Plant co-ordination and Response

Why do plants need to respond to their environment? Avoid stress Avoid being eaten Survive long enough to reproduce The main way that plants can respond to their environment is by growth – plant hormones, also known as plant growth factors regulate this.

Features of plant growth factors Regulate plant growth and response to environmental biotic and abiotic factors Produced in the growing regions of the plant e.g. movement of shoots and tips Meristems - cells still capable of diving. Undifferentiated cells.

Plant Growth Factors Plants have plant growth hormones respond to stimuli such as light, water or gravity and parts of the plant such as the stem or the roots to grow in a certain way They are made by cells located all over the plant Some plant growth hormones affect the tissues that release them rather than a distant target organ.

Plant Growth Factors Plant growth factors are produced in small quantities. An example of a plant hormone is auxin which causes plant cells to elongate. The cell walls become loose and stretchy so the cells become longer.

Plant Growth Factors Hormone Auxins e.g. IAA Cell elongation, inhibition of side shoots; inhibits leaf abscission Cytokinins Promote cell division Gibberellins Seed germination and growth of stems Abscisic Acid Causes leaf fall; inhibits seed germination and growth; stomatal closure when lacking water Ethene Promotes Fruit Ripening

Growth movements of plants are important for plant survival It is essential for plant shoots to grow towards light for photosynthesis and roots to grow down into the soil to absorb water and nutrients.

IAA is an important Auxin When light shines all around a plant, the IAA is evenly distributed and the plant grows straight upwards. When light shines from one side the IAA moves to the shaded side. This causes the cells to elongate on the shaded side causing the shoot to bend towards the light. Experiments using plant shoots were used to establish what caused the shoot to move.

How IAA causes cell elongation The presence of IAA promotes the active transport of hydrogen ions through the ATPase enzyme, into the cell wall. This decreases the pH and allows optimum conditions for the wall loosening enzymes (expansins) to work. These enzymes break bonds within the cellulose, so the walls become less rigid and can expand as the cells take in water.

Plants in the dark Plants grown in the dark: auxin evenly distributed (plant shoots grow straight up). Chloroplasts do not develop in darkness. Plants: tall, yellow and spindly. The leaves are often further apart than on a normal plant (etiolated). If the shoots reach the light the chloroplasts will develop and the plant will grow normally. If not, they will die (lack of photosynthesis).

Auxins as weedkillers Auxins can be selective – so that they affect some plants and not others. Auxins can be sprayed onto weeds making them grow very fast – this causes them to die.

Ethene A plant hormone. Used by farmers when they want the fruit to ripen. Tomatoes are picked when they are green and hard. When the supplier wants the fruit to ripen he exposes the tomatoes to ethene. (This prevents the fruit going bad in transport). Ethylene: old IUPAC name

Comparing hormones with plant growth factors PFGs Produced by… Endocrine glands Various tissues and cells Move by… Blood Mass flow/ active transport/ diffusion in phloem or xylem

Similarities between hormones and plant growth factors Only bind to specifically shaped receptors Can move to other areas in the organism Chemical messengers Long lasting response on the organism e.g. bananas ripening; secondary sexual characteristics in animals

Cytokinins High cytokinins prevent leaf fall Low cytokinins leaf fall occurs Low cytokinins; less auxin; more sensitive to ethene; cellulase action increases allowing the leaf to break from the stem

Plenary Plant Growth Hormones Practice Exam Qs

Plenary Give three similarities between IAA and animal hormones.