Tomato Hydroponics: Interveinal Yellowing and Starch Testing
A school greenhouse grows tomato plants in two hydroponic beds. The nutrient solution in bed A contains nitrate, phosphate, potassium, calcium, sulfate and iron, but the bottle of magnesium sulfate was left out for six weeks. Bed B received the complete solution. Bed A plants are shorter and their older leaves show yellowing between the veins, while the veins remain green; younger leaves are less affected. Bed B plants are dark green and flowering. A student suggests adding more nitrate to bed A to make the leaves green. Another student says the yellowing is caused by a virus. The teacher asks the class to use the evidence to identify the most likely cause and to design a simple test to show that light is needed for starch production in a healthy leaf.
1. Identify the most likely mineral deficiency in bed A. Justify your answer with two pieces of evidence from the setup and explain why adding nitrate is unlikely to correct the yellowing.
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Most likely magnesium deficiency. Evidence: magnesium sulfate was omitted for six weeks, and the older leaves show interveinal yellowing while veins stay green. Magnesium is needed to make chlorophyll, so low magnesium causes chlorosis. Nitrate is already supplied, and nitrate is mainly needed for amino acids and proteins, not for chlorophyll directly. Adding more nitrate would not replace the missing magnesium, so chlorophyll cannot be rebuilt. A virus is less likely because the pattern is uniform across bed A, follows the missing nutrient, and bed B with complete solution is healthy.
2. Explain why the older leaves show the symptoms before the younger leaves.
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Magnesium is a mobile nutrient in the phloem. When supply is low, the plant redistributes magnesium from older leaves to young growing tissues and seeds. The older leaves therefore lose chlorophyll first and yellow between the veins. Younger leaves are supplied at the expense of older ones, so they remain greener for longer. This mobility is why a deficiency of magnesium is described as showing first on older leaves.
3. Describe a controlled test using a healthy bed B leaf to show that light is needed for starch production. Include one controlled variable and one safety precaution.
Read the worked answer
Destarch the plant by keeping it in darkness for 24–48 hours so existing starch is used up. Cover part of one leaf with opaque foil, leaving the rest exposed, and place the plant in bright light for 4–6 hours. Remove the leaf, boil it in water to kill tissue, then boil in ethanol in a water bath to remove chlorophyll. Rinse and add iodine solution. The exposed area should turn blue-black, showing starch; the covered area should stay brown-yellow, showing no starch. Control: same leaf, same plant, same time and temperature; only light is varied. Safety: ethanol is flammable, so heat it in a water bath, away from flames.