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2001 Paul Barlow


All the organic constituents of a chrysanthemum, such as sugars, proteins, fats, cellulose, etc., contain the element carbon, and one function of photosynthesis is to bring new carbon into the plant. It has been estimated that 200 billion tons of carbon are taken from the air each year by the photosynthetic activity of plants. This is done by combining carbon dioxide from the air with water already in the plant, to form sugars.

Photosynthesis requires energy, for the sugars have a higher energy content than the simple compounds from which they are formed, and this energy is obtained from light which is absorbed by the pigments (chlorophylls and carotenoids) in the leaves. Plants consist of more than sugars, however, and these compounds have then to be converted into structural materials such as cellulose and proteins. These conversions also require energy to drive them and this is obtained by breaking down some of the energy-rich sugars into carbon dioxide and water again, in the presence of oxygen. This energy-releasing process is termed respiration and it is similar to the respiratory processes in animals. The second function of photosynthesis, therefore, is to capture energy and to store it in the form of sugars where it is available to power the process of growth.

photosynth.jpg (22995 bytes)

The major problem facing a plant living on land is how can it allow carbon dioxide from the air to pass freely into its leaves without at the same time losing excessive quantities of water by evaporation to the surrounding atmosphere which is usually relatively dry? In the chrysanthemum the outer surfaces of the leaf are protected by a thin waterproof layer (cuticle), and the main pathway by which water leaves the leaf is through the stomata--small pores in the upper and lower leaf surface (see diagram 1.4).

photosynth2.jpg (59346 bytes)

The stomata open during the day to admit carbon dioxide to the large air spaces in the leaf and close at night, when photosynthesis ceases, so as to minimise this loss of water.

In cross-section (see diagram 1.4) can be seen that the chrysanthemum leaf is very thin. This ensures that carbon dioxide entering through the stomata has only a short distance to travel before it can pass into a photosynthesising cell in the middle of the leaf. These cells are packed with chloroplasts containing the green pigment chlorophyll. On the other hand, the thinness of the leaf means that some light passes right through and is not absorbed by the chloroplast pigments.

Light absorption is greatest when the leaves present a broad, fiat, dark-green surface to the incoming light. This can be achieved by avoiding wilting at any stage and by supplying the correct nutrients, especially magnesium which is an important constituent of chlorophyll, at the right concentrations. The spiral arrangement of leaves around the stem also helps to form a broad column of light-absorbing tissue. Indeed, if the object is to get maximum photosynthesis per unit area of ground, the plants should be closely spaced so that all the incoming light is absorbed by foliage and none reaches the soil.

Growers of exhibition blooms, however, are usually more concerned with getting maximum production from each plant. For this purpose the plants should be well spaced so that they cast relatively little shade on one another, and the growing area should be sited where the plants will not be shaded by any neighbouring trees, buildings or other objects. Having arranged for the plants to receive the maximum of light it is also essential to keep their leaf surfaces free of dusts, powders and spray residues, otherwise some of the light will be unable to reach the chlorophyll-containing cells in the middle of the leaf and will be wasted.

photosynth1.jpg (29411 bytes) The uptake of nutrients and water
The organ which is responsible for extracting nutrients and water from the growing medium is the root. The area which is mainly responsible for this lies behind the root tip where many of the surface cells have a long, finger-like protrusion from their outer walls. These root hairs, which are shown in diagram 1.5 increase the area of absorbing surface.

The concentration of nutrients in the growing medium is usually much lower than that in the root hairs so that if they did not have a semi-permeable membrane, the nutrients would pass out again. The accumulation of nutrients from a weaker solution needs energy to carry them across the cell membranes and into the cytoplasm and vacuole, and this energy is obtained from the respiratory breakdown of sugars in the root cells. Respiration requires a continual supply of oxygen, and the levels of oxygen in the growing medium can be depleted quite rapidly if the air spaces around the roots are eliminated by over-watering or by unnecessary compaction of the medium. In these circumstances, respiration is inhibited, nutrient uptake ceases and the plant may show signs of nutrient deficiency.

A further feature of nutrient uptake is that the accumulation is selective and some nutrients can be absorbed in preference to others, even though they may be present in lower concentration in the growing medium. The best policy, however, is to ensure that the nutrients are present in the medium in the appropriate proportions.

In a well-aerated medium, nutrient uptake proceeds normally and the root hairs have a higher concentration of nutrients than in the growing medium. As a result, water passes across the semi-permeable membranes of the root hairs and enters the root by osmosis. It then passes across to the transport tissues at the centre of the root which are continuous with those in the stem and the veins of the leaf.

The arrangement of transport tissues in the stem is shown in the diagram. Water and nutrients move together in the xylem; a group of large, dead cells with thick walls which form a series of continuous hollow tubes that run from the roots to the leaves. Organic materials are carried in the small, living, thin-walled cells of the phloem. Each phloem cell is a separate unit, but they have many connections with their neighbours.

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Last updated on 26 June, 2002