– tropic movement are growth movement in response to external stimulus
– usually stimulus acts from a given direction
– direction of movement is determined by direction of stimulus
– plant hormones are involved in tropic responses
– E.g phototropism, geotropism, hydrotropism, thigmotropism, chemotropism
Nastic movement are also growth movements in response to external stimulus
– plant hormones may not be involved
– environmental factors such as humidity, temperature may be involved
– E.g opening of flowers, sleep movement in leaves
– Tropic responses generally last longer than nastic movement
Phototropism:
– movement of shoots in response to light
– caused by unequal distribution of auxin
– auxin on illuminated side is photo-oxidized
– reduces concentration on the side
– results in less growth on illuminated side
– more growth on dark side causes bending toward direction of light
– its significance to the plant is that it allows for maximum exposure of photosynthetic parts to light source
Geotropism
– movement of roots in response to gravity
– also caused by unequal distribution of auxin
– in a horizontally placed seeding
– auxin accumulates on lower half of seeding
– auxin inhibits growth in roots
– results in lower growth on lower side
– more growth on upper side causes root to bend downward
– its significance to plant is that it allows for establishment and maintenance of polarity
Hydrotropism:
– movement of roots toward moisture
– caused by soil moisture tension
– its significance is that it enables roots to locate sources of water for proper plant growth
Chemotropism:
– movement in response to certain chemical substances
– E.g in growth of pollen tube towards ovary in flowering plants
– allows for fertilization of ova
Thigmotropism:
– movement in response to touch
– e.g tendrils of climbing plants, touch sensitive plants ( Mimosa pudica), and carnivorous plants ( sundew, Venus flytrap)
– movement enable plants to obtain support, protect delicate tissue and obtain nutrients
– nastic movement generally involves different cell division in one part of the plant
– may also involve expansion of certain cells
– result in localized bending movements in the appropriate parts of the plant
– method of stimulus transmission is not very clear
– opening of flowers timed to coincide with the most likely period that pollination agent may be available
– E.g morning glory
– ensures pollination
– diurnal/sleep movements in leaves correspond to daily fluctuations in intensity
– Allows maximum absorption of available sunlight.