Lockhart and Wiseman’s Crop Husbandry Including Grassland

By Alison Samuel, Steve Finch and Gerry P. Lane

Lockhart and Wiseman’s Crop Husbandry Including Grassland, Tenth Edition delivers the latest developments in crop varieties, crop protection products and environmental schemes. This new edition reflects the changing world around us, with sections covering the principles of crop production and chapters on plants, climate, soil management, fertilizers, manures, weeds and diseases that threaten farm crops. Other chapters focus on crop husbandry techniques and the integration of sustainability across the board in crop production. This update also includes an additional focus on the principles of plant breeding, seed production and certification considerations necessary for today’s agriculture.

• Features contributions from leading experts that are specifically structured to help students see the whole picture of crop husbandry
• Presents a fully revised and updated resource that reflect the latest scientific advances and current approaches
• Includes expanded coverage on World Agricultural Systems
• Provides a summary of recommended websites and references for expanded knowledge

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 29, 2022
• ISBN: 9780323984386

Book preview

1: Plants

Abstract

This chapter describes the biology of plants, the most important organisms on the planet. It covers plant physiology and important biochemical processes such as photosynthesis and respiration. It describes the grouping of plants depending on their life cycle. It discusses the structures which are of most importance to crop production including seeds, roots and leaves. The chapter stresses the importance of the leguminous plants and their role in improving soil fertility. It describes the requirements of plants for growth and development, and how these are controlled by plant hormones.

Keywords

Nutrients; Photosynthesis; Physiology; Plant hormones; Plant structure; Water

1.1. Introduction

Plants are living organisms consisting of many specialised individual cells. They differ from animals in many ways and a very important difference is that they can build up valuable organic substances from simple materials such as carbon dioxide and water. The most important part of this building process, called photosynthesis, is the production of carbohydrates such as sugars, starch and cellulose, along with oxygen, using energy provided by light. They have rigid cell walls enclosing a selectively permeable cell membrane which allows the passage of water through it (osmosis). They have specialised organs such as roots, stems and leaves, are mostly immobile and are primary producers of food in most land-based ecosystems.

Most plants consist of roots, stems, leaves and reproductive parts and need a medium in which to grow. These media could be soil, compost, water where plants are grown hydroponically or even air, where the bare roots are sprayed with a fine mist of nutrients and water (aeroponics). In addition to functioning in the uptake of water and nutrients, roots can help to anchor the plant in the growth medium. Stems support the plant, helping to hold the leaves in the optimal positions for light capture and provide routes for transport for substances around the plant. The leaves are the main site of photosynthesis, a process which produces sugars and other molecules for use by the plant.

1.2. Plant groups

There are many ways of classifying plant groups but, from an agricultural and horticultural point of view, a useful way is to divide them into annuals, biennials and perennials according to their total length of life.

1.2.1. Annuals

Annuals are plants that complete their life cycle in one growing season, i.e., starting from a seed, in 1year they develop roots, stem and leaves and then flower and set seed before dying. Typical examples of annuals grown as crops are wheat, barley and oats.

1.2.2. Biennials

These plants grow for 2years. They spend the first year producing roots, stem and leaves, and the following year producing the flowering stem and seeds, after which they die. Sugar beet, swedes and turnips are typical biennials, although the grower treats these crops as annuals, exploiting their life cycle by harvesting them at the end of the first year when most of the foodstuff is stored up in the root, and before the plant moves on to produce the seed head. Those biennial plants that do behave as annuals and produce a seed head in their first year are called ‘bolters’ and are effectively treated as weeds in a biennial crop.

1.2.3. Perennials

Perennials live for more than 2years and, once fully developed, they usually produce seeds each year. Many of the grasses and forage legumes are perennials, as are many of the horticultural fruit crops such as raspberries or apples, and some energy crops such as willow and Miscanthus.

1.2.4. Monocotyledons versus dicotyledons

Plants can also be classified as monocotyledons (monocots) or dicotyledons (dicots). Monocotyledons have one embryonic leaf, or cotyledon and dicotyledons have two. Monocot crops include wheat, maize, rice and other cereals and grasses. Dicot, or broadleaf, crops include potatoes, beans, sugar beet, leafy salads, fruits and vegetables amongst others. The main differences between monocots and dicots are summarised in Table 1.1.

