Responses by plants towards environmental stimulation

Tropisms:

As we mentioned earlier, a tropism is a growth movement in response to a unidirectional stimulus...

Usually:

• Can't involve mature tissues, so confined to meristematic regions ( apical tips, intercalary areas)
• Usually involves elongation rather than cellular division... faster... hormone?
• It is usually difficult to separate out the stimulus... is it light or gravity, if a plant grows up?

Example: Gravitotropism in which when a seed germinates, the stem grows up and the roots down...

Response time: 30- 60 minutes

Presentation time: seconds to 25 minutes

Reaction time: 10- 120 minutes

Perception: Statolith Theory 1900

• Statoliths ( starch bodies) are found in the root cap and endodermis of stem
• They are temperature sensitive... under low temperatures or with treatment with gibberellins, they can disappear and there is no reaction to gravity.
• In some plants, instead of statoliths, there are calcium oxalate crystals instead.

How do these cellular inclusions perceive gravity? several hypotheses were defined...

Is it the position of the statolith? or is it the movement of the statolith? or is it a case of pressure release?

The change in pH can result in changes in osmotic potential, concentrations of reducing sugars, enzyme activities, respiration rate.

Other theories relating to the statoliths and alternative mechanisms:

• Starch could serve as glucose substrate for cell wall synthesis
• Fall of statoliths brings down enzymes which release IAA/ gibberellins
• Statoliths act as pressure probes... microfilaments stretch open channels so hormones flow
• Alternatively, since some mutants do not have statoliths but still respond to gravity... theorize it may be a pressure response, where proteins between the cell wall and cell membrane may be required for sensing..

Second stage ---> Once sensitized, how is the response initiated?

The original explanation suggested above has since been modified:

1. Differential sensitivity on the sides in relation to Ca+ and auxin

2. Must be other hormones involved... perhaps ABA ( inhibitory) and Gibb ( +)

3. Other physiological factors may be involved --> polarization resulting in metabolic changes.

Why do plants bend to light?

We know that plant bending is due to the side of the plant toward the light source not growing as rapidly or at all relative to the shaded side which results in the bending....

a. Is it that light destroys IAA along the lite side?

b. Or does light cause the transport of IAA toward the shaded side only?

Briggs in 1950's showed coleoptiles grown in light or dark have equal amounts of auxin on both sides, so a is not a viable mode....

1. Further experiments suggest that the effect is independent of light intensity, but the action spectrum of the reaction matches with the absorption spectrum of B carotenoids and (ribo)flavin

Based on this observation, it has been hypothesized that:

Carotenoids ---> broken down into xanothin ( abscin) which is a known inhibitor of growth --->

Xanothin may:

a. inhibit the transport of auxin ( however the rate of transport seems to slow for reaction time) or

b. destroy auxin or

c. inhibit auxin (( light may repress the release of free auxin from bound forms)). This would be a quick response...

However at this time the complete answer is not known.

We know that many organisms respond to changes in light hours ( god know I;m trying to recuperate from Sunday's change)... some people exhibit the SAD syndrome

In plants, photoperiodism was studied intently by Garner and Allard who experimented with early mammoth tobacco strains.

A mutant strain did not bloom till December, no matter when it was planted. They grew this strain under different environmental conditions - the only consistent factor which affected blooming time was the length of day... it had to be shorter than a specific number of hours.

From these and other experiments with mutant soybean strains they came to the conclusion that:

short day plants	neutral day plants	long day plants
need a long night include mums, primroses and strawberries	cucumbers, sunflowers	summer plants... spinach, lettuce ( which bolt as summer commences)

Time is precise! different genotypes respond to differences of 10 minutes...

Continued experimentation with cockleburs indicated that if the 'night-period' was interrupted with just a minute of light, flowering could be interrupted:

Short-day plants:

------------------------------------------------|---------------------| No flower

16 hours light ......................................... 8 hrs nite.......

------------------------|---------------------------------------------| Flower

8 hours light ......................................... 16 hrs nite.......

------------------------|-----------------------|*|-------------------| No flower

8 hours light ...................8 hrs nite...........8 hrs nite.......

Long-day plants:

------------------------------------------------|---------------------| Flower

16 hours light ......................................... 8 hrs nite.......

------------------------|-----------------------|*|-------------------| Flower

8 hours light ...................8 hrs nite...........8 hrs nite.......

They refined the experiments and found that:

red light 660 nm was most effective

far red 730 nm reversed the effect of red so that if when in sequence i.e., red -> fr -> red ->... whatever was last was induced either the flowering or not.

From this they suggested the following:

Phytochrome Pr 660 - inactive form	sunlight 660 nm ------------------------------>	Pfr ( biologically active form):
	<----------------------------- 3-4 hrs of 730 nm or natural reversion during nite	promotes flowering of long day plants and inhibits of short-day plants

During the day, both wavelengths are given off by the sun but at night, Pfr drops.

Besides flowering, what else is impacted by these phytochromes?

1. Seed germination and seedling growth...lettuce and weeds germinate when exposed to light ( so plow at night!)...

It promotes leaf expansion and straightening of seedlings

It promotes chlorophyll synthesis and plastid development as well as anthocyanin... plants grown in dark are etiolated ( white and spindly)

2. Shading effects: as light hits leaf, plants absorb <700 wavelength... but 700-800 passes through which then converts Pfr into Pr... this results in rapid internodal elongation ( reach for sun effect)