Effects of Climate Change on Effects of Climate Change on Mechanisms That Initiate FloweringMechanisms That Initiate Flowering

Donald GeigerDonald Geiger

University of DaytonUniversity of DaytonMarianist Environmental Education CenterMarianist Environmental Education Center

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Exposure to warm temperatures over time is the main driver Exposure to warm temperatures over time is the main driver for springtime plant development, particularly for perennial for springtime plant development, particularly for perennial plants in temperate zonesplants in temperate zones.

Plants require a certain amount of exposure to warm Plants require a certain amount of exposure to warm temperatures before leaf out or flowering can occur.temperatures before leaf out or flowering can occur.

The The required heat sum,required heat sum, a quantified requirement that controls a quantified requirement that controls development, is a genetically controlled adaptive trait.development, is a genetically controlled adaptive trait.

One of the most obvious ways ecosystems are affected by One of the most obvious ways ecosystems are affected by global climate change is through alteration of organisms’ global climate change is through alteration of organisms’ developmental timing, their developmental timing, their phenologyphenology..

Onset of flowering is an Onset of flowering is an important element of phenology.important element of phenology.

Basis For Study of Flowering Phenology

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Climate Change and Flowering Phenology

The effect of climate change on the phenology of flower initiation was reported in the following publication: McEwan RW, RJ Brecha, DR Geiger, GP John 2011 Flowering McEwan RW, RJ Brecha, DR Geiger, GP John 2011 Flowering phenology change and climate warming in southwestern Ohio. phenology change and climate warming in southwestern Ohio. Plant Ecology 212,(1) 55-61.Plant Ecology 212,(1) 55-61.

The above study, based on observations of first flowering dates for an array of species from 1976 to 2003, provided evidence that climate change is affecting plant development.

Change in first flowering date over a 28–year (1976 to 2003) observation period in southwestern Ohio, USA. Change in flowering date represents the slope of the linear regression between first flowering date and year. Darkened symbols represent slopes that were different from zero (P<0.05). Horizontal dashed line represents no change.

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SpringSpring EphEph

Spring EphSpring Eph

Woody FruitWoody Fruit

Quantitative L-DQuantitative L-D

McEwan et al. Plant Ecology (2011) 2112:55-61

In spring

In summer

Effect of Climate Warming on Flowering Phenology

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Qualitative L-DQualitative L-D

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Woody OrnamentalWoody Ornamental

DevelopmentDevelopmentQuantitative L-DQuantitative L-DDevelopmentDevelopment

DevelopmentDevelopment

Quantitative L-DQuantitative L-D

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Basis For Flower Phenology Studies This study revealed a trend of progressively earlier flowering progressively earlier flowering

time for a number of speciestime for a number of species.

The two earliest flowering species showed the largest advance two earliest flowering species showed the largest advance in flowering timein flowering time; advance decreased with later flowering date.

Two species that bloomed in mid summer also had advanced bloomed in mid summer also had advanced flowering dates.flowering dates.

Flowering is known to be initiated by a number of different different physiological mechanisms.physiological mechanisms.

Hypothesis: The two clusters of advanced bloom dates may Hypothesis: The two clusters of advanced bloom dates may result from climate change acting on two different mechanisms result from climate change acting on two different mechanisms that regulate initiation of flowering.that regulate initiation of flowering.

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Physiology of Floral Induction:

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Background: A Survey of Physiological Mechanisms of Flowering Phenology

In their discussion the authors of the flowering phenology paper “suggest that future work is needed that bridges the gap between pattern (plant phenology response) and process (plant development physiology) in relation to particular climatic cues”.”.

The study that follows is a survey of process… mechanisms that initiate flowering and investigates possible ways in which climate change may advance initiation mediated by these mechanisms… pattern.

