Nutrition in Plants, revision notes

Overview

Plants make their own food by photosynthesis and take up mineral nutrients from the soil. This chapter covers the mineral nutrients a plant needs, how it absorbs water and minerals, and the diversity of plant nutrition.

Photosynthesis itself is explored in depth on the process pages; here the focus is on nutrients and uptake.

Each macronutrient has a specific role and a recognisable deficiency symptom. Too little nitrogen, which builds proteins and chlorophyll, causes stunted growth and yellowing of older leaves; too little phosphorus, needed for roots and energy transfer, gives poor root growth and purplish leaves; too little potassium causes yellow, curling leaf margins; and too little magnesium specifically prevents new chlorophyll from forming, giving yellow leaves with the veins still green (interveinal chlorosis).

Plants also obtain some nutrients through partnerships with other organisms. Rhizobium bacteria living in the root nodules of legumes such as peas and beans fix nitrogen gas from the air into compounds the plant can use, while the plant supplies the bacteria with carbohydrates, a mutualistic relationship exploited in crop rotation.

Many plants also form a mycorrhizal association with soil fungi, which greatly increases the root surface available for absorbing water and mineral ions, especially phosphorus, in exchange for sugars.

This chapter is often assessed by giving a picture or description of a deficiency symptom and asking candidates to name the missing mineral and explain its role, or by describing an unusual plant, such as one that traps insects or grows on a tree, and asking how it obtains its nutrients. Naming the specific ion or the specific relationship, not just 'lack of nutrients', is what earns full marks.

18.1 Main Inorganic Nutrients

Plants require a set of inorganic nutrients from the soil in addition to the carbohydrates they make by photosynthesis. Nutrients needed in large amounts, called macronutrients, include nitrogen, phosphorus, potassium and magnesium, while nutrients needed only in tiny amounts, called micronutrients, include iron, manganese, zinc and boron.

Each nutrient plays a specific role, so a shortage of any one of them produces a recognisable deficiency symptom rather than simply weaker growth in general.

Nitrogen is a component of proteins and chlorophyll, so a lack of it causes stunted growth and yellowing of older leaves. Phosphorus is needed for root growth and energy transfer, so a lack of it gives poor root development and purplish leaves.

Potassium deficiency causes yellow, curling leaf margins. Magnesium is specifically a component of chlorophyll, so a lack of it prevents new chlorophyll from forming while the veins stay green, producing interveinal chlorosis.

Exam questions on this content standard commonly give a photograph or written description of a deficiency symptom and ask candidates to name the missing nutrient ion and justify the answer by linking the symptom to that nutrient's specific role; naming the ion precisely, rather than writing a general answer such as a lack of nutrients, is what separates a full-mark answer from a partial one.

18.2 Uptake of Water and Mineral Salts

Water enters a root mainly through root hair cells by osmosis, moving from the more dilute soil solution into the more concentrated cell sap. Each root hair cell is a long, thin extension of an epidermal cell, which greatly increases the surface area in contact with the soil, while its thin wall keeps the distance water has to travel short; with a very large number of these cells covering a root system, absorption becomes efficient overall.

Mineral ions, in contrast, are usually absorbed by active transport rather than osmosis, because the concentration of a particular ion in the soil solution is often lower than its concentration inside the root hair cell. Moving an ion against its own concentration gradient requires energy, which is supplied by respiration in the root cells; this is why root cells contain a comparatively high number of mitochondria.

A common exam distinction is that water uptake does not require metabolic energy, since it follows a water potential gradient, while mineral ion uptake does require metabolic energy, since it is often working against a concentration gradient; a root deprived of oxygen or respiratory substrate can therefore still take up some water but takes up far fewer mineral ions.

Investigating Mineral Requirements: Water Culture Experiments

The specific role of each macronutrient was established experimentally using water culture, commonly called hydroponics, in which a plant is grown with its roots in a nutrient solution containing every known mineral ion instead of soil. By preparing a set of identical setups and deliberately leaving one named ion out of only one solution, any deficiency symptom that develops can be attributed to that missing ion, since every other variable is controlled.

A properly designed water-culture investigation needs a control set-up containing the complete nutrient solution, plants of the same species and initial size in each set-up, and identical conditions of light, temperature and aeration throughout, so that the only difference between the control and the test set-up is the single ion being investigated.

Hydroponics is also used commercially to grow crops such as lettuce and tomatoes without soil, particularly in cooler highland areas, by supplying all the necessary mineral ions directly and continuously in a nutrient solution; this application is often used in exam questions asking candidates to explain an advantage of growing crops this way, such as more precise control over the mineral ions a plant receives.

18.3 Diversity in Plant Nutrition: Parasitic and Epiphytic Plants

Not every plant obtains all of its nutrients through ordinary root absorption combined with photosynthesis. A parasitic plant such as Rafflesia has no chlorophyll of its own and draws water, minerals and food directly from a living host plant through specialised absorptive structures, usually harming the host in the process.

An epiphyte, such as a species of orchid, grows on the branch or trunk of a tree purely for physical support and does not draw nutrients from the host tissue at all. Instead, an epiphyte absorbs the water and minerals it needs from rainwater running down the bark, from moisture in the surrounding air, and from decaying plant debris that collects around its own roots, so the supporting tree is not harmed.