Table 1.1

1.3. Plant structure

The plant can be divided into four parts: the root system, stem, leaf and flower.

1.3.1. The root system

The root system is concerned with the parts of the plant growing in the soil; there are two main types:

1.3.1.1. The tap root or primary system

Dicotyledonous plants possess a primary root called the tap root with lateral secondary roots branching out from it and sometimes additional tertiary roots branching from the secondary roots. This can form, in some cases, a very extensive root system (Fig. 1.1). The root of the bean plant is a good example of a tap root system. If this is split it will be seen that there is a slightly darker central woody core, the skeleton of the root, which helps to anchor the plant and transport foodstuffs. The lateral secondary roots arise from this central core (Fig. 1.2). Carrots and other true root crops such as sugar beet have very well developed tap roots. These biennials store food, usually as carbohydrates, in their roots during the first year of growth to be used in the following year for the production of the flowering shoot and seeds. However, they are normally harvested after one season and the roots are used as food for humans and livestock.

Figure 1.1  Tap root or primary root system.

Figure 1.2  Tap root of the bean plant.

1.3.1.2. The adventitious root system

This is found on all grasses and cereals, and it is the main root system of most monocotyledons. The primary root is soon replaced by adventitious roots, which arise from the base of the stem (Fig. 1.3). This forms a large mass of similarly-sized roots. These roots can, in fact, develop from any part of the stem, and they are found on some dicotyledons as well (but not as the main root system), e.g., underground stems of the potato.

1.3.1.3. Root structure

Roots are divided into different sections that perform a range of functions. At the tip of the root is the root cap. This acts to protect the growing root. Its cells can respond to gravity, light and soil particle pressure to guide root growth. It produces a substance known as mucilage which helps the root move through the soil. Behind the root cap is an apical meristem. Cells in the meristem divide and develop into the different tissues found within the root. 4–10mm behind the root cap is found the zone of elongation. The cells in this part of the root can elongate as much as 150 times. This helps to push the root cap and meristem through the soil. Behind this is the zone of differentiation where the different tissues of the root start to form and then the zone of maturation which possesses a large number of root hairs.

Figure 1.3  Adventitious root system.

Figure 1.4  (a) Section of root tip and root hair region. (b) Cross-section of root showing the root hairs as tube-like elongations of the surface cells in contact with soil particles.

Root hairs (Fig. 1.4) are very small white, hair-like structures. They increase the absorptive surface of the root several thousand-fold. As the root grows, the hairs on the older parts die off and others develop on the younger parts of the root. They play a very important part in the nutrition and water uptake of a plant.

1.3.2. The stem

The second part of the flowering plant is the shoot that normally grows upright above the ground. It is made up of a main stem and branches. Stems are either soft (herbaceous) or hard (woody) and in UK agriculture it is usually only the soft and green herbaceous stems which are of importance. These usually die back every year.

Turgor pressure and thickened cell walls in the stem helps to provide a structure that supports the leaves, allowing the best possible access to light, and also the flowers, for presentation to pollinators. Stems provide a route for the transport of water and nutrients up from the roots to the leaves and for sugars and other compounds from the leaves. They also allow the movement of hormone-based signals around the plant to direct responses to the environment and to cause developmental changes. In dicots, the vascular bundles, which carry out this transport, are arranged in a ring close to the edge. This allows growth of the centre of the stem and so sideways (secondary) expansion. This allows the growth of very large stems such as the trunks of trees. In monocots, the vascular bundles are scattered across the stem cross-section, meaning that monocots lack secondary stem growth. Stems are also able to store large amounts of starch and water in parenchyma cells.

Figure 1.5  Jointed stem.

Stems are usually jointed with each joint forming a node. The part between two nodes is called the internode. At the nodes, the stem is usually solid and thicker, and this swelling is caused by the storing up of material at the base of the leaf (Fig. 1.5).

All stems start life as buds and the increase in length takes place at the tip of the shoot called the terminal bud. If, for example, a Brussels sprout is cut longitudinally and examined it will be seen that the young leaves arise from the bud axis, where an apical meristem is present. This meristem is continually making new cells for the growth of the stem (Fig. 1.6). Axillary buds are formed in the angle between the stem and leaf stalk. These buds, which are similar to the terminal bud, develop to form lateral branches, leaves and flowers.

