Multiplicative and sequential models of dormancy and germination

of Striga hermonthica

Israel Dzomeku and Alistair Murdoch

University for Development Studies, Tamale, Ghana

University of Reading, UK.

11h World Congress on Parasitic Plants, Martina Franca, Italy. 7-12 June 2011

Prizes for the least attention-grabbing title!!

This must be one of the front-runners!

“To germinateor not to germinate?That is the question!”

Dormancy in the Orobanchaceae

Four types of seed dormancy

1. Primary dormancy (dry seeds)– Acquired on mother plant

– Effects:

»prevents vivipary

»seeds do not respond to stimulants

– Relieved faster in warm dry conditions

Four types of seed dormancy in Striga

2. Primary dormancy (imbibed seeds)– Acquired on mother plant (?)

– Effect:

»prevents or delays response to germination stimulants

– Relieved by “conditioning”

»seeds become more sensitive to stimulant

Four types of seed dormancy in Striga

3. Secondary dormancy (imbibed seeds)– Induced after dispersal

by prolonged conditioning

– Effect

»seeds become less sensitive to stimulant

– Relieved by “drying” followed by reconditioning

Do host root exudatesterminate dormancy or stimulate germination?

The last step in breaking dormancy?

OR

The first step in germination?

4. Stimulants remove a physiological or metabolic block to germination and so terminate dormancy. The seed is then free to germinate.

Focus on the second and third types seen in germination after seed conditioning

(Striga)C

0

20

40

60

80

100

0 45 90 135

Conditioning period at 25°C in water, days

Percentage

germinationafter 7 days

at 35°C in 3ppm GR24

C

0

20

40

60

80

100

0 45 90 135

Conditioning period, d

Germination (%)

after conditioning

What you seeInitial increase followed by a decrease in sensitivity to germination stimulant

What you get: 2 or 3 processes1) loss of primary dormancy and2) induction of secondary dormancy3) possible loss of viability

What is going on?1.Loss of primary dormancy 2.Induction of secondary dormancy

•Are these processes independent & concurrent? •Are they sequential?•Is there a lag period?

Conditioning (Striga)

Approach

1. Ultimate goal of this researchTo predict what will happen in the field

2. ApproachEmpirical models

3. Previous published researchAssume independent & concurrent processes

4. Here we tests that assumption statistically

Loss of 1ry

dormancy

Processes during conditioning

Conditioning period, days

Pro

port

ion o

f seed

pop

ula

tion a

ffecte

d, %

100

80

60

40

20

00 30 60 90 120 150 180 210

Seeds withNO secondary dormancy

Simplifiedmultiplicative modelG = Φp

-1. Φns-1

additive (sequential) modelG = Φp

-1- Φs-1

Φs-1

Φp-1

Φns-1

Secondary dormancy

Φ-1 : transformation from probits to proportions

C

0

20

40

60

80

100

0 45 90 135

Models

Multiplicative (independent & concurrent)1. All seeds, no lag period

2. Active seed fraction, no lag period (could not fit to data)

3. Active seed fraction, lag period

Sequential4. All seeds, no lag period

5. Active seed fraction, lag period (difficult to fit)

6. Active seed fraction, no lag period

Evaluated on seeds conditioned at different temperatures, water potentials and urea solutions

MultiplicativeNo lag

MultiplicativeWith lag

Sequential

No lagActive seeds

SequentialNo lagAll seeds

Sequential Lag timeActive seeds

Conditioning period, days

Perc

enta

ge g

erm

ination

Conditioned up to 135 days in water, no urea,

30°C

All seeds in secondary dormancy by 84 days

MultiplicativeWith lag timeAll seeds

SequentialNo lagAll seeds

Conditioning period, days

Perc

enta

ge g

erm

ination

Conditioned up to 135 days-1.5 Mpano urea20°C

All seeds in secondary dormancy by 84 days

No lagActive seeds

Sequential

MultiplicativeNo lag All seeds

MultiplicativeWith or without lag

All seeds

No lagActive seeds

Sequential

Models

Multiplicative (independent & concurrent)1. All seeds, no lag period (fairly good fit)

2. Active seed fraction, no lag period (could not fit to data)

3. Lag period before secondary dormancy starts (includes active fraction)

Rejected as no consistent pattern in lag periods

Sequential4. All seeds, no lag period (poor fit)

5. Active seed fraction, lag period (could not fit to data)

6. Active seeds, no lag period in each seed before secondary dormancy starts

Evaluated on seeds conditioned in 48 treatments

(temperature, water potential and urea)

Result

The sequential model

without a lag period

including active fraction of seeds

fitted the data with a lower residual

in 35 out of 48 treatments

The multiplicative model was often better at lower water potentials (-0.75 and -1.5 Mpa)

Validation of sequential model

�

Predicted lines from main experiment

�

Independent data: seeds conditioned at

20, 25, 35 °C�

0.0, -0.25, -0.75, –2.25 MPa

�

water or 0.083 mM urea

20°C

No N

20°C

+urea

25°C

No N

25°C

+urea

35°C

No N

35°C

+urea

Germ

inatio

n,

% in 3

ppm

GR

24

Conditioning period, days

0 28 56 84

100

806040200

100 806040

200

100 80

6040200

100 806040

200

100

806040200

100

806040200

0 MPa -0.25 -0.75 -2.25

0 28 56 840 28 56 840 28 56 84

Summary ...

n The sequential model wins!

n The previous model for Orobanche (published 1999) and other species, should be re-evaluated

n The model could help to predict what happens in soil (results not shown)

n Implied mechanistic hypothesis:– processes are linked and sequential

– Gene expression studies could help to test that hypothesis

Thank you!

Work was carried out at Reading by

Israel Dzomeku with a Commonwealth

Scholarship