Floral InductionShoot Apex Phase Changes

Shoot apical meristems have

three developmental phasesthree developmental phases:

11. . Juvenile phase Juvenile phase

22.. Adult vegetative phaseAdult vegetative phase

33.. Adult reproductive phaseAdult reproductive phase

4. 4. Flower formation

11

22

33

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Shoot Apex Stages for Flowering

Adult Adult ReproductiveReproductive

statestate

emphasis on developmental states Floral evocation in the adult reproductive phaseadult reproductive phase involves three statesthree states on the way to flowering:

Competent- Shoot apex becomes competent to respond appropriately Shoot apex becomes competent to respond appropriately

to a given developmental signal. to a given developmental signal. E.g.- able to respond to photoperiodable to respond to photoperiod

Determined- able to follow some developmental program even if shoot is able to follow some developmental program even if shoot is

removed from its normal position and nurtured by grafting or rooting.removed from its normal position and nurtured by grafting or rooting. E.g.- able to respond to plant growth regulator.able to respond to plant growth regulator.

Expressed- apical meristem undergoes physiological steps that lead to apical meristem undergoes physiological steps that lead to

morphogenesis to become a flower. morphogenesis to become a flower. (Process on slide 10)(Process on slide 10)

bractbract

shoot shoot apexapex

epalepal

shoot pex shoot pex

etaletal

a.a.

b.b.

c.c.

d.d.

stamen stamen primordiaprimordia

Floral Induction and Development of the FlowerFloral Induction and Development of the Flower

Signal to apex in determined state

Vegetative shoot apexVegetative shoot apex

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Survey of Factors Involved In Altering Flowering Time

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Mikhail Chailakhyan in 1937 demonstrated that floral induction floral induction can be transmitted through a graft indicating that a signal substance is being sent.

The hypothetical substance was named florigen.

In 2005 an RNA was mistakenly identified as florigen. In 2007 the paper was retracted; In 2008 florigen was found to be a protein a translocated floral initiation signal, florigen.

Florigen proteins FTFT , Hd3aHd3a and PIF4PIF4 have been identified as biochemical signals known to be translocated to the apical meristem and to initiate flower formation.

Biochemical Signaling Is Able To Initiate Flowering

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Biochemical Signaling Involved in Flowering

Multiple development pathways for floral evocation in Arabidopsis. Part 2

Flowering isexpressed byaction of the florigenflorigen signal in response to photoperiodphotoperiod or warming warming stimulusstimulus.

Mitigating factors:Mitigating factors:

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Temperature- warmingwarming after chilling

Temperature- vernalization = = period of chillingchilling

Photoperiod- day lengthday length; dark period lengthdark period length

Biennial life cycle- vernalization followed by lengthening daysvernalization followed by lengthening days

Carbohydrate pathway- sucrosesucrose

Plant growth regulators- gibberellin, ethylenegibberellin, ethylene pathway-

Autonomous initiation- age, size, leaf numberage, size, leaf number.

Significance:- Existence of Existence of multiple pathwaysmultiple pathways provides provides flexibility to produce seeds under a variety of conditionsflexibility to produce seeds under a variety of conditions.

Which of these mechanisms are affected by climate change?Which of these mechanisms are affected by climate change?

Some Factors That Initiate Flowering:Candidates for Climate Responses

Photoperiodism: Monitoring Day

Length

15 Photoperiodic regulation of flowering showing effects on short-day plants and effects on short-day plants and long-day plants.long-day plants. Bar graphs show the effects of the duration of dark periods on flowering; data show the critical variable is length of the dark perioddata show the critical variable is length of the dark period.

Day long enough Night long enough

Short-Day plants Long-Day plants

“day”

“night”

too long

too short

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Biochemical Signaling Involved in Flowering

[pp 660 – 667]

FT protein, the “florigen” is transported to the FT protein, the “florigen” is transported to the shoot apex to evoke flowering.shoot apex to evoke flowering.