1.3.2.1. Modified stems

1. A stolon is a stem that grows along the ground surface.

Adventitious roots are produced at the nodes, and buds on the runner can develop into upright shoots, and separate plants can be formed, e.g., strawberry plants.

2. A rhizome is similar to a stolon but grows under the surface of the ground, e.g., common couch (Fig. 1.7). If a rhizome is detached from the parent plant or cut into sections, the resulting parts can generate a new plant. This is a form of vegetative reproduction and used in the propagation of certain plants.

3. A tuber is really a modified stolon. The ends of the stolons swell to form tubers. The tuber is therefore a swollen stem. The potato is a well-known example with ‘eyes’ (buds) which develop shoots when the potato tuber is planted. Potato tubers will also turn green by producing chlorophyll when exposed to light; this is another characteristic of stems.

4. A tendril is found on certain legumes, such as the pea. The terminal leaflet is modified as in the diagram (Fig. 1.8). This is useful for climbing purposes to support the plant.

Corms and suckers are other examples of modified stems.

Figure 1.6  Longitudinal section of a Brussels sprout.

Figure 1.7  Modified clover and couch grass stems.

Figure 1.8  Modified pea stems.

1.3.3. The leaf

Leaves in all cases arise from buds. They are extremely important organs, being not only responsible for the manufacture of sugar and starch via photosynthesis for the growing parts of the plant, but they are also the organs through which transpiration of water takes place.

A typical leaf of a dicotyledon consists of three main parts:

1. The blade.

2. The stalk or petiole.

3. The basal sheath connecting the leaf to the stem. This may be modified (as with legumes) into a pair of wing-like stipules (Fig. 1.9a).

The blade is the most obvious part of the leaf and it is made up of a network of veins formed from vascular bundles, with leaf tissue in between.

There are two main types of dicotyledonous leaves:

1. Those with a prominent central midrib, from which lateral veins branch off on either side. These side veins branch into smaller and smaller ones (Fig. 1.9a).

2. Those with no single midrib, but several main ribs spread out from the top of the leaf stalk; between these, the finer veins spread out as before, e.g., horse-chestnut leaf (Fig. 1.9b).

The veins are the essential supply lines for the process of photosynthesis.

Their vascular bundles consist of two main parts: the xylem, for bringing the required raw materials (water and nutrients) up to the leaf from the roots and the phloem, which carries the finished product (sugars and other carbohydrates) away from the leaf, to growing and/or storage regions.

Leaves can show great variation in shape and type of margin, as in Fig. 1.9. They can also be divided into two broad classes as follows:

Figure 1.9  (a) Simple leaf; (b) compound leaf.

1. Simple leaves. The blade consists of one continuous piece (Fig. 1.9a).

2. Compound leaves. Simple leaves may become deeply lobed and when the division between the lobes reaches the midrib it becomes a compound leaf, and the separate parts of the blade are called the leaflets (Fig. 1.9b).

Monocotyledonous leaves are generally simpler but still contain a network of (usually parallel) veins for transport.

The leaves are the main parts of the plant where photosynthesis occurs (Fig. 1.10). The leaf surface is often covered with a waxy cuticle produced by the epidermal cells near the surface. The cuticle helps to reduce water loss and protect against pathogens. The blade surface may be smooth (glabrous) or hairy, according to variety. This is important in legumes because it can affect their palatability. Below the epidermis are two types of mesophyll cells. The palisade mesophyll cells contain many chloroplasts and are the main site of photosynthesis, while the spongy mesophyll tissue contains many air spaces between the cells which as important for the exchange of gases into and out of the cells. Below this is the lower epidermis.

A very important feature of the leaf structure is the presence of large numbers of tiny pores (stomata) on the surface of the leaf, in the epidermis layers (Fig. 1.11). There are usually thousands of stomata per square centimetre of leaf surface. Each pore (stoma) is oval-shaped and surrounded by two guard cells. Carbon dioxide to be used in photosynthesis diffuses into the leaf through the stomata. Most of the water vapour leaving the plant, as well as the oxygen produced from photosynthesis, diffuses out through the stomata.

Monocotyledonous leaves are dealt with in Part IV, Chapter 19 (Grassland).

