Download Plant Tropism and Defenses

Plant Tropism and Defenses
• Lights important affect on plants
– Photosynthesis… conversion of CO2 & H2O to energy containing
carbon compounds and O2
– Phototropism… movement of plants toward light
– Photomorphogenesis… the induction of important key development
and growth stages due to the presence, intensity, and duration of light
Light Frequencies
• Action spectrum… the wavelengths useful to the plant
– Red and blue light for most plants
• blue-light photoreceptors
– Active in phototropism & slowing hypocotyl elongation when seedlings break
ground
• Phytochromes
– Activate de-etiolation process
– Signals germination in many seed types depending on type of red light
present (near vs far)
» Allows plants to alter metabolism at different times of the day (shade
avoidance)
• Circadian clocks (rhythms) or
biological cycles that are
independent of extraneous
environmental signals
– Are internal to all Eukaryotic
organsims
• Can be reset by outside stimulus…
light (Pr D Pfr)
• Duration of night in some flowering
plants
– May be due to transcription factors
that inhibit further production
through allosteric inhibition.
• Causes an increase in the morning
and a fall in the evening
– Clock will operate in the absence
of day night although timing may
be off
Circadian Clock
Response to External Stimuli
• Gravitropism… causes stem
to travel up and root to
travel down
– Orchestrated through
statoliths (heavy starch
grains) that settle in the
bottom of cells
• Causes a concentration of
Ca2+ and auxin increasing
growth on the upper
portion resulting in bending
of root
• Thigmomorphogenisis
(Mechanical stimuli)…
– causes tree trunks to
thicken and growth to be
stunted
– Plant leaves to curl when
touched… action potentials
resembling nerve impulses
– Vines to grasp…
thigmotropism
Environmental Stresses
• Drought… closes stomata and inhibits shallow root growth (deep
root growth continues)
• Flooding… fills pore spaces and drowns roots
– Oxygen in the air spaces of soil is needed to support cellular
respiration by root systems
• Salt stress… reduces water potential of soil (becomes hypertonic to
roots) dehydrates plant and reduces the plants ability to bring water
to the stems and leaves
– Some plants can produce solutes to counteract increases in salinity
• Heat stress… denatures needed enzymes
– Plants common to hot regions produce heat-shock proteins
(chaperonins) to reduce the effect of the heat
• Cold stress… affects solute transport as membranes become rigid
– Production of unsaturated lipids increases (like cholesterol) to reduce
crystallization and keep the membrane fluid
Plant Defenses
• Herbivory
– Thorns
– Toxins
• kill of disable insect or animal
• Attract predators that feed on possible eaters
• Released to start production of toxins in neighboring
plants