Effects of Light Limitation on Legume-Mycorrhizae Interactions
Jess A. Millar*, Daniel J. Ballhorn
Department of Biology, Portland State University
Portland State University
Abstract
§  Full factorial randomized block experiment using Lima bean plants (Phaseolus lunatus)
§  Two levels of light: full light & 75% shading
§  Two levels of microbial inoculation: Sterile soil and Mycorrhiza (powder inoculant,
BioOrganics)
§  Measurements taken over the course of the experiment: plant height, leaf number, flowers
and seed pod number
§  Measurements taken at the end of the experiment: shoot:root ratio, percentage mycorrhizal
colonization, number and weight of seeds, days to germination
Plant Biomass and Shoot:Root Ratio
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MF
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Control
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2
1
4
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No Shade
2
1
Shade
No Shade
Shade
J. A. Millar
Flower and Seed Pod Production
MF
Control
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2
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Δsqrt(Number of
Seed Pods)
Δsqrt(Number of
Flowers)
4
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MF
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Control
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4
3
2
No Shade
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Shade
No Shade
Shade
Seed Weight and Germination Rate
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§  Inoculation with mycorrhiza fungi did not affect
total plant biomass, but it did result in a higher
shoot:root ratio.
§  It is possible that the only effect was a shift to
more aboveground vegetative growth, but did not
cause sink stimulation in either light treatment.
§  Under full light conditions, mycorrhiza fungicolonized plants had significantly fewer flowers
and seed pods than controls. Shaded plants
showed no significant difference between microbe
treatments, but had altogether fewer flowers and
pods.
§  It is possible that in full light conditions,
inoculated plants allocated more resources into
vegetative growth and had less available for
reproduction.
§  Inoculation with mycorrhiza fungi resulted in
significantly increased seed weight. Shaded plants
produced overall heavier seeds.
§  It is possible that under light limiting conditions,
plants produce fewer but larger seeds.
§  In full light, despite an increase in seed weight,
mycorrhiza fungi did not reduce germination
time. In shade, though the seed weight was the
same, mycorrhiza lengthened germination time.
§  It appears that under both conditions mycorrhiza
has a negative effect on plant fitness.
0
Days to Germination
3) Does maintaining symbiosis under
low light conditions affect plant
fitness, such as weight and
germination rate of viable seeds?
Control
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J. A. Millar
Avg. Seed Weight (g)
2) Does maintaining symbiosis under
low light conditions result in
r e d u c e d r e p r o d u c t i o n ( i . e.
production of flowers and seed
pods)?
MF
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Research Questions
1) Do plant-associated mutualistic
microbes (mycorrhizal fungi) cause
constraints in plant growth under
low light conditions?
J. A. Millar
Results
Plant Biomass (g)
§  Mutualisms between plants and microbes are
important components in the determination of
plant diversity and ecosystem productivity
§  Mycorrhizal fungi colonize plant roots and
contribute nutrients, mainly phosphorus
§  Mycorrhizal fungi may consume up to 16% of
photosynthetically fixed carbon from the plant1
§  Plants compensate for symbiotic costs by
increasing photosynthesis, a process known as
sink stimulation2
§  Sink