Iron Chlorosis in Hydroponic Tomatoes at High pH
Tomato plants are grown in two recirculating hydroponic tanks. Both receive the same nutrient solution, except that tank A is maintained at pH 5.5 and tank B at pH 7.8 by daily KOH addition. Iron is supplied as 20 µM Fe-EDTA in both. After 3 weeks, tank B plants show interveinal chlorosis of young leaves; tank A plants are green. Leaf iron is 18 mg kg−1 dry mass in B and 55 mg kg−1 in A. Root ferric-chelate reductase activity is 2.1 µmol g−1 h−1 in A and 0.7 in B. Solution bicarbonate is 0.2 mM in A and 2.5 mM in B. The grower wants to add more Fe-EDTA, switch to Fe-EDDHA, or lower the pH. The budget allows only one change before the next harvest.
1. Diagnose the disorder and explain why chlorosis occurs despite 20 µM iron in both tanks.
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The symptoms and leaf analysis indicate iron deficiency chlorosis. Total iron in solution is adequate, but availability is not. At pH 7.8, Fe-EDTA is less stable and ferric iron tends to form insoluble hydroxides, so the concentration of usable Fe3+ at the root surface falls. The high bicarbonate concentration also inhibits the root ferric-chelate reductase that tomato roots use to reduce Fe3+ to Fe2+ for uptake; the measured reductase activity is 0.7 versus 2.1 µmol g−1 h−1 at pH 5.5. Because iron is poorly mobile in the plant, young leaves become chlorotic while older leaves may remain greener. The low leaf iron of 18 mg kg−1 confirms a true deficiency rather than a dilution effect. Adding more Fe-EDTA without changing pH may simply increase precipitation and will not necessarily raise leaf iron.
2. Compare adding more Fe-EDTA, switching to Fe-EDDHA, and lowering the pH; recommend one change for the next harvest.
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Adding more Fe-EDTA is the least reliable option because the chelate is already unstable at pH 7.8; extra iron may precipitate and remain unavailable. Switching to Fe-EDDHA is a reasonable short-term fix because Fe-EDDHA remains stable at higher pH and keeps iron in solution, but it is more expensive and does not correct the bicarbonate-induced inhibition of root reduction. Lowering the pH toward 5.5–6.0 addresses the root cause: it improves Fe-EDTA stability, reduces bicarbonate, and restores ferric-chelate reductase activity. With only one change allowed, acidification is the best long-term investment if the system can safely control pH. If acidification is technically difficult, Fe-EDDHA is the best immediate rescue. The recommendation should be justified by the leaf iron and reductase data, not by the label concentration of iron alone.
3. Predict the effect of adding 10 mM bicarbonate to tank A while keeping pH initially unchanged, and explain the reasoning.
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Adding 10 mM bicarbonate would raise the solution pH and, more importantly, increase bicarbonate stress at the root surface. Even before the bulk pH fully equilibrates, bicarbonate can inhibit the root ferric-chelate reductase and interfere with iron uptake. The plants would be expected to develop interveinal chlorosis of young leaves, and leaf iron would fall, despite the same 20 µM Fe-EDTA in the tank. The effect would be similar to, but faster than, the chronic high-pH condition in tank B. This prediction follows from the lower reductase activity and lower leaf iron already observed at 2.5 mM bicarbonate. It also shows why total iron concentration is a poor predictor of iron status in hydroponics; chelate stability, pH, and bicarbonate must be considered together.