1.3.3.1. Modified leaves

1. Scales are normally rather thin, yellowish to brown membranous leaf structures, very variable in size and form. On woody stems they are present as bud scales which protect the bud; they are also found on rhizomes such as common couch.

2. Leaf tendrils. The terminal leaflet on the stem can be modified into thin thread-like structures, e.g., the pea plant (see Fig. 1.8).

Other examples of modified leaves are leafspines and bracts.

Figure 1.10  Cross-section of green leaf showing gaseous movements during daylight.

Figure 1.11  Stomata on leaf surface.

1.3.4. The inflorescence

Special branches of the plant are modified to bear the flowers, forming what is called the inflorescence. There are two main types of inflorescence:

1. Indeterminate fluorescence. Here the branches bearing the flowers continue to grow, so that the youngest flowers are nearest the apex and the oldest farthest away (Fig. 1.12a). A well-known example of this type of inflorescence is the spike found in many species of grasses such as wheat.

Figure 1.12  (a) Indeterminate inflorescence; (b) determinate inflorescence.

2. Determinate fluorescence. Here the main stem is terminated by a single flower and ceases to grow in length; any further growth takes place via lateral branches, and they themselves eventually terminate in a single flower and growth is stopped (Fig. 1.12b). An example of a species with this type of inflorescence is linseed.

There are many variations of these two main types of inflorescence. For example, some plants, such as sunflowers, have a head called a capitulum, while others, such as carrots and parsley have inflorescences called umbels. The inflorescences of oats and many grass species, meanwhile, are referred to as panicles.

1.3.5. The flower

Plants use a number of different approaches to reproducing via the use of flowers. The majority of plants possess hermaphrodite flowers that contain both male and female parts. However, a small number of plants possess separate male and female flowers on the same plant (referred to as monoecious species) e.g., maize, hazel and squashes, while around 6% of flowering plants species carry male and female flowers on separate plants (referred to as dioecious species) e.g., holly.

The remainder of this section refers to the structure and reproduction of a typical hermaphrodite flower.

In the centre of the flower is the axis that is simply the continuation of the flower stalk. It is known as the receptacle and on it are arranged four kinds of organ. Whilst many flowers are more complicated in appearance than the one described here, they still consist of these four main parts:

1. The lowermost is a ring of green leaves called the calyx, made up of individual sepals. The sepals form the outside of the closed flower bud and so protect the remainder of the flower at this stage.

2. Immediately above the calyx is a ring of petals, known as the corolla. The number, shape and colour of petals on a flower can differ widely between species. Petals are usually brightly coloured and their function is to attract pollinators (usually insects). Flowers can also use scent to attract pollinators. At the base of the petals are often found modified structures called nectaries. These, as their name suggests, produce the sweet nectar that acts as a reward for a visiting pollinator.

3. Above the corolla are the stamens, again arranged in a ring. These are the male part of the hermaphrodite flower. They are similar in appearance to an ordinary match, the swollen tip called the anther sitting on top of the filament, which holds the anther in place. When ripe, the anther bursts open to release the pollen grains. The filaments can be of varying sizes, either keeping the anther inside the flower or allowing it to hang outside (useful in plants which are pollinated by wind rather than e.g., insects).

4. The highest position on the receptacle is occupied by the pistil which consists of one or more small green bottle-shaped bodies called carpels. These are comprised of three parts: the stigma, style and the ovary. The style supports the stigma, which is the site of pollen binding and germination. The ovary contains ovules that, when fertilised, will form the future seeds (Figs. 1.13 and 1.14).

Figure 1.13  Longitudinal section of a simple flower.

Figure 1.14  Carpel detail.

1.4. Plant physiology

1.4.1. Transpiration

The evaporation of water from plants is called transpiration. It mainly occurs through the stomata and has a cooling effect on the leaf cells. Water in the cells of the leaf can pass into the pore spaces in the leaf, such as in the spongy mesophyll tissue, and then out through the stomata as water vapour (Fig. 1.10).