Determined byDetermined byphotoperiod signalphotoperiod signal

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Determined byDetermined bywarming signalwarming signal

Florigen gene Florigen gene expression regulated by expression regulated by warm temperature . warm temperature . PIF4 protein, a florigenPIF4 protein, a florigen

Long Day Short Day

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Case Studies of Flower Types: 1. Warming Activates Florigen Gene

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Warmth Activates Flower Initiation

A period of warming over a period of time A period of warming over a period of time drives spring flower development, particularly for perennial plants in temperate zones.

Plants require a critical amount of exposure to warm temperatures require a critical amount of exposure to warm temperatures before leaf out or flowering can occur; required heat sum (RHS)required heat sum (RHS)

RHSRHS for spring development is a genetically controlled, adaptive trait.genetically controlled, adaptive trait.

EEnables plants to survive unpredictable early onset of warmth at the start of the growing season that does not persist.

Warmth activates PIF4 gene which acts as a florigen; exerts control over the flowering pathway (slide 16slide 16).

This mechanism is likely to be encountered in a number of species of plants but has not been studied extensively since its recent discovery (Kumar et al. 2012).

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Mechanisms That Can Modulate Flowering Time

The thermal control time The thermal control time can be modulatedmodulated by conditions conditions that affect the rate of accumulation rate of accumulation of the required heat sumrequired heat sum. RHSRHS

Different environments may cause plants to require more or less time to accumulate the effective heat sum under the same input rate. effective heat sum under the same input rate.

CConditions known to modulate RHS mechanism modulate RHS mechanism include:

snow pack depth photoperiod winter cold treatment snow pack depth photoperiod winter cold treatment

For example, photoperiod photoperiod can delay bud break, protecting against responding to the threat of unusually early but intermittent warm temperature. This effect can complicate RHS/flower induction studies.

.

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Case Studies of Flower Types:2. Spring Ephemeral Pattern-

Altered Rate of Flower Development

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Background: Spring Ephemerals

Spring ephemerals are species that have an active spring growth period limited to 40 - 60 days… adapted to take adapted to take advantage of a two-month high-light before canopy closureadvantage of a two-month high-light before canopy closure. Examples: fawn lily, Dutchman’s breeches, crocuscrocus,, bleedingbleeding heartheart,, snow dropssnow drops,

tulip, hyacinth, Cardamine spp (redred = reported in McEwan et al. = reported in McEwan et al.)

Annual life cycle, Erythronium americanum- yellow fawn lily

17 April17 April 20 April20 April19 April19 April 21 April21 April 22 April22 April

Spring Ephemeral Flowering Cycle

40 to 60 days above ground40 to 60 days above ground

Spring Ephemeral Developmental Life Cycle Diagramic life cycle of Erythronium americanum .

The perennial organ is shown in radial section to show bud developmentThe perennial organ is shown in radial section to show bud development

summersummer autumnautumn winterwinter springspring

1 2 3 4 5 6 7

BudBud

Soil temperature drives life cycle development that determines flowering time.Soil temperature drives life cycle development that determines flowering time.22

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Spring Ephemeral Life Cycle

Species that have a 40 to 60 day growth period in spring.

Annual life cycle, Erythronium americanum- yelllow fawn lily

Dormancy is broken in autumn;Dormancy is broken in autumn; bud and root growth continues through the winter at a very slow rate due to the low soil temperatures. (stages 2, 3)(stages 2, 3).

As the soil warms shoot expansion is rapid (stages 3, 4)shoot expansion is rapid (stages 3, 4), the long underground growth period ends and the above ground phase beginsabove ground phase begins. This process is hastened by the warming due to climate change.

Sexual reproduction Sexual reproduction takes place over a 5- to 6-day period (stages 4,5,6)(stages 4,5,6)

Senescence of the above ground plant (stage 6)Senescence of the above ground plant (stage 6) followed by senescence of the root (stage 7) (stage 7) and a stage of apparent dormancy of the perennial apparent dormancy of the perennial organorgan

stage 1)… the end of the 40 – 60 day period.stage 1)… the end of the 40 – 60 day period. Review previous slideReview previous slide..

