Hydroponics Deficiency: Young or Old Leaves First?
A student grows tomato seedlings in aerated complete nutrient solution at pH 6.0. Four treatments each lack one element: nitrogen, magnesium, iron or calcium. After 20 days the observations are: Culture 1: the oldest leaves are uniformly pale yellow; the whole plant is stunted. Culture 2: the oldest leaves show yellowing between the veins while the veins remain green. Culture 3: the youngest leaves show yellowing between green veins; older leaves look normal. Culture 4: the youngest leaves are distorted with necrotic margins; root tips are short and brown; older leaves look normal. The student writes: 1 = iron, 2 = nitrogen, 3 = magnesium, 4 = calcium, and adds that all deficiencies appear first in young leaves because young leaves are the strongest sinks. The teacher marks three of the four assignments wrong and the general statement wrong.
1. Correct the four assignments and justify each using element mobility and function.
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Culture 1 is nitrogen deficiency, not iron. Nitrogen is mobile in the phloem, so it is withdrawn from older leaves and moved to growing points. Because nitrogen is needed in proteins, including chloroplast proteins, the oldest leaves become uniformly pale and growth is stunted.
Culture 2 is magnesium deficiency, not nitrogen. Magnesium is also mobile, so older leaves are affected first. It is the central atom of chlorophyll, and its deficiency produces interveinal chlorosis while veins remain greener.
Culture 3 is iron deficiency, not magnesium. Iron is immobile in the plant and is needed for chlorophyll synthesis, so the youngest leaves show interveinal chlorosis while older leaves remain normal.
Culture 4 is calcium deficiency, which is correct. Calcium is immobile and is needed for cell walls and membranes. Deficiency damages young expanding leaves and root tips, producing distortion, necrosis and short brown roots. Only one assignment was therefore correct.
2. Explain why the student's general statement that all deficiencies appear first in young leaves is wrong.
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The statement confuses sink strength with mobility. Young leaves are strong sinks and receive nutrients delivered by the phloem, but whether a deficiency appears there first depends on whether the element can be remobilised from older tissue.
Mobile elements such as nitrogen, magnesium and potassium can be withdrawn from older leaves and exported to young growing leaves. The older leaves therefore show deficiency symptoms first, while the young leaves may look normal for some time. Immobile elements such as iron and calcium cannot be withdrawn efficiently from older tissue. When supply stops, the young expanding leaves and root tips are affected first.
The observed patterns fit this rule. Cultures 1 and 2 affect old leaves, so the missing elements are mobile. Cultures 3 and 4 affect young leaves, so the missing elements are immobile. Sink strength alone does not predict the pattern.
3. Design a simple control to confirm that culture 4 is caused by missing calcium rather than by the distilled water or pH. State the predicted result and why old leaves would not recover.
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Set up two parallel cultures at pH 6.0. Test culture: the same nutrient solution lacking calcium. Control culture: the same solution with calcium restored, for example as 2 mM calcium nitrate, with the nitrate contribution balanced so that only calcium differs. Observe the new leaves produced over the next 10 days.
If calcium is the missing element, the test culture will continue to produce distorted, necrotic young leaves with short brown root tips, while the control will produce normal new leaves. If the problem were the distilled water or pH, both cultures at pH 6.0 would behave similarly, and adding calcium would not specifically correct the new growth.
Old leaves would not recover because calcium is immobile. The calcium already present in those leaves cannot be redistributed to the young leaves, and the damage to the young tissue is already fixed. Recovery must be looked for in new growth, not in the old leaves.