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FLORAL EVOCATION: INTERNAL AND EXTERNAL CUES
Flowering is regulated by:
ØInternal
factors controlling the switch to reproductive development:
•Age
•Size
•Number of leaves
Ø External
cues controlling seasonal response:
•Photoperiodism
(Light/dark cycle) and light quality
(wavelength and intensity)
•Vernalization (Low temperature)
•Total light
radiation
•Water availability
•External cues and internal developmental
factors together fine-tune flower evocation in most plants.
•When flowering occurs strictly
in response to internal developmental factors and does not depend on any environmental
conditions, it is referred to as autonomous regulation.
THE SHOOT APEX AND PHASE CHANGES
•In higher plants,
developmental
changes occur in a single, dynamic region, the shoot apical meristem.
•During postembryonic
development, the shoot apical meristem passes through three well-defined
developmental
stages:
ØThe juvenile
phase
ØThe adult
vegetative phase
ØThe adult
reproductive phase
•The transition
from one phase to another is called a phase change.
•Juvenile tissues are
produced first and are located at the base of the shoot.
Competence and determination are two stages in floral evocation
•To initiate floral development, the cells
of the meristem must first become competent i.e.
capable of responding to floral stimulus
(induction).
•A competent
vegetative meristem responds to
a floral stimulus (induction) to becoming
florally determined (committed to producing a
flower).
•Once determined, it will flower even
after removal of the floral stimulus.
•Competent vegetative
shoot (scion) grafted
onto a flowering stock will flower.
•Reason: It is capable
of
responding to floral stimulus
present in the stock.
•The grafted scion will fail to flower
before
attaining competence.
•Reason: Its shoot
apical meristem is not yet able to respond to the floral
stimulus present in the stock.
•A bud is said to be determined if it
progresses to the next developmental stage (flowering) even after being removed
from its normal context.
•Thus a
florally determined bud will produce flowers even if it is grafted onto a
vegetative plant that is not producing any floral stimulus
Demonstration of the determined state of axillary buds in tobacco
•In a
day-neutral tobacco, plants typically
flower after producing about 41 leaves or
nodes.
•If a flowering tobacco plant is decapitated just above the 34th
leaf, the axillary bud of the 34th
leaf grows out, and flowers after producing 7 more leaves (for a
total of 41).
•If
the
34th bud
is excised from the plant and either rooted
or grafted onto a stock without leaves near the base, it produces
a
complete set of 41 leaves before
flowering.
•Reason:
The 34th
bud was not yet florally determined.
•If a flowering tobacco plant is decapitated just above the 37th
leaf, the 37th
axillary bud flowered after producing 4
more leaves in all three situations.
•Reason:
ØThe
37th
bud was already
florally
determined.
ØThe
number
of nodes a meristem produces before flowering is a function of two factors:
•Strength
of
the floral stimulus from the leaves
•Competence
of
the meristem to respond to the signal
PHOTOPERIODISM: MONITORING DAY LENGTH
•Photoperiodism
is the ability of an organism to detect day length
which makes it possible for an event to occur
at a particular time of year, thus allowing for a seasonal response.
•Circadian
rhythms (Biological
clock) and photoperiodism have the common property of responding to cycles of light and
darkness.
Plants can be classified by their photoperiodic responses
•The classification
of plants according to their photoperiodic responses is usually based on flowering.
•The two
main photoperiodic response categories are:
ØLong-day
plant (LDPs)
ØShort-day
plants (SDPs)
Critical day length
•Flowering in
LDPs is promoted only when the day length exceeds a certain duration, called
the critical
day length, in every 24-hour cycle.
•Promotion of
flowering in SDPs requires a day length that is less than the critical day
length.
•The absolute
value of the critical day length widely varies among species.
LDPs flower: Day length > critical day length
SDPs flower: Day length < critical day length
Both
Xanthium (a SDP) and Arabidopsis (a LDP) flowers under 12 hours of photoperiod.
•Critical day length of Xanthium is
15 hours and it flowers when photoperiod is less than 15 hours).
•Critical day length of Arabidopsis is
11 hours and it flowers when photoperiod is greater than 11 hours.
Since, 12 hours of photoperiod is less than 15 hours and greater than 11 hours, both Xanthium and Arabidopsis can flower.
ØLong-day
plants (LDPs):
•Flower only in long days (qualitative LDPs)
or flowering is accelerated by long days (quantitative LDPs).
•Flowering promoted only when the day
length exceeds critical
day length, in every 24-hour cycle.
•LDPs
measure the lengthening days of spring or early summer and delay
flowering until the critical day length is reached.
•Ex: Triticum
aestivum
ØShort-day
plants (SDPs):
•Flower only in short days (qualitative SDPs)
or flowering is accelerated by short days (quantitative SDPs)
•Promotion of flowering in SDPs requires a
day length that is less than the critical day length.
•SDPs flower in fall, when the days
shorten below the critical day length.
•Ex: Chrysanthemum morifolium
•However,
day length alone is an ambiguous signal because it cannot distinguish between
spring and fall.
•For avoiding
the ambiguity of day length signal, plants often couple a temperature requirement to a
photoperiodic response.
•Other plants avoid seasonal ambiguity by
distinguishing between shortening and lengthening days
and are called “dual–day
length plants”.
The “dual–day length plants” fall into two categories:
•Long-short-day
plants (LSDPs): flower
only after a sequence of long days followed by short days. LSDPs,
•Ex: Bryophyllum, Kalanchoe,
and Cestrum nocturnum
flower
in
the late summer and fall, when the days are shortening
•Short-long-day
plants (SLDPs): flower
only after a sequence of short days followed by long days. SLDPs
•Ex: Trifolium repens,
Campanula medium,
and
Echeveria
harmsii, flower
in the early spring in response to lengthening days.
•Day
neutral plants
(DNPs): Species
that
flower under any photoperiodic condition and are insensitive to day length.
•Flowering in
DNPs is typically under autonomous regulation—that is, internal developmental
control.
•Ex: Phaseolus vulgaris, Castilleja chromosa, and
Abronia
villosa
Plants Monitor Day Length by Measuring the Length of the Night
Night breaks can cancel the effect of the dark period
•The
dark
period can be
made ineffective by interruption with a short exposure to light, called a night break.
•But
interrupting a long day with a brief dark period does
not cancel the effect of the long day.
•When given during a long dark
period, a night break promotes flowering in LDPs and inhibits flowering in SDPs.
•A
night
break was found to be most effective when given near the middle of a dark period.
Leaf Is the Site of Perception of the Photoperiodic Stimulus
•Treatment
of
a single leaf of
a
SDP
with
short
photoperiods are sufficient to cause flowering when the
rest of the plant is exposed to long days.
•But,
treatment of the shoot apex with
short photoperiods don’t induce flowering if
the rest of the plant is exposed to long days
•In
response
to photoperiod, leaf
transmits a signal that regulates the transition to flowering at the shoot apex.
The photoperiod-regulated processes that
occur in the leaves resulting in the transmission of a floral stimulus to the
shoot apex is referred to collectively as photoperiodic induction.
Floral stimulus is transported via the phloem
•Once
produced, the floral stimulus is transported to the meristem via phloem,
and it appears to be chemical in nature.
•Treatments
that
block phloem transport, such as girdling or localized heat-killing
prevent
movement of the floral signal.
•The
floral
stimulus is translocated along with sugars in the phloem
and it is subject to source-sink relations.
•An
induced
leaf positioned close to the shoot apex is more likely to cause flowering than
an induced leaf at the base of a stem, which normally feeds the roots.
(Images kindly shared by Oxford University Press and are being used for teaching purpose only)
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