Control and coordination in plants

Plants lack nerves and therefore only possess chemical coordination. Plants show movements of two types: Movement of locomotion and movement of curvature. The latter is more prominent in plants.

Movement of locomotion- changes in position of the whole organism or cellular components. They are of two types:

a) Autonomic or spontaneous movements of locomotion- They occur automatically due to intrinsic reasons. For eg. Flagellar movements in unicellular algae like Chlamydomonas.

Taxis Small algae such as Euglena and Chlamydomonas can exhibit movements of the whole organism and such movements are called taxis

Chemotaxis- Locomotory movement in response to chemicals. b) Paratonic or Tactic movements of Locomotion- They are locomotory movements of complete cells or cell organelles in response to external stimuli. Chemotaxis- Locomotory movement in response to chemicals. Phototaxis- Locomotory movements in response to light.

Movements of curvature- Movements of plants parts in relation to others. They are of two types:

a) Turgor movements- They occur in plant organs which have regions of bending where cells shrink or swell up due to loss or gain of turgidity. These movements are reversible. b) Growth movements- unequal or differential growth in different parts of an organ results in its bending. They are autonomic or paratonic.

Trophic Movements Trophic Movements are further classified into the following- Phototropism – Direction growth movement of curvature in response to light. Shoot apices are positively phototropic while the root apices are negatively phototropic. The light effective for phototropic response is blue light.

Trophic Movements Geotropism-– Direction growth movement of curvature in response to gravity. Shoot apices are negatively geotropic while the root apices are positively geotropic

Trophic Movements Hydrotropism- Directional growth movement of curvature which occurs in response to unilateral stimulus by water. Hydrotropism is generally shown by roots, which are positively geotropic. Activity for testing hydrotropism: Place some germinated seeds in moist saw dust contained in a sieve. The radicles will move down the pores of the seed in response to gravity. However after some time they will curve and re-enter back into the saw dust showing a response towards water, which is greater than gravity.

Trophic Movements Thigmotropism- Directional growth movements of curvature which occurs in response stimulus of contact. In twiners and tendrils – which respond by turning around a support. Chemotropism- Directional movement of growth in response to a chemical stimulus.

Trophic Movements Activity for testing Chemotropism: Take some seeds and germinate them in minimal medium. A drop of nutrient medium containing boron will result in bending of pollen tubes towards it from all directions.

Nasties Another type of plant movement is called nastic movement (plural nasties) – direction of response independent of direction of stimulus.

Plant Hormones

FUNCTIONS OF AUXINS Stimulates cell elongation Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture Stimulates differentiation of phloem and xylem

Stimulates root initiation on stem cuttings and lateral root development in tissue culture Mediates bending in response to gravity and light Delays leaf senescence

The auxin supply from the apical bud suppresses growth of lateral buds The auxin supply from the apical bud suppresses growth of lateral buds Can inhibit or promote leaf and fruit abscission

FUNCTIONS OF GIBBERELINS Stimulate stem elongation by stimulating cell division and elongation. Stimulates flowering in response to long days. Breaks seed dormancy in some plants.

FUNCTIONS OF CYTOKININS Stimulates cell division. Stimulates the growth of lateral buds-release of apical dominance. Stimulates leaf expansion resulting from cell enlargement. May enhance stomatal opening in some species.

FUNCTIONS OF ABSCISIC ACID Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis). Inhibits shoot growth Induces seeds to synthesize storage proteins. Has some effect on induction and maintanance of dormancy.

FUNCTIONS OF ETHYLENE Stimulates the release of dormancy. Stimulates shoot and root growth and differentiation (triple response) Stimulates leaf and fruit abscission.

Stimulates flower opening. Stimulates flower and leaf senescence. Stimulates fruit ripening