The rate of transpiration varies considerably depending upon the environmental conditions and status of the plant. It is greatest when the plant is well supplied with water and the air outside the leaf is warm and dry, creating a large water vapour gradient. When the guard cells are turgid (full of water) the stomata are open. When the plant is under drought stress, the guard cells lose water and the stomata close, slowing down the loss of water vapour (transpiration) from the plant. However, this also slows down the rate of photosynthesis. The stomata also close in very cold weather, e.g., 0°C. Transpiration is also retarded if the humidity of the atmosphere is high because there is only a very small water vapour gradient between the inside of the leaf and the outside atmosphere. The stomatal guard cells also close (and so transpiration ceases) during darkness. This is to minimise water loss when photosynthesis ceases and the benefits of CO2 uptake are lost.

1.4.2. Conduction

The conductive flow of water through the plant takes place in the xylem tissue that runs in bundles along the length of the root and stem and into the organs of the plant. The xylem vessels, which carry the water and mineral salts from the roots to the leaves, are tubes made from dead cells called tracheids and vessel elements. The cross walls of these cells are no longer present and the longitudinal walls can be thickened with lignum to form wood. These tubes help to strengthen the stem.

1.4.3. Translocation

The movement of food materials through the plant is known as translocation. The phloem forms a branched system throughout the plant and transports assimilates from sites of production (sometimes called sources) to sites of demand or consumption (sinks). The phloem tubes therefore carry organic material through the plant, e.g., sugars and amino acids, from the leaves to storage parts or growing points. In contrast to the xylem, the phloem vessels are chains of living cells, are not lignified, and possess cross walls which are perforated. They are sometimes referred to as sieve elements which, along with companion cells that provide energy and proteins, form the sieve tube members.

1.4.4. Osmosis

Much of the water movement into and from cell to cell in plants is due to osmosis. This is a process in which a solvent, such as water, will flow through a selectively permeable membrane (e.g., a cell membrane) from a weak solution to a more concentrated one. The cell membrane only allows the water to pass through, as the molecules in solution are too big. The force exerted by such a flow is called the osmotic pressure. In plants the normal movement of the water is from the soil solution into the cells. However, if the concentration of a solution outside the cell is greater than that inside, there is a loss of water from the cell, and it contracts; this is called plasmolysis.

1.4.5. Uptake of water

Water is taken into the plant from the soil. This occurs mainly through the root hairs near the root tip. Their function is to increase the surface area available for absorption of water. There are thousands of root tips on a single healthy crop plant (Fig. 1.4). As the root grows the hairs are constantly replaced.

A number of processes are involved in the uptake of water into and their subsequent flow through the plant. As the cell contents and intercellular spaces of the roots are usually more concentrated than that of the external rootzone, water moves into roots via osmosis. This creates pressure that can help drive water flow up through the xylem. In addition, transpiration of water from leaves creates a negative pressure higher up the plant, providing an additional driver for xylem sap flow. Finally, water movement along the narrow vascular tubes is aided by capillary action arising from interactions between the water molecules and the walls of the tubes.

The rate of absorption of water into roots is slowed down by:

1. A shortage of water in the soil.

2. A lack of oxygen for root respiration (e.g., in waterlogged soils).

3. A high concentration of salts in the soil water near the roots.

Normally, the concentration of the soil solution does not interfere with water absorption. High soil water concentration can occur in salty soils and near bands of fertiliser. Too much fertiliser near developing seedlings may damage germination and subsequent emergence by restricting the uptake of water.

1.4.6. Uptake of nutrients

The absorption of chemical substances (nutrients) into the root cells is partly due to a diffusion process but it is mainly due to the ability of the cells near the root tips to accumulate such nutrients. Nutrients are taken into the root in the form of charged ions through the root hairs, along with water. The water and solutes move through the cells into the inner ring of xylem. They are prevented from leaking back into the soil between the cell walls by a waxy layer of cells called the Casparian Strip. In this way an electrochemical gradient is produced which allows the flow of nutrient ions into the plant from the soil solution.

1.4.7. Photosynthesis

In photosynthesis, specialised pigments such as chlorophyll A and B use light energy (normally sunlight but sometimes artificial) to change carbon dioxide and water into sugars (carbohydrates) and oxygen in the green parts of the plant. Indeed, it is these pigments which give these parts of the plant their green appearance, because chlorophylls primarily absorb red and blue light but reflect green light. The cells that contain chlorophyll can also possess orange/yellow pigments such as xanthophyll and carotene, and brown pigments called phaeophytins, which absorb different wavelengths of light to the chlorophylls. Crop plants can only build up chlorophyll A and B in the light, and so any leaves that develop in the dark are yellow and cannot efficiently produce carbohydrates. The yellowing of leaves (chlorosis) can also be caused by disease attack, nutrient deficiency or natural senescence (dying off).

