Chapter 28

External factors and Plant growth

Nastic Movement

• Nastic Movements- plants movement that occur in response to a stimulus independent of position of stimulus ( leaves light and dark cycle)

• Phototropism- response to light- caused by elongation- under the influence of Auxin

• Gravitropism- response to gravity• Thigmotropism- response to touch

External Factors and Plant Growth

• Tropism- the growth response involving bending, or curving, of a plant part toward or away from an external stimulus determines the direction of movement

Went’s experiment- chemical produced by growing tips influences direction of growth

What role does the light play in the phototropic response?

• Light decreases the auxin sensitivity of the cells on the lighted side

• Light destroys auxin• Light drives auxin to the shaded side

‘Light eliminates auxin activity’

Tropism video

Gravitropism- starch-statolith plastids hypothesis

Auxin and root gravitropism

Gravitism video

Thigmotropism is growth in response to touch

• Tendrils of bur cucumber

• Enables shoots and roots to navigate, cling and climb

• Thigmotropism in Opuntia acanthocarpa flowers video

Thigmotorpism- tendrils of bur cucumber, twistingCaused by different growthRates on the inside and Outside of tendril

Phototropism

• Common response to light

Diurnal movements of Wood sorrel (Oxalis)

Diurnal movement- biological clock is the organism

keeping itself in time with some external stimulus?

Other types of Movement

• Thigmonastic (seismonastic) Movement are results of mechanical stimulation

• Most night closure (nyctinastic movement) are results from changes in the size of perenchyma cells in the jointlike thickening (pulvinus) structure at the base of each leaf.

Pulvini Mimosa pudica vascular tissue suroundedBy a cortex which consists largely of thin-walledParenchyma cells

Mimosa pudica

Results from changes in turgorPressure in pulvini

Entire leaf drops in responseTo movement, shock, thermal Stimulation, touch

Touch reponse in the venus flytrap (Dionaea muscipula)

Thigmomorphogenesis- The inhibition of growth by touch Arabidopsis thaliana both six weeks of age

Solar tracking Lupine (Lupinus arizonicus)

Sunflower solar tracking