18.3 Diversity in Plant Nutrition: Insectivorous Plants and Mutualism

An insectivorous plant such as the pitcher plant or the Venus flytrap still photosynthesises normally to make its own carbohydrates; it traps and digests insects mainly to obtain extra nitrogen and other minerals that are scarce in the poor, often waterlogged soils where these plants typically grow. The Venus flytrap has sensitive trigger hairs on hinged leaves that snap shut when touched twice in quick succession, while the pitcher plant has a deep, slippery, nectar-lined trap that insects fall into and cannot climb out of; both then secrete digestive enzymes to break down the prey and absorb the nutrients released.

Some plants obtain extra nutrients through a mutualistic partnership with another organism instead of by trapping prey. Rhizobium bacteria living inside root nodules on legumes such as peas, beans and groundnuts fix nitrogen gas from the air into compounds the plant can absorb, while the plant supplies the bacteria with carbohydrates, so both organisms benefit.

Plant roots commonly also form a mycorrhizal association with soil fungi, whose fine threads greatly extend the effective surface area for absorbing water and mineral ions, particularly phosphorus, in exchange for sugars made by the plant.

Exam focus: telling similar deficiency symptoms apart

Two pairs of deficiency symptoms are frequently confused in examination answers. The first pair is nitrogen deficiency and magnesium deficiency, both of which turn leaves yellow: nitrogen deficiency turns an entire older leaf yellow because nitrogen is mobile and is withdrawn from older leaves first, while magnesium deficiency causes interveinal chlorosis, yellowing between the veins while the veins themselves remain green, because magnesium is specifically needed to make new chlorophyll.

The second pair is iron deficiency and magnesium deficiency, both of which involve chlorosis but appear on different leaves: iron cannot move easily from older to younger parts of a plant, so an iron deficiency shows first as chlorosis in the youngest leaves near the growing point, whereas magnesium is mobile and is withdrawn from older leaves first, so a magnesium deficiency appears there instead. Naming which leaves are affected first is usually worth a separate mark from naming the missing ion itself.

Key concepts to master

• Main inorganic nutrients, Macronutrients (e.g. nitrogen, phosphorus, potassium) are needed in large amounts; micronutrients are needed in small amounts.
• Role of nutrients, Nitrogen is needed for proteins, magnesium for chlorophyll; a lack of a nutrient causes a specific deficiency symptom.
• Uptake of water, Water enters root hair cells by osmosis, moving down the water potential gradient.
• Uptake of mineral salts, Mineral ions are absorbed by active transport, which needs energy because the soil is often more dilute than the cell.
• Root hair adaptations, Root hair cells are long and thin with a large surface area to speed up absorption.
• Diversity in plant nutrition, Some plants are parasitic, epiphytic or insectivorous, gaining nutrients in unusual ways.
• Macronutrient deficiency symptoms, A lack of nitrogen causes stunted growth and yellowing of older leaves because nitrogen is needed for protein and chlorophyll; a lack of phosphorus gives poor root growth and purplish leaves; a lack of potassium causes yellow, curling leaf margins; and a lack of magnesium causes interveinal chlorosis because magnesium is a component of chlorophyll.
• Nitrogen fixation and root nodules, Rhizobium bacteria live inside swellings called root nodules on the roots of legumes such as peas, beans and groundnuts. The bacteria fix nitrogen gas from the air into nitrate and ammonium compounds the plant can absorb, while the plant supplies the bacteria with carbohydrates, making this a mutualistic relationship rather than parasitism.
• Mycorrhizal association, Many plant roots form a partnership with soil fungi called mycorrhiza. The fungal threads greatly extend the effective surface area of the root system, absorbing extra water and mineral ions, especially phosphorus, from a larger volume of soil, and in return the fungus receives sugars made by the plant in photosynthesis.
• Named examples of diverse nutrition, Rafflesia is a parasitic plant with no chlorophyll of its own that draws water and food directly from a host vine; the pitcher plant and Venus flytrap are insectivorous, trapping and digesting insects mainly for nitrogen while still photosynthesising; and many orchids are epiphytes, using a tree only for physical support and absorbing water and minerals from rain and air.
• Investigating mineral requirements, Growing plants in water culture (hydroponics), a nutrient solution with one mineral ion deliberately left out, lets a specific deficiency symptom be observed and linked to that missing ion, which is how the role of each macronutrient was first established experimentally.
• Micronutrients, Micronutrients such as iron, manganese, zinc and boron are needed only in tiny amounts but are still essential; for example, iron is required to make chlorophyll, and a shortage of it also causes yellowing of leaves, similar in appearance to a magnesium deficiency but affecting young leaves first rather than old ones.
• How the Venus flytrap and pitcher plant trap insects, The Venus flytrap has sensitive trigger hairs on its hinged leaves that snap shut when touched twice in quick succession, trapping an insect, while the pitcher plant has a deep, slippery, nectar-lined trap that insects fall into and cannot climb out of; both then secrete digestive enzymes to break down the prey and absorb the released nutrients.

Quick recall checklist

1. Can you define and explain Main inorganic nutrients?
2. Can you define and explain Role of nutrients?
3. Can you define and explain Uptake of water?
4. Can you define and explain Uptake of mineral salts?
5. Can you define and explain Root hair adaptations?
6. Can you define and explain Diversity in plant nutrition?
7. Can you define and explain Macronutrient deficiency symptoms?
8. Can you define and explain Nitrogen fixation and root nodules?
9. Can you define and explain Mycorrhizal association?
10. Can you define and explain Named examples of diverse nutrition?
11. Can you define and explain Investigating mineral requirements?
12. Can you define and explain Micronutrients?
13. Can you define and explain How the Venus flytrap and pitcher plant trap insects?

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