Mineral Deficiency Symptoms and a Potometer Reading
Tomato plants were grown for four weeks in aerated nutrient solutions: complete, minus magnesium, and minus nitrate. The complete plants were healthy. The minus-magnesium plants had yellow tissue between green veins on the older leaves, while the young leaves looked normal. The minus-nitrate plants were short, had few leaves, and all the leaves were uniformly pale.
A shoot from a healthy plant was then set up in a potometer and the distance moved by the air bubble in 5 minutes was recorded:
| Condition | Bubble movement / mm |
|---|---|
| Still air | 12 |
| Fan directed at the shoot | 46 |
| Lower leaf surface smeared with Vaseline, still air | 3 |
| Upper leaf surface smeared with Vaseline, still air | 10 |
| Whole shoot enclosed in a clear polythene bag, still air | 1 |
1. Explain why magnesium deficiency shows first in older leaves, whereas nitrate deficiency affects the whole plant.
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Magnesium. Magnesium is the central atom of every chlorophyll molecule, so without it chlorophyll cannot be made and the leaf appears yellow (chlorosis). Magnesium is also phloem-mobile: when supply is short, the plant exports magnesium from older leaves and redirects it to growing points and young leaves. The older leaves therefore lose chlorophyll first and show interveinal chlorosis, while the veins remain green because they retain some chlorophyll and the young leaves are protected by the recycled supply.
Nitrate. Nitrogen is needed for amino acids, proteins, nucleic acids, chlorophyll and the whole apparatus of growth. A plant short of nitrate cannot build new cells, so growth is stunted, leaves are small and few, and all leaves are pale because nitrogen is also recycled from older tissue to the growing points. Unlike magnesium, the deficiency is not confined to a single visible pigment change - it limits the plant's entire capacity to grow, which is why the whole plant looks pale and small.
The contrast therefore illustrates two points: symptoms appear where the element's function fails first, and the site of the symptom depends on whether the element can be remobilised through the phloem.
2. Use the potometer data to decide which leaf surface has more stomata and to explain the effect of the fan and the polythene bag. Support your answer with calculated values.
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Which surface has more stomata. Smearing the lower surface reduces the reading from 12 to 3 mm, a fall of 9 mm or 75%. Smearing the upper surface gives 10 mm, a fall of only 2 mm, about 17%. If the smeared surface loses the pores through which most water vapour escapes, the surface whose treatment causes the greater fall must carry the greater number of stomata: here, the lower surface. The residual 3 mm is loss through the upper surface, cut edges and any uncovered areas.
The fan. Air movement raises the reading from 12 to 46 mm, a 3.8-fold increase (about 280% more). Moving air removes the layer of still, humid air next to the leaf surface, steepening the water-vapour gradient between the air spaces inside the leaf and the outside air, so diffusion out of the stomata is faster.
The polythene bag. The reading collapses to 1 mm because the air around the shoot quickly becomes saturated with water vapour. The gradient between the leaf interior and the surrounding air is almost abolished, so net diffusion out of the leaf nearly stops. This shows that transpiration is driven by a diffusion gradient, not simply by stomata being open.
3. A classmate concludes that the potometer readings measure the rate of transpiration of the plant. Evaluate this claim and suggest how the apparatus could be used more validly.
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What the potometer actually measures. The bubble moves as water is drawn into the shoot, so the instrument measures water uptake by a detached shoot. Uptake equals transpiration only if all the absorbed water is transpired, which is not the case: some water is used in photosynthesis, some maintains turgor and fills cells, and some may be lost through cut surfaces. The rubber connections can also leak.
Further problems. The shoot is cut, so the normal pull from root pressure and the whole-plant regulation of stomatal opening are absent; the shoot may close its stomata in response to cutting, giving artificially low readings. The bubble is subject to friction in a narrow tube, readings are affected by temperature changes that expand or contract the air bubble, and the small distance moved is hard to read precisely. The rates recorded (12 mm in 5 minutes) are small, so a 1 mm error is an 8% error.
A more valid use. Use the apparatus comparatively rather than absolutely: measure the same shoot under several conditions, allowing a recovery period between readings and keeping light and temperature constant; use a whole rooted plant in a sealed pot if total water loss is required; or measure transpiration directly by weighing a potted plant enclosed around the pot, so that mass loss over time is due to transpiration alone.