chronic photoinhibition plays only a minor role in the...

Tomáš Hájek1,2, Eeva-Stiina Tuittila3, Raija Laiho3 and Mati Ilomets4

1 Institute of Botany of ASCR, Dukelská 135, 379 82 Třeboň, CZ, � hajek@butbn.cas.cz2 Faculty of Biological Sciences, Branišovská 31, 370 05 České Budějovice, CZ3Department of Forest Ecology, University of Helsinki, P.O. Box 27, 00014 Helsinki, FIN4 Institute of Ecology, Tallinn University, Kevade tn. 2, Tallinn 10 137, EST

Chronic photoinhibition plays only a minor roleChronic photoinhibition plays only a minor role

in the worldin the world’’s most successful bryophyte genuss most successful bryophyte genus

Photosynthesis and StressPhotosynthesis and Stress Brno, 15 Brno, 15 -- 16 September 200516 September 2005

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 400 800 1200 1600 2000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 400 800 1200 1600 2000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

PPFD [µmol m–2 s–1]

Gross photosynthesis [m

gC

O2g

DW

–1h

–1]

Gross photosynthesis[µ

mol C

O2m

–2s

–1]

Losers – growing only in the pristine open mire

S. magellanicum

S. angustifolium

Survivors – growing in both habitats

Winners – colonizing the drained mire

S. fallax

S. flexuosum

S. papillosum

S. russowii

Pleurozium s.S. russowii

Pleurozium s.

Introduction and MethodsIntroduction and Methods

Peat mosses (Sphagnum) cover vast areas of earth’s

land surface. They often induce water-saturated

unfavourable soil conditions for vascular plants and so

form treeless, sun-exposed habitats.

This evokes the hypothesis that they show a long-term

adaptation to full sun irradiance. When pristine mires

are drained and their tree cover increases, bryophyte

species composition changes only partially, thus

indicating that some species are able to acclimate to

shade conditions.

To test the mosses’ ability to adapt/acclimate to sun and

shade habitats, we measured:

• light response of CO2 assimilation (IRGA)

• chlorophyll fluorescence (photochemical parameters)

• contents of chlorophylls and total carotenoids

in:

• six Sphagnum species and Pleurozium schreberi

• growing in open and/or forested mire

The mosses were collected in central Finland and kept

under mild irradiation for 10 days before experiments. pristine –

wetand o

pen –

MIR

E –

dra

inedand s

haded Results & DiscussionResults & Discussion

Light curves of Sphagnum spp. and Pleurozium:

Light curves in Sphagnum mosses showed:

• large differences in maximum rate of photosynthesis,

• large span of max. quantum yield of photosynthesis (α)

= indication of strong photoinhibition in species low in α

• high PPFD saturation point (~ 2000 µmol m–2 s–1)

• ← maximum quantum yield of photosynthesis

determined the photosynthetic rate in Sphagnum

= indication of reduced amount of PS2 reaction centres

Chlorophyll fluorescence:

• the maximum quantum yield of

photosynthesis correlated well with that

of ΦPSII (FV/FM, r = 0.82) ↓

= support for the gas-exchange data

ConclusionsConclusions

� losers, sun-grown Sphagna of open mires, are

very inefficient in utilizing any light level

� losers suffer from chronic photoinhibition

(due to photodamage to photosystem II), which is

unrecoverable on a time scale of weeks

� a survivor, Sphagnum angustifolium, was the

most photosynthetically active and less

photoinhibited species in both habitats why ?

• FV/FM correlated closely with ФPSII

(at PPFD of 220 µmol.m–2.s–1, r = 0.94)

and less with qP (r = 0.74)

Maximum quantum yield of photosynthesis

[mgCO2 g–1 h–1 : µmolPAR m–2 s –1]

FV/F

M

0.3

0.4

0.5

0.7

0.8

0.00 0.01 0.03 0.04 0.05 0.06 0.08 0.09

S. angustifolium

S. magellanicum

S. papillosum

S. flexuosum

Sphagnum fallax

Pristine part:

S. angustifolium

S. magellanicum

S. russowii

Drained part:

Pleurozium s.

Sphagnum spp.

Pleurozium s.

Pigment content results:

• no clear correlation between chlorophyll

content or Chl a/b ratio and gas exchange and

any chlorophyll fluorescence parameter

• carotenoid content is (slightly) inversely

proportional to ΦPSII or FV/FM (r = 0.50)

�sun-grown plants are richer in

carotenoids but their photoprotective

function is insufficient

� we postulate that the long-lasting photoinhibition in sun-grown Sphagna can be

ascribed to the ambient levels of both PPDF and/or UV-B radiation

Sphagnum fallax Sphagnum russowii Sphagnum papillosum Sphagnum angustifolium

Sphagnum magellanicumSphagnum flexuosum

Max. quantum yield of photosynthesis [µmolCO2 µmolPAR–1]

Maximum gross photosynthesis

0

1

2

3

4

5

6

7

8

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

Sphagnum spp.

Pleurozium s.

µmol CO2m

–2s–1