Flowering time was advanced in crocus (Crocus flavus) and bleeding heart (Dicentra specta sp.)the study of phenology affected by climate warming (McEwan et al. 2010).

next

year

next

year

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Case Studies of Flower Types:3. Flower Bud Dormancy Broken by Spring Chilling; Woody Perennials

and Geophytes

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Breaking of Bud Dormancy: ChillingFor a wide range of spring flowering perennials a temperature For a wide range of spring flowering perennials a temperature signal typically breaks dormancy of the renewal (winter) buds.signal typically breaks dormancy of the renewal (winter) buds.

Renewal or winter buds Renewal or winter buds form in early summer form in early summer after leaf after leaf development ends and development ends and initiate next spring’s floweringinitiate next spring’s flowering..

In woody perennials (apple) formation next year’s leaf and flower In woody perennials (apple) formation next year’s leaf and flower primordia occurs in this year’s renewal buds in fall and early winter. primordia occurs in this year’s renewal buds in fall and early winter. !!!!!!

Flowering is initiated by chilling during spring warming period.Flowering is initiated by chilling during spring warming period.

Ex. Woody ornamental:- rhododendron; tree fruit species:- apple, pear

Flowering time was advanced in apple (Malus domestica) in the study of phenology affected by climate warming (McEwan et al. 2010)

In the geophytes renewal buds form on the roots; flowering is In the geophytes renewal buds form on the roots; flowering is initiated by chilling during the spring warming period.initiated by chilling during the spring warming period.

Ex. Geophytes:-Geophytes:- tulip, peonie

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In woody perennials breaking of bud dormancy in spring breaking of bud dormancy in spring is a factor determining the timing of flowering... Example apple, pear

Bud dormancy in fall is initiated by low temperatures and short low temperatures and short photoperiodphotoperiod. Fruit treesFruit treesIn spring bud dormancy is broken by environmental signals, especially chilling during the spring warming period.especially chilling during the spring warming period.

Linear accumulation of chilling hours occurs below 7Linear accumulation of chilling hours occurs below 7o o C. 45C. 45o o F.F. Short photoperiod promotes blooming. Phenology of spring woody perennial flower activation is complex and

still not well understood. Remember: flowers formed in late fall, early winter in woody Remember: flowers formed in late fall, early winter in woody

perennials. It is bud break that determined flower activation.perennials. It is bud break that determined flower activation.

Breaking of Bud Dormancy: Chilling

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Case Studies of Flower Types:4. Flowering in Qualitative, Obligate Long-Day Plants

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Qualitative Obligate Long-Day Plants

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Annual PhloxPhlox (Phlox drummondii) is a qualitativequalitative long-day plant.

qualitativequalitative L-D plant:- flowers ONLY on long daysONLY on long days… obligate; obligate; day period must be at least a certain critical length at least a certain critical length to initiate flowering

quantitativequantitative L-D plant:- flowering ACCELERATED by long d. ; ACCELERATED by long d. ; facultative:- long days are not required for flowering. facultative:- long days are not required for flowering.

Under long-day conditions leaf processes mediated by phytochrome produce florigen that initiates flower formation by the apical meristem. (Revisit slides 16, 15, slides 16, 15, and 1010))

At 11 hrD- 98 days to flowering; 13 hrD 87 d, 15 hrD 75 d, 17 hrD 74 d

In Phlox drummondi Phlox drummondi flower development did not increase beyond 15hr day.

Flowering time was advanced in annual phlox (Phlox drummondi) in the study of phenology affected by climate warming (McEwan et al. 2010)

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Case Studies of Flower Types:5. Flowering in Quantitative Facultative Long-Day Plants

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Shortest time to flowering in Ranunculus asiaticus was achieved under 16 hr photoperiod at 1616 hr photoperiod at 16ooC .C .

