Flowerin' in general...
Flowering is the reproductive procedure of plant(s).
The more flowers produced ,the more chances for plant to bear seeds.
Since a flower gets pollinated ,it stops growth and prepares for seed construction.
Flowering sets in ,at different rates* ,as "vegetative growth" ,at different rates* again, ceases....
*
Those rates depend in a wide variety of genome expressions but also in enviromental parameters (stimulant / stimulus ) ....
When referring to "vegetative growth" ,what is meant is the general biological "mode" or "stage" ,where the plant
produces new roots,stalk,stems,leaves,shoots,nodes & branches......Growth,at general..
At a certain point ,plant(s) under the "pressure" of
genome expression or/and external stimulant(s),cease to grow ( gradually )
and begin to produce reproductive organs ( flowers ) ...Again gradually....
*As long ,as flowers do not get pollinated ,plant(s),continue to produce new flowers at high rates,most of cases... *
Regarding annual plants (1 year/season total plant lifetime ),as "grow season"(? ) reaches to end ,
unpollinated flowers wither,and plant dies .
When flowering has set in completely ,plant ceases to produce
new roots,stalk,stems,leaves,shoots,nodes & branches...
Plant produce only flowers at that case ....
So with no new roots ,plant might exaust /spend all natural assimilates ,than can be assimilated by it's root reach ...
If soil nutrients exausted
(take into account autumn seasonal rainfalls [rain=almost distilled water ] which " leach" nutrients ,deep into earth..),
plant turns to own " nutrient storages" (old /"fan" leaves ) translocating mobile minerals (N-P-K-Mg ) from leaves to flowers
and consuming leaf energy resources at same time( starches/sugars=carbonhydrates).
Leaves then turn yellow (chlorosis ),wither and fall ..
Before natural senescence (=growing old ) occurs .
Short Day Plants
As Short Day plants ,are characterised those which possess a special biochemical mechanism ,an "oscillator",
whose main function is to measure the
photoperiod . Daylight hours vs Night hours.
As winter approaches ,"days get shorter" and an
Annual Short Day plant will be setting reproductive (flowering) procedures.
All vegetative metabolic procedures will cease gradually,as mentioned already.
Under Long Days ( i.e daylight >12 hours ) ,or Short Days( i.e daylight <12 hours )increasing to Long Days ( spring->summer ) ,
Short Day plants do not flower.
The " oscillator "...
Photoperiod is measured by plant " sensing " light.Complex natural pressures have led,SD plants to sense "daylight" using certain wls of sunlight ...
If one has a look at
natural sunlight's (reaching Earth's surface )
relative spectrum ,it's easy to see that the deep red wls around 650-680 nm ,
are quite of the lowest in relativepower...!!!! ...???? ...
Being at same time the wls which provide over 5.5 umoles /sec per Watt ..
(large photon flux )...
Also ,Chlorophyll A [
ChA ] has both max "absorption" and "action " peaks close/around that range of wls ...
ChA being the main photosynthetic pigment at Photosystem I [ PS I] .
The "Sun Photon Harvesting System " .
It's not by luck or just a coinsidence that the most abundant Photosynthetic pigment ,
harvests most efficiently the rarest of red -photon rich- wls of natural sunlight....
...
Still under intense sunlight ,those deep reds ,do have the lowest relative power of PAR 400-700 nm light....
So,ChA is made to harvest efficiently ,those "rare",easy absorbable by atmospheric vapor or seawater ,
650-680 nm photons.....
So,for many different reasons ," working daylight period " for plants , is sensed if deep red light ( 650 -680 nm ) is harvested ...
Likewise the "photoperiod measurement system " of SD plants utilise those " rare "deep reds to measure daylight duration...
With peak around 660 -680 nm [
R ] .
....
Using
-likewise photosynthesis uses ChA -a " photo-sensitive " pigment...The phytochrome [
Phy ].
Unlike ChA the Phy doesn't harvest photons to turn them into electrons .
Thus it doesn't not produce energy (or biomass),as a photoreceptor pigment.
It can contribute heavily on that ,but in " indirect ways ",acting on many biological procedures (i.e. at the rate of them).
When Phy harvests a deep red photon ,it just changes "state"...More like a switch..."on" & "off " ..0 or 1 ...
That "states" of Phy,detonate a series of metabolic procedures ,which have serious impact on plant.
Phy is not used for flowering...
At least,solely...
Phy just measures
light duration and
light quantity(irradiance ) ,
by sensing a certain
quality of light.
....its ..."rareness" ,sunlight's "weakest point"- but most photon rich wls
...The most Wanted ?...Do not rush ,to jump into such conclusions,about it...
...
Plants use less blue wls for driving PS ,although blue photons,do carry great amounts of energy...
And they are most abundant in natural daylight...
But photosynthetic pigments are more "receptive" to the lower power reds...
