#42 Plant nutrition (English)

Plant Nutrition

1. Photosynthesis

The fundamental process by which plants manufacture carbohydrates from raw materials using energy from light.

Carbon dioxide + Water  ------(light)------> Glucose + Oxygen

6CO₂             +   6H₂O  ------(light)------>  C₆H₁₂O₆  +    6O₂

 Photosynthesis occurs in the leaves of a plant.

How photosynthesis happens

•         Carbon dioxide and water enter the cell

•         The cell traps light energy using chloroplasts.

•         The energy is used to split water (H₂O) into hydrogen and oxygen

•         The oxygen is excreted outside the leaf to the atmosphere as a waste product

•         The hydrogen reacts with carbon dioxide forming glucose.

Carbon dioxide supply

Land plants get their carbon dioxide from the air. There is only about 0.04% of it in the air but since the plants continuously carry out photosynthesis, there is always a concentration gradient. The carbon dioxide diffuses into the plants through tiny holes in the leaf called stomata. Then these molecules move through the air spaces between the spongy cells, through the cell walls, cell membranes and cytoplasm of the spongy and palisade cells, and finally into their chloroplasts.

Water supply

The water is absorbed by the roots of the plants. Then it is transported upwards through a hollow tube called the xylem vessel, until it reaches the leaf where photosynthesis takes place. It enters the leaf through holes in the xylem. Excess water leaves the cell through the stomata; this is called “transpiration.”

Light supply

The leaves are always exposed to sunlight at daytime. The green pigment, chlorophyll, found in the chloroplasts of plant cells absorbs light energy from the sun. The light penetrates the transparent layers on the leaf until it reaches the mesophyll layer, where photosynthesis takes place. Palisade cells are nearer to the surface of the leaf than the spongy cells, so they receive more of the light and make more photosynthesis. Guard cells are also able to photosynthesize.

Uses of produced glucose

-  For respiration

-  Converted into starch and stored

-  Turned into sucrose and transported to other parts of the plants

LIMITING FACTORS

Something present in the environment in such short supply that it restricts life processes.

There are several factors affecting the rate of photosynthesis. The most important ones are:

-   Light intensity

-   Concentration of carbon dioxide

-   Temperature

Light intensity

In the first part of the curve, as light intensity increases, the rate of photosynthesis increases. It is then a limiting factor as the rate of the reaction is being kept low or limited by the lack of light.

However, it eventually reaches a point where the plant is photosynthesizing as fast as it can. Light stops being a limiting factor as even if more light is shone on the plant, the rate of the reaction is still the same. This is probably because another limiting factor is holding it back.

Carbon dioxide

CO2 is a key limiting factor. In perfect conditions, a lack of CO₂ holds back the rate or reaction. Like light intensity, as a plant is given more carbon dioxide it will photosynthesize faster. This is so until it reaches its maximum. From here onwards, something else must be the limiting factor.

Temperature

The chemical reactions of photosynthesis can only take place very slowly at low temperature, so a plant can photosynthesize faster on a warm day than on a cold one. However, plants can’t photosynthesize either if it gets too hot.

Greenhouses

A green house is a placed covered by transparent polythene. In green houses, the limiting factors of photosynthesis are eliminated, and the plants are provided the most suitable conditions for a healthy, rapid growth.

The soil in green houses is fertilized and very rich in mineral ions, assuring healthy, large yields. More carbon dioxide is supplied to the crops for faster photosynthesis. The polythene walls and ceiling allow heat waves and light rays only to enter and prevent harmful waves, thus providing a high light intensity and optimum temperature, sometimes a heating system is used too. A watering system is also present. The disadvantages of green houses are that it is too small to give a large yield and that it is expensive.

2. Leaf structure

Upper Epidermis: it is a layer of cells that covers the leaf and protects it. It is covered by a layer of wax called cuticle.

Mesophyll Layer:

·         Palisade Mesophyll: a layer of palisade cells which carry out most of photosynthesis

·         Spongy Mesophyll: a layer of spongy cells beneath the palisade layer, they carry out photosynthesis and store nutrients.

Vascular Bundle: it is a group of phloem and xylem vessels that transport water and minerals to and from the leaves.

Lower Epidermis: similar to the upper epidermis, only that it contains a special type of cells called guard cells. Guard cells are a specialized type of cells that control the passage of carbon dioxide into the cell and the passage of oxygen out of the cell by opening and closing the stomata (a hole in the leaf through which gases pass) so guard cells are responsible for the gas exchange.

How guard cells work

At daytime, the guard cells open the stomata to allow gaseous exchange, this occurs according to the following steps:

•         Sunlight increases the potassium concentration in the vacuoles of the guard cells, the water potential decreases making a gradient between the guard cells and the surrounding epidermal cells.

•         Water moves by osmosis into the guard cells from the epidermal cells.

•         The water raises the pressure inside the guard cells.

•         The cell wall adjacent to the stomata is thicker and less stretchable then the cell wall on the other side.

•         The pressure expands the whole cell except for the inner cell wall, creating a curve and a pore between the two guard cells.

•         The stoma opens.

At night however, the mechanism is opposite:

•         Potassium level decreases in the vacuole of the guard cells,

•         Water potential increases in the cell and water diffuses out of it,

•         The guard cells straighten up because of low pressure closing the stoma.

3. Mineral requirements

The plant is also in need for mineral ions to control chemical activities, grow, and produce materials. The most important minerals are:

-  Mg⁺² (magnesium ions): they are important for the production of chlorophyll (chlorophyll synthesis). A lack of it results in lack of photosynthesis and wilting of the leaves.

-  Nitrates: these are the sources of nitrogen; they are required to make amino acids and proteins by combining with glucose. A lack of it results in deformation of the plant structure making it small and weak.

Fertilizers:

Sometimes the soil is lacking of the mineral ions needed. This problem can be solved by adding fertilizers to the soil. Fertilizers are chemical compounds rich in the mineral ions needed by the plants. They help the plants grow faster, increase in size and become greener. They simply make them healthier and increase the crop yield. But there are disadvantages of fertilizers, such as:

- Excess minerals and chemicals can enter a nearby river polluting it and creating a layer of green algae on the surface of it, causing lack of light in the river, thus preventing the aqua plants photosynthesizing.

- When living organisms in the river or lake die, decomposers such as bacteria multiply and decay, respire using oxygen. Eutrophication takes place eventually.

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