A quantitative (LD accelerates flowering), facultative (LD not quantitative (LD accelerates flowering), facultative (LD not required) long day plant required) long day plant

Both increased temperature + increased temperature + longer daylengthdaylength reduce time to flowering.

The decreased time to flowering by lengthening the photoperiod was The decreased time to flowering by lengthening the photoperiod was due to warming advancing development triggered by photoperiod due to warming advancing development triggered by photoperiod induction of flowering. induction of flowering. RanunculusRanunculus is a quantitative long day plant. A is a quantitative long day plant. A day of a certain length is not required for flowering to occur. day of a certain length is not required for flowering to occur.

Effect of Photoperiod and Temperature on Flowering in Quantitative L-D Plants

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Control of time to flowering in ViolaViola:

Progress to flowering increases with temperature up to 28oC. This pattern promotes earlier flowering as a result of higher springtime This pattern promotes earlier flowering as a result of higher springtime

temperatures.temperatures.

Progress to flowering also increases with daylength.Progress to flowering also increases with daylength. Not a photoperiod induction of flowering but the requirement pattern for the requirement pattern for

growth of a growth of a quantitativequantitative long day plant. long day plant.

Response to longer photoperiod is adaptive, lessening the lag Response to longer photoperiod is adaptive, lessening the lag imposed by cooler early season temperatures. imposed by cooler early season temperatures.

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Effect of Photoperiod and Temperature on Flowering in Quantitative L-D Plants

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A quantitative long-day plant but long photoperiod is not used as a cue. Lengthening days promoter stem elongation.

Flowers are formed underground during the previous winter or earlier Flowers are formed underground during the previous winter or earlier when the plant forms its rametsramets.

In mountainous habitats the first date of bare ground in spring is a good predictor of first flowering date, reminiscent of the winter/spring maturation of the developing flower stalks..

Working hypothesis: increasing soil temperature promotes the formation increasing soil temperature promotes the formation and maturing of flowers initiated with the formation of ramets in the previous and maturing of flowers initiated with the formation of ramets in the previous fall/winter.fall/winter.

Effect of a Variety of Factors on Timing of Flowering in in Quantitative L-D Plants

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Case Studies of Flower Types:6. Flowering in Biennials

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Biennials: First year rosette of sugar beet is unable to form unable to form reproductive shootsreproductive shoots during the first year of growth. Duplicate genes BvFT1BvFT1 and BvFT2BvFT2, homologs of the FT gene mediate

regulation of flowering time: BvFT2BvFT2 promotespromotes while BvFT1BvFT1 repressesrepresses. Winter vernalizationvernalization lowers expression of BvFT1BvFT1, which renders

shoots competentcompetent to reproduce.

Exposure to lengthening days (quantitative long day) quantitative long day) upregulates BvFT2BvFT2 and maintains BvFT1 in downregulated state.

Subsequent exposure to increasing day length (qualitative qualitative long day) long day) causes the shoot to become determineddetermined.

The dependence on LD puts flower induction outside the springtime window of earlier increased temperatures.

Biennial Sugar Beet Flowering

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Case Studies of Flower Types: 7. Flower Initiation Part of a

Genetically Determined Development Program

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Some species of plants initiate flowering at a specific node (leaf location on the stem).

For a certain variety of cotton it occurs just after nodes number 5 or 6.

The program development is driven by heat measured in heat units. (next slide)

Determination by stage of plant development seems to be a common mechanism.

Flowering Initiated At a Specific Development Stage

Node 4Node 4Node 3Node 3Node 2Node 2Node 1Node 1

Node 5Node 5Node 6Node 6

11stst Flower Flower

Main StemMain Stem

Vegetative BranchVegetative Branch

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Flowering Initiated At a Specific Development Stage

Growth Stage Days Heat Units – DD60s

Planting to Emergence 4 to 9 50 to 60

Emergence to First Square

27 to 38 425 to 475

Square to Flower 20 to 25 300 to 350

Planting to First Flower

60 to 70 775 to 850

Flower to Open Boll 45 to 65 850 to 950

Planting to Harvest Ready

130 to 160 2200 to 2600

Table 1. The average number of days and heat units required for various growth stages of cotton in the Mid-South.