Thus in such a way ,to gather light from all wls almost equally ,depending on their "abudance" in sunlight ,maybe ? ....
Think about it,for a moment...
Phy "sensing & measuring " system is used by plant(s) for many other purposes,beyond setting "time to flower "....
But more about some of them,later on...
Let's stick to flowerin' ,for the moment being...
Phytochrome..How this switch works ?
Well,for starters it must have an " off position "...
Where nothing happens ...
This is called "Phytochrome Ground State " [
Pr ] ..
At this state
Phy is ready to accept
red photons...
Once red photons accepted ,Phy goes to "active state " ....[
Pfr ] .."On position"..
The more R (red light 650-680 nm ) energy { power x duration } ,
the more of ground state Phy - Pr -will rise to active Pfr state....
More intense stimulus....
...
And it is about time ,for statin', that this 'switch' has rather "fluid " final position..
Neither completely "off " or "on " ,---
regarding natural environment,at least..
How's that possible ?
Well ,the
'mystery' lies ,to the way(s) Phy drops back to ground state.....
From Pfr back to Pr....
While there's only one possible way for Pr to 'rise' into Pfr state ,---
being 650-680 nm irradiation...
There are more than one ways,to drop back to Pr state...
Why ?
Because in that way ,Phy measures light power ( or irradiance ),at same time as duration...
So Phy actually, is plant's Light Energy Meter ....
How exactly does that work ?
....
Simplified enough :
1)-
Pfr drops back to Pr ,during total darkness...
The more hours of darkness the more of plant's Pfr will drop to Pr,again....
2)-
Pfr ,partially,decomposes during darkness,while at same time new amount of Phy
(at state Pr ,of course..) is synthesised by plant...
3) -
Far Red [ FR ] irradiation aka Near Infra Red [ NIR ] of sunlight ,turns the active Pfr back to Pr ground state.
The more FR irradiation of both
duration or
power ( total energy,in fact ) ,the more Pfr will 'drop' to Pr state...
Most Phy active range of FR is around 720-740 nm .
.....
-Pr: It absorbs preferentially red light with peak absorbance at 666 nm and when concentrated appears blue. It has minimal absorbance of FR, so that when phytochrome is saturated with FR, there is 97% Pr and 3% Pfr.
So, the maximum possible Percentage of Pr in plant ,
would be 97 % ,if irradiated with FR.
-Pfr: This isomeric form when concentrated appears green in color, and is the active form. It absorbs at a peak of 730 nm (in far red) but also absorbs some in red light, therefore preventing complete conversion of Pr to Pfr. Because of this absorption spectrum overlap, when phytochrome is exposed to saturating amounts of R, there is 85% net conversion to Pfr and 15% is Pr, the “photostationary state”.
Max Pfr percentage under intense R : 85% .
..
Phy also absorbs somewhat in the blue range, and BL also results in conversion of Pr to Pfr. But the contribution of phytochrome (as opposed to other blue-sensitive photoreceptors such as cytochrome) to blue-light exposure can be determined by the degree to which the effect is reversible by FRL.
...
The responses to Pfr include
rapid and
slower effects, and are subdivided as follows, based on total
fluence (total photons expressed as μmol m[SUP]-2[/SUP]),
and the
rate of fluence or
irradiance (expressed as μmol photons m[SUP]-2[/SUP] s[SUP]-1[/SUP]).
Although FR can reverse the effects of exposure to RL,
after a certain period of time the reversal no longer occurs—this is “escape from photoreversibility”.
Very Low Fluence Responses [ Phy VLFR ]: These can be initiated by fluences of RL as low as 0.0001 μmol m[SUP]-2[/SUP]
(1/10 of the amount of light emitted by one firefly flash) and are independent of the rate of exposure.
These include the de-etiolation response in the oat coleoptile and mesocotyl, and the stimulation of
Arabidopsis seed germination by R (0.001 μmol m[SUP]-2[/SUP] to 0.1 μmol m[SUP]-2[/SUP]). These levels of light exposure
convert < 0.02% of Pr to Pfr, and the effect is not photoreversible (since there is always at least this
amount of Pr). Seeds close to the surface and therefore positioned for optimal germination would benefit
from this response arising from the very low level of R reaching them. Interestingly, it has been shown
that tilling fields in the darkness of night, leads to less germination of preexisting underground weed seeds,
apparently because even transient exposure to sunlight during tilling activates them to germinate.
The action spectrum for VLFRs matches the absorption spectrum for Pr, confirming that the Pfr which results is the active form.
Low-Fluence Responses [Phy LFR ]: These are photoreversible and occur with exposures of R > 1 μmol m[SUP]-2[/SUP]
and saturate at about 1000 μmol m[SUP]-2[/SUP]. They include lettuce seed germination, regulation of leaf
movements, etc. The action spectrum for Arabidopsis reveals a peak response at 660 nm and a peak of
inhibition at 720 nm, consistent with Pr and Pfr absorption peaks, respectively.
