Photosynthesis and Plant Nutrition: Question 1
Syllabus 6.1
In a school greenhouse, two rows of pepper seedlings are grown side by side in identical trays of nutrient solution, except that each row's solution is missing one different mineral ion.
Row P: the seedlings reach a normal height, and their leaves stay a healthy dark green colour, but very few new leaves appear and the stems and leaves remain thin, as though the plant cannot make enough material for new cell growth.
Row Q: the seedlings also reach a normal height and produce plenty of new leaves, but their older leaves turn pale and yellowish between the veins, while the veins themselves stay a darker green.
Which mineral ion is missing from the nutrient solution given to Row P, and which is missing from the nutrient solution given to Row Q?
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Worked solution
Step 1: Recall the roles of nitrate ions and magnesium ions
Plants need nitrate ions to make amino acids, which are then built into proteins. Proteins are needed for making new cells and new tissue, so a shortage of nitrate ions leads to poor growth, even though the leaves that are already present can stay green.
Plants need magnesium ions to make chlorophyll, the green pigment inside chloroplasts. A shortage of magnesium ions means less chlorophyll can be made, so leaves (especially older leaves, which have had longer to use up their existing chlorophyll) turn pale and yellowish, while the plant can otherwise still grow to a normal size.
Step 2: Match each row’s symptoms to the correct ion
Row P grows to a normal height with healthy dark green leaves, but produces very little new leaf and stem material. The green colour shows chlorophyll is being made normally, so magnesium is not the problem. The lack of new growth points to a shortage of the material needed to build new cells, which points to a nitrate ion deficiency (not enough amino acids for protein synthesis).
Row Q grows to a normal height and produces plenty of new leaves, so protein synthesis for growth is not the problem. But its older leaves turn pale and yellowish between the veins, which is the classic sign of a magnesium ion deficiency, since less chlorophyll is being made.
Step 3: Why the other options are wrong
- B swaps the pairing, giving the wrong reason for each row’s symptoms.
- C and D both assume the two rows are missing the same ion, but the two rows show clearly different symptoms, so they must be missing two different ions.
Final answer
Row P is missing nitrate ions and Row Q is missing magnesium ions, option A.