Modified from Oosterhuis, D.M. 1990. Growth and development of the cotton plant. In: W.N. Miley and D.M. Oosterhuis (eds) Nitrogen Nutrition in Cotton: Practical Issues. Proc. Southern Branch Workshop for Practicing Agronomists. Publ. Amer. Soc. Agron., Madison, WI

Daytime high °F + Nighttime low °FDaytime high °F + Nighttime low °F -60 = DD60 -60 = DD60 22

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Species that initiate flowering at a specific stage of development that is promoted by spring warming would advance flowering as a consequence of climate warming.

Species that flower in early spring would be most reliably advanced due to climate warming. Those blooming later likely would show more variability.

For tomato plants, the first flower is initiated at node 6 to 8 depending on the earliness of the variety. In a commercial variety of tobacco the first flower was reported to occur at or about node 41.

In the phenology study Ageratum, Centauria, Dicentra, Hemerocallis, and Hosta flowering appears to be regulated by developmental stage.

Flowering Initiated At a Specific Development Stage

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Summary: Possible Mechanisms for

Responses of Flowering To Climate Change

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Flowering Phenology and Climate Warming

McEwan R, R Brecha, D Geiger, G John 2011 McEwan R, R Brecha, D Geiger, G John 2011 Flowering Phenology Change and Climate Warming in Southwestern Ohio. Plant Ecol Plant Ecol 212: 55-61.

Global climate change affects timing of plant development.

Thirty years of annual botanical surveys were used to examine impacts of climate change on plant development.

Authors of the study found a significant relationship between warming winter and spring temperatures and first flowering time for 60% of the species studied.

Change in first flowering date over a 28–year (1976 to 2003) observation period in southwestern Ohio, USA. Change in flowering date represents the slope of the linear regression between first flowering date and year. Darkened symbols represent slopes that were different from zero (P<0.05). Horizontal dashed line represents no change.

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SpringSpring EphEph

Spring EphSpring Eph

Woody FruitWoody Fruit

Quantitative L-DQuantitative L-D

McEwan et al. Plant Ecology (2011) 2112:55-61

In spring

In summer

Effect of Climate Warming on Flowering Phenology

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DevelopmentDevelopmentQuantitative L-DQuantitative L-DDevelopmentDevelopment

DevelopmentDevelopment

Quantitative L-DQuantitative L-D

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1. 1. WarmingWarming activates florigen gene. activates florigen gene. Arabidopsis thaliana; newly discovered; not researched

2. 2. Spring ephemeral pattern of flower developmentSpring ephemeral pattern of flower development.. Erythronium americanum, Cardamine sp., Galanthus sp., Allium sp.,

Anemone sp., Crocus sp.3. Flower bud summer dormancy broken in spring 3. Flower bud summer dormancy broken in spring by by chilling.chilling.

Woody trees and shrubs: Malus sp. Rhododendron sp. Geophytes: Paeonia, Tulipa

44. Long days requiredLong days required in qualitative, obligate long-day plantsqualitative, obligate long-day plants Phlox sp.

5. 5. Long days accelerate floweringLong days accelerate flowering; ; quantitative L-D plants quantitative L-D plants Ranunculus asiaticus, , Viola sp, Delphinium sp.