High-irradiance responses HIRs: These responses require a high exposure rate (not just a high total photon exposure), are proportional to the irradiance and the duration, and are not photoreversible.
These responses include synthesis of anthocyanin in various dicot seedlings, induction of flowering in henbane, enlargement of cotyledons in mustard, and production of ethylene in sorghum. The action spectrum peak at 720 nm in the FR part of the spectrum for darkgrown lettuce seedling hypocotyl elongation inhibition, which is attributable to phytochrome, is due to a photoequilibrium of Pr and Pfr at FR.
(Other peaks at BL and UVA are attributable to cryptochrome CRY1 and CRY2 effects).
There are different forms of phytochrome,
light-labile Type I and
light-stable Type II.
In Arabidopsis, phytochromes are encoded by 5 different genes, PHYA, PHYB, PHYC, PHYD, and PHYE...
PHYA encodes light-labile Type I phytochrome, whereas PHYB encodes
the most abundant light-stable phytochrome...
Genetic analysis of Phytochrome function (in Arabidopsis):
•
Phytochrome A mediates responses to continuous FRL. In particular, VLFR and FR-HIR responses.
•
Phytochrome B mediates responses to continuous White light or RL.
Specifically, certain R-HIR and LFR responses.
•
Roles for Phytochromes C, D, and E are not yet worked out .
• Phytochrome gene functions have diversified during evolution .
...
Other phytochrome-related photoreversible phenomena
include-(when Phy=>Pfr ),regarding SD plants :
*
promotes de-etiolation at seedlings.
*
promotes formation of leaf primordia ,
at seedlings
*
inhibits internode elongation .
*
inhibits flowering.
*
enhances Chl accumulation .
*
promotes growth .
*
promotes replications of plastids.
*
promotes directional orientation of chloroplasts to optimize light capture .
*adaptation To Light Quality Changes of Sun-Seeking Plants:
Some plants respond
to the ratio of red light RL (660 nm) to far red light FRL (730 nm).
This ratio R:FR is 1.2 in daylight, 1.0 at sunset, only 0.13 under a foliage canopy of ivy, 0.88 in soil at 5 mm, etc..
Sun-seeking plants (growing normally in open fields) can detect by the low ratio of R:FR ,
that they are being shaded by other plants[
neighbor detection ability ],and exhibit a shade avoidance response,
by growing taller stems (with a greater internode elongation rate) usually at the expense of reduced leaf area and branching.
This behavior is not seen in shade-tolerant plants, which expect to grow in shaded environments.
Phytochrome is thus important in shade detection.
Understanding the mechanisms for shade avoidance and growth inhibition has helped
commercial manipulation of phytochromes, so that plants such as maize have been made more tolerant of
shading, allowing for higher crop yields arising from higher planting density.
(...meaning from plants which nevertheless ,receive less light .....Keep that in mind..
Higher crops,through Phy manipulation and not by adding more light power...)
*
Germination Effects:
Small seeds may be inhibited from germination even when moist if they are exposed
to light with low R:FR ratio—they effectively sense that they will not succeed when strongly shaded by the
foliage of other plants. (They also sense if they are buried so deep as to be in darkness, and thus unlikely
to succeed upon germination.)
Germination of larger seeds is less affected by shading plants, because they
have more reserve with which to send up longer shoots. The inhibitory effect of low R:FR ratio on
germination of small seeds (such as the trumpet tree) can be reversed by filtering out the FRL, indicating
that it is the detected ratio that is inhibitory and not just the low absolute amount of RL involved, since the
RL remains low with FRL filtering.
Phytochrome effects are important early in germination... PhyB mediates the de-etiolation of a seedling
emerging from darkness into open sunlight (in which the R:FR ratio is relatively high). However, PhyA
mediates the de-etiolation of a seedling emerging from darkness into canopy shade (in which the R:FR is
low). “Because phyA is labile, however, the response is taken over by phyB... In switching over to phyB,
the stem is released from growth inhibition ... allowing for the accelerated rate of stem elongation that is
part of the shade avoidance response”.
*
Modulating Effects—Cryptochrome [ Cry ] and Phototropin [ PHOT ]: cry2 mutants promote flowering in blue light by
repressing phyB function... CRY1 and CRY2 interact with phyA...
But where is that " oscillator " ,then ?
Well how exactly this " oscillator " works in plants ,for regulating circadian rythms is still pretty much a mystery..
Not fully understood to it's whole extend...
As many other parameters ,light-independant ,take place ...
More on them later on...
What is known ,is that Phy ,
entrains the "oscillator"...
Gives the rythm,to the whole tune...
Enough for now about the Phy....
More later on...
( Still there is much to ..note down ,regarding flowering & growing,using leds...
Easy,now..
Step by step... )
.... )
.....