Growth Responses and Regulation of Growth

 Growth- the increase in size of a plant  Development- the gradual changes over the life of the plant  Both are controlled by  Genetics  Hormones (ex. Auxins, gibberellin…)  Environment (ex. Tropism)

 Organic compounds that act as chemical signals between cells  Regulate growth & development

1. Auxin 2. Gibberellin 3. Cytokinin 4. Ethylene 5. Abscisic Acid  Basically, these are just signaling molecules

 Hormone involved in  Stem elongation  Apical dominance (inhibition of lateral growth)  Encourages root formation on cuttings  Helps plant grow towards light  Targets cells that are not reached by light (in shade)  Responsible for Phototropism Animation

 Coleoptile = the first leaf of a monocot seedling

HIGH AUXIN CONCENTRATION Control (roots placed in water) LOW AUXIN CONCENTRATION

 Hormone involved in  Stem elongation  Flowering  Seed germination

 Hormone involved in  REQUIRED FOR CELL DIVISION!!!  Delay of senescence (aging)  Interacts with auxin in the control of apical dominance (encouraging primary growth)

 A gaseous plant hormone involved in  Leaf abscissions (the normal falling off of leaves/fruits/flowers)  Ripens fruits  Weakening cell walls during autumn  Leaf senescence (aging)

 Plant Hormone involved in  Dormancy (temporary state of arrest/growth does not occur)  Response to stress  Ex. Drought – triggers closing of stomata

 The response of a plant to the relative lengths of daylight and darkness (such as flowering)  Short-day plants (long night)  Flower when the night length is = or > ~ 12 hours  Late Summer or fall  Intermediate-day plants  Do not flower when day is either too long or too short  Spring or Fall  Long-day plants (short night)  Flower when night length is = or < 12 hours  Late spring or summer  Day-neutral plants  photoperiod does not affect flowering

 Vernalization  The low-temp requirement for flowering in some plant species  Some plants need to be exposed to low temp for several weeks for flowering to occur after seeds germinate and grow  Example of plant with low T requirement:  Winter Wheat:  planted in fall and germinates.  Seedlings exposed to cold winter  Flower after resuming growth in spring

 A tropism is a directional growth response  Permanent change in position  3 types of tropism  Phototropism - Stimulus  photo = “light”  Gravitropism – Stimulus  grav = “weight”  Thigmotropism – Stimulus  thigmo = “touch”

 PHOTOTROPISM  Directional growth of a plant caused by light  GRAVITROPISM  Plant growth in response to direction of gravity  THIGMOTROPISM  Growth in response to contact with a solid object