The process of photosynthesis may be set out as follows:

(a) The light reactions (light-dependent)

    This takes place in the thylakoid membranes inside the chloroplast, an organelle found inside the cells of green plant tissue. The thylakoid membranes, which are the site of proteins containing chlorophyll, are stacked into structures called grana. Light provides energy for the chlorophyll molecules which causes the release of electrons. By passing through a series of proteins found in the thylakoid membranes, these electrons provide energy to form the energy storage molecule adenosine triphosphate (ATP), with water as a by-product. The chlorophyll molecules then regain electrons from the splitting of water into oxygen and hydrogen.

    The hydrogen and ATP are then used in the next stage:

(b) The dark reactions (light-independent)

    This takes place in the watery stroma of the chloroplast. Here, via a series of chemical reactions known as the Calvin Cycle, and using energy released by ATP molecules, the hydrogen is combined with carbon dioxide to form carbohydrates.

The overall reaction for the two stages of photosynthesis can be written:

CO2 +2H2O → (CH2O)+O2 +H2O

Photosynthesis therefore not only provides the basis for all food production but it also supplies the oxygen which animals and plants need for respiration The carbohydrates (CH2O) are simple sugars, which can be moved through the vascular system of the plant in solution to wherever they are needed. The simpler carbohydrates, such as glucose, may be built up to form starch for storage purposes or as cellulose for building cell walls. Fats and oils (lipids) are formed from carbohydrates by a process of esterification which produces mostly triglycerides. These are usually found in seeds and are a form of concentrated energy. Protein material, which is an essential part of all living cells, is made from carbohydrates and nitrogen compounds and also frequently contains sulphur. These form from amino acids which are held together in proteins by peptide bonds. Proteins can carry out structural roles, but many act as enzymes which catalyse chemical reactions in the plant.

The amount of photosynthesis that takes place each day is limited by the duration and intensity of sunlight, and the ability of the green parts of a plant to capture it. It will therefore depend upon weather conditions (such as cloud cover) and factors affecting day length, such as latitude and time of year, and shading by other plants, an important consideration in crop canopy and weed management. The amount of carbon dioxide available can also be a limiting factor. Shortage of water, low temperatures and leaf disease or damage can also reduce photosynthesis (Fig. 1.15).

The light reactions and the Calvin Cycle are common to all photosynthetic plants. However, some plants have evolved variations which aid photosynthesis under different conditions. The majority of plants carry out what is known as C3 photosynthesis, where all steps take place in the chloroplasts of mesophyll and other cells. This is so-called because the first product of carbon fixation is a compound containing three carbon atoms. While this method of photosynthesis works well in most environments, in hot and dry conditions, when stomata close to reduce water loss, it performs poorly as oxygen builds up inside the leaf and photosynthetic enzymes bind this oxygen instead of carbon dioxide.

Figure 1.15  Overview of transport and photosynthesis in a plant.

In order to deal with this, some plants have evolved variations, known as C4 (where the first product of carbon fixation is a compound containing four carbon atoms) and crassulacean acid metabolism (or CAM) photosynthesis. In C4 plants, photosynthetic processes are split between mesophyll and bundle sheath cells which surround the vascular bundles. This helps to reduce oxygen binding to the photosynthetic enzymes. A number of important crops are C4 plants, such as maize. In CAM plants, the stomata are closed in the day to reduce water loss and open at night to allow carbon fixation. Both of these approaches help to increase photosynthesis and reduce water loss in hot and dry environments.

1.4.8. Respiration

Plants, like animals, carry out respiration i.e., they break down, or metabolise, sugars such as glucose in order to release energy, stored again as ATP. To do so, they also need to take in oxygen to act as an acceptor for some of the electrons released during respiratory processes, which results in the production of water. Respiration also results in the release of carbon dioxide. Respiration therefore appears, superficially, to be the reverse of photosynthesis, with carbohydrates and oxygen as the inputs and carbon dioxide and water (plus energy) as the resulting products. However, it is actually more complicated than this and involves different metabolic processes taking place.

