BIO2030 Chap.5 Plant Nutrition and the Nutritional Quality of Food
Plant Nutrition and the Nutritional Quality of Food
Plant Nutrition and the Nutritional Quality of Food is built on discriminations. A macronutrient is separated from a micronutrient by a stated quantity, not by importance. A mobile element is separated from an immobile one by whether the plant can withdraw it from old tissue, which is why the position of a deficiency symptom identifies the class of element that is short.
And plant nutrition is separated from human nutrition by the observation that closes the theme: a crop grown under a higher carbon dioxide concentration can gain mass while losing concentration of the elements that make it food. The chapter also settles the nitrogen question, which is why an element making up most of the atmosphere is nonetheless the commonest limit on yield.
What this chapter covers
- 01
The quantitative definition that separates macronutrients from micronutrients, on a dry weight basis
- 02
Why every definition in this area is stated on dry weight, and what a fresh weight figure would measure instead
- 03
Criteria that put an element on the essential list at all
- 04
Mobile against immobile elements, and reading a deficiency by where the symptom appears
- 05
Nitrogen: abundant in air, unavailable to plants, and largely spent on one enzyme
- 06
What the plant can actually take up, and how those forms reach the root
- 07
How much fertiliser is enough, and the shape of the response curve
- 08
Nutrient dilution under elevated carbon dioxide, the mechanisms behind it, and the crop group that escapes
Diagnose two deficiencies from the position of the symptom
- +1State the mechanism: a mobile element can be withdrawn from mature tissue and redirected to new growth when supply is short.
- +1Apply it to the first case: symptoms on the oldest leaves with a healthy growing point mean the plant is stripping old tissue to supply new tissue, so the element in short supply is mobile.
- +1Apply it to the second case: an element that cannot be moved once deposited leaves old leaves unaffected and starves the newest tissue, so symptoms in new growth indicate an immobile element.
- +1Name the distinguishing property explicitly as mobility within the plant, and note that it is independent of how much of the element the plant needs.
- +1Say what a soil test misses: it measures what is present in the soil, not what the plant can take up or move once inside, so a soil with adequate total supply can still produce either symptom if uptake or transport is limited.
Key terms
- Macronutrient
- An element required at more than one thousand milligrams per kilogram of plant dry weight. The definition is a quantity, so a macronutrient is not more important than a micronutrient, only needed in larger amounts.
- Micronutrient
- An element required at less than one hundred milligrams per kilogram of plant dry weight. A shortage produces symptoms as severe as any macronutrient deficiency.
- Mobile element
- An element the plant can withdraw from mature tissue and redirect to new growth. Its deficiency symptoms appear first in the oldest leaves.
- Immobile element
- An element that cannot be relocated once deposited. Its deficiency symptoms appear first in the newest tissue while older leaves stay normal.
- Nutrient dilution
- The fall in the concentration of nutritionally important elements in plant tissue when biomass rises faster than element uptake, observed under elevated carbon dioxide.
Plant Nutrition and the Nutritional Quality of Food FAQ
Why are all the nutrient definitions given on a dry weight basis?
Because fresh weight moves with water content, and water content moves with how recently the plant was watered and what time of day it was cut. A concentration expressed per unit of fresh weight would therefore change without any change in the plant's nutrition, and two treatments could differ on paper because of irrigation rather than nutrient supply.
That is also why the growth experiment dries every sample at a set temperature for a set time before weighing: it removes the term that would otherwise contaminate every comparison.
If nitrogen makes up most of the atmosphere, why is it the commonest limit on yield?
Because plants cannot use it in the form it takes in air. Atmospheric nitrogen is held in a bond that plants have no way to break, so they depend on forms supplied through the soil, which are produced by soil organisms, by symbiotic partners in the case of legumes, or by fertiliser.
Demand is also unusually high, because a large share of a leaf's nitrogen goes into the carbon fixing enzyme, which is abundant precisely because it is slow. Photosynthetic capacity is therefore tied to nitrogen supply, which is what connects this chapter to the carbon one.
Does elevated carbon dioxide make food less nutritious?
The unit's position is that it dilutes it, and it distinguishes two mechanisms rather than asserting one. Extra fixed carbon adds carbohydrate mass, so a fixed amount of an element is spread through more tissue and its concentration falls. Separately, changes in water flow through the plant alter how much of some elements is delivered to the shoot at all.
The chapter also names an exception, and the exception is the useful part of the answer, because a crop group that does not show the effect points to which mechanism is doing the work.
Assessment move
Build two tables and keep them side by side. The first has the essential elements sorted by the quantitative macronutrient and micronutrient definitions, with the numbers written in, because the definition is a threshold and questions test whether you know it is a threshold. The second sorts the same elements by mobility, because that is the table you will actually use to answer a diagnosis question.
Practise diagnosing from a described symptom rather than from a named element, since that is the direction the question comes in. Learn the nitrogen story as a chain from the atmosphere through the soil to the carbon fixing enzyme, because it links this chapter to the photosynthesis chapter and to the fertiliser treatments in your own experiment.
Finally, be able to state both dilution mechanisms and the exception, since an answer that gives only one mechanism cannot explain why the exception exists.
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