•A growth response towards or away from an
environmental stimulus coming from one
direction
A shoot growing towards the light is positively
phototropic
A root growing towards gravity is positively
geotropic.
Roots in soil grow away from copper pipes, we
say these are negatively chemotropic.
•The response of plants to diffuse stimuli that
do not come from any particular direction, e.g.
temperature, humidity and light that surround a
plant.
E.g. the opening and closing of flowers to
different light intensities.
Photonasty – a response to alterations in the
light intensity.
Thermonasty – a response to changes in the
temperature.
•Plants have a “guidance system” or
mechanism designed to make sure they grow
in a suitable manner.
•This presupposes 3 things:
A sensing system
A response system
A mechanism connecting the 2.
•Plants have at least 3 distinct photoreceptors.
•Phytochrome – (blue green) that controls the
development of plants and flowering.
•Chlorophyll – (green) and other yellow
substances called Carotenoids, that run
photosynthesis.
•Riboflavin (yellow) and Beta Carotene
(orange)
•Coleoptiles contain riboflavin and beta
carotene.
•When pieces of membrane from coleoptile
cells were examined they had the same
absorption spectrum, and the only chemical
found in the membrane was flavin.
•So it seems the light detector in phototropism
is a membrane-bound flavin pigment
system.