leaves and meristems.
Nitrogen, magnesium and potassium are all mobile. When supply falls short, the plant dismantles the older leaves: nitrogen is salvaged from proteins and chlorophyll, magnesium is withdrawn from chlorophyll in the lower leaves, and potassium is redistributed from mature tissue to young growing points. The older leaves therefore show symptoms first and most severely, while the young leaves stay green until the shortage becomes extreme.
Iron is immobile in the phloem. Once deposited in a leaf it cannot be recovered and re-exported, so a plant starved of iron cannot supply its new leaves at all. The young leaves, which must build chloroplasts from scratch, become chlorotic, while the older leaves that were supplied earlier remain green.
The clinical rule: symptoms on old leaves point to mobile elements (N, P, K, Mg); symptoms on young leaves point to immobile elements (Fe, Ca, B, Mn). Position of the symptom on the shoot is therefore diagnostic information, not an accident of appearance.
3. If iron is restored to Tank 4 now, will the yellow leaves turn green? Explain, and state how the school could have made the trial more reliable.
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Partly, and not on the leaves already damaged. Iron is needed for the enzymes of chlorophyll synthesis, so once it is supplied, new growth and the youngest expanding leaves will green up within a week or two. Leaves that are already severely chlorotic have lost chloroplast structure and may remain pale for the rest of their life; the honest way to judge recovery is to watch the new leaves, not the old ones. Recovery also depends on iron staying available: at pH 6.0 with aeration it should remain soluble, but iron readily precipitates as hydroxide at higher pH, which is why the students must not change the pH mid-trial. Supplying iron as an EDTA chelate would improve uptake.
Sources of unreliability in the present design: each treatment exists in a single tank, so any accidental difference is confounded with the treatment; there is no replication; root aeration and solution renewal should be identical; and the seedlings should be of the same variety and age. The school should run at least three replicate tanks per treatment, randomise their positions on the bench, replace solutions regularly and record the position of symptoms on old versus young leaves as the primary data, so that the same conclusion can be demonstrated rather than asserted.