Plant responses

Role of auxins

Controls cell elongation, prevents lead fall and maintains apical dominance. Involved in the coordination of directional growth and stimulating ethene production

Role of gibberellins

Causes cell elongation, triggers the mobilisation of food in germination and stimulates pollen tube growth in fertilisation

Role of cytokinin

Promotes cell growth and inhibit senescence

Ethene

Causes fruit ripening, promotes abscission in deciduous trees

ABA (abscisic acid)

Maintains seed/bud dormancy, stimulates cold protective responses and stimulates stomatal closing

Plant hormone mechanism

They are released by active transport, diffusion or mass flow into the xylem or phloem. They have varied uses, particularly when working in conjunction with other plant hormones. It uses a decentralised production with various tissues detecting and independently releasing hormones which bind to receptors on the target cells

Mechanism of fruit ripening

In climacteric fruits, ethene is used commercially, with fruits picked unripe and kept in cold conditions until needed. This reduces the risk of damage during transportation due to firmness. Use of ethene allows the ripening process to be completely controlled

Mechanism of seed germination

Species tightly regulate germination time until certain abiotic factors are exact, this increases risk of survival. Seeds can remain dormant for long periods of time, and are ultimately germinated by the action of gibberellins

Response to darkness as an abiotic stress

Many seeds only germinate in darkness as this indicates they are properly buried where they have proper access to water and minerals and are hidden. This means that seeds only have limited food reserves to reach the surface - allocates all resources to stem growth (no chloroplasts, less leaf and root growth and thin cell walls) - this is called etiolation

Mechanism of leaf loss

Deciduous trees lose their leaves in autumn as negatives>benefits. Loosing leaves reduces chance of wind or storm or frost damage. Controlled leaf loss allows plants to reabsorb nutrients for spring growth (senescence) and suberization and release of cellulase to break down cell walls and block of leaves (abscission)

Response to onset of freezing conditions

As day length shortens and temperatures decrease, certain genes are supressed or activated leading to the production of polysaccharides and proteins where they are stored in the vacuole to lower its freezing point. Sustained longer temperatures reverses this change

Response to water deficit

ABA is released in times of stress which binds to receptors on guard cells leading to the release of potassium and chlorine ions out of cells, causing water to move out and the cells to go turgid, closing the stomata

Tannins

Type of chemical plant defence with a bitter taste which are toxic to insects as they bind to the digestive enzymes in their saliva and disable them

Alkaloids

A nitrogenous chemical plant defence with a very bitter taste which are toxic and effect animal metabolism (e.g caffeine)

Terpenoids

Type of chemical plant defence acting as toxins and repellents to insects (e.g pyrethrin which is an insect neurotoxin or citronellal which repels insects)

Pheromones

Airborne molcules which trigger responses within plants, for example Volatile Organic Compounds (VOCs) are detected by receptors on other parts of the plant or other plants and cause chemical defences and can attract predators or parotoids to kill and scare of herbivores