6. Biennial species, 6. Biennial species, chilling periodchilling period and and lengthening dayslengthening days Beta vulgaris

7. Development 7. Development + + warming hastens floweringwarming hastens flowering.. Dicentra, Hemerocalis, Hosta

Seven Flower Induction Mechanisms

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Advanced

Mar 2: Galanthus sp.- Spring ephemeral Spring ephemeral

Mar 6: Crocus flavus- Spring ephemeral Spring ephemeral

Apr 18: Viola pubescens- Quantitative L-D plant; Quantitative L-D plant; LD? + warming LD? + warming

Apr 19: Malus sp.- Woody fruit, Woody fruit, bud dormancy broken by chillingbud dormancy broken by chilling

Apr 25: Delphinium consolida- Quant. L-D plant; Quant. L-D plant; LD? + warming LD? + warming

Apr 27: Dicentra spectabilis- Quant L-D plant; Quant L-D plant; LD? + warming LD? + warming

May 19: Hemerocalus lilioasphodelus- Development;Development; Dev + warmDev + warm

Jul 14: Phlox sp- Qualitative, long-day plant; Qualitative, long-day plant; LD + warmingLD + warming

Aug 11: Hosta plantaginea- Development;Development; Dev + warmDev + warm

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Did Not Advance

Mar 27: Forsythia sp- Flower bud dormancy broken by chilling.Flower bud dormancy broken by chilling.

Apr 23: Ajuga reptans- Development Development ?? ??

Apr 27: Geranium maculatum- facult L-D plant; LD + warming facult L-D plant; LD + warming

May 5: Ranunculus sp- Development Development

May 13: Centaurea cyanus- Development Development

Aug 8: Ageratum sp- Development Development

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Flowering physiologyFlowering physiology Taiz L, E Zeiger 2006 Plant Physiology 4th Ed. Sinauer Associates Inc. Zeevaart, JAD. 2008 Leaf-produced floral signals. Current Opinion in Plant Biology 11:541–

547. McDaniel CN, SR Singer and SME Smith. 1992 Developmental states associated with the

floral transition. Developmental Biology 153:59-69. Flowering phenology and climate warmingFlowering phenology and climate warming McEwan RW, RJ Brecha, DR Geiger, GP John 2011 Flowering phenology change and

climate warming in southwestern Ohio. Plant Ecology 212,(1) 55-61. Conover D, S Pelikan 2010 Earlier flowering in a restored wetland-prairie correlate with

warmer temperatures (Ohio). Ecological Restoration. 28(4) : 428-430. Bertin RI 2008 Plant phenology and distribution in relation to recent climate change. J.

Torrey Bot. Soc. 135, 126–146. Lapointe L (2001) How phenology influences physiology in deciduous forest spring

ephemerals. Phsiologia Plantarum 113:151-157 Bradley NL, AC Leopold, J Ross,. W huffaker 1999 Phenological changes reflect climate

change in Wisconsin. Proceedings of the National Academy of Science USA 96:9701-9704 Floral induction mechanism triggered by warming Kumar SV, Lucyshyn D, Jaeger KE, Alos E., Alvev E, Harberd NP, Wigge PA. 2012

Transcription factor PIF4 controls the thermosensory activation of flowering Nature March

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Release of bud dormancy in woody perennials Arora A, Rowland LJ and Tanino K 2003 Induction and release of bud dormancy in woody

perennials: a science comes of age. Hort Science 38:911-921. Photoperiod and flowering phenology Karlsson, M 1996 Control of Ranunculus asiaticus flowering by photoperiod and

temperature. Hort Science 31:680-68 Adams SR, Pearson S and Hadley P 1997 The effects of temperature, photoperiod and light

integral on the time to flowering of Pansy cv, Universal Violet. Annals Bot 80;107-12. Baloch J, M Munir, M Abid and M Iqbal. 2011 Effects of different photoperiods on flowering

time of qualitative long-day ornamental annuals. Pakistan J Bot.43(3): 1485-1490. Modulation of flower induction mechanismsModulation of flower induction mechanisms Inouye et al. 2002 Oecologica 130:543-550 investigate the roles of snowpack, frost, La Nina,

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