There are three main processes. The first is glycolysis, performed in the cell cytoplasm, where simple sugars such as glucose are split to release energy (as ATP) and to form pyruvate, water and an electron carrier molecule, called nicotinamide adenine dinucleotide (NADH). The second stage is carried out in the mitochondria and is called the ‘Citric Acid’ or ‘Krebs’ cycle, where the pyruvate is converted to citric acid, which cycles within the system through intermediate molecules, releasing carbon dioxide and generating more NADH and some further ATP. In the third and final stage, known as oxidative phosphorylation, the NADH molecules donate electrons to proteins found in the inner membrane of the mitochondria. Like in photosynthesis, the electrons pass through a series of these proteins to release energy for the formation of more ATP. Oxygen then acts as an electron acceptor and water is produced.

The ATP molecules can then be moved around the plant to provide it with the energy it requires for metabolism, growth and development.

Oxygen availability is important for the growth of plants. Indeed, their growth may be limited by the availability of oxygen in the rootzone. If the air spaces in the soil fill with water and it becomes waterlogged, this lowers oxygen availability and the roots may not be able to access sufficient oxygen for growth. This is termed hypoxia. If the situation persists and the soil has a complete lack of oxygen, caused by long term flooding, this is called anoxia.

1.4.9. Reproduction

1.4.9.1. Pollination

The first stage in reproduction is pollination. Pollen from the anther is transferred to the stigma via pollinators or by the wind. This may take the form of self-pollination, where the pollen is transferred from the anther to the stigma of the same flower, or cross-pollination when it is carried to a different flower, most often on a different plant of the same species (Fig. 1.16). The vectors of pollen transfer can also vary. Some flowers are insect pollinated (entomophilous) and are usually scented and brightly coloured. The pollen itself is usually sticky or oily. Other flowers are wind pollinated (anemophilous) and do not need to be brightly coloured. They produce huge amounts of pollen (as most of it is lost) and the pollen grains are smooth, light and small, aiding their transfer by the wind. The flowers of wind pollinated plants are often unisexual with a predominance of male flowers. Stamens and stigmas often hang outside the flowers, and the stigmas are often feathery to give them a better chance of trapping a pollen grain as it blows past.

Figure 1.16  Self- and cross-pollination.

Figure 1.17  (a) Bean seed attached to the inside of the pod by the funicle. (b) Bean seed showing the hilum and micropyle.

1.4.9.2. Fertilisation

After a pollen grain attaches to the stigma, it soon germinates and a pollen tube containing three nuclei (one tube nucleus and two male nuclei) begins to grow. The pollen tube grows down the style of the carpel, towards the ovary sac. When the ovary is reached, the pollen tube bursts. This releases the nuclei. One male nucleus fuses with the egg cell in the ovule and the other joins with a second nucleus to form the primary endosperm nucleus. This double fertilisation is unique to flowering plants.

The ovule itself goes on to form the seed. The ovary also changes after fertilisation to form the fruit which encloses the seed.

With grasses, and therefore cereals, there is only one seed formed in the fruit and, being so closely united with the inside wall of the ovary, it cannot easily be separated from it. This one-seeded fruit is called a grain.

1.4.9.3. Asexual reproduction

Some plants can also reproduce without the processes of pollination and fertilisation. Rhizomes and stolons can generate new plants by forming roots and shoots from their nodes. Plants can also be propagated by the use of cuttings made from stems or leaves. This is sometimes used in horticulture to obtain many genetically identical plants from a single parent. One drawback of these asexual forms of reproduction is a lack of the genetic diversity that is introduced via pollination and fertilisation and which can cause problems with susceptibility to issues such as pests and diseases.

1.5. Seeds

1.5.1. Dicotyledon

A good example of a dicotyledon seed is the field bean. If its pod is opened when nearly ripe it will be seen that each seed is attached to the inside of the pod by a short stalk called the funicle. All the nourishment that the developing seed requires passes through the funicle from the bean plant. When the seed is ripe and has separated from the pod, a black scar, known as the hilum, can be seen where the funicle was attached. Near one end of the hilum is a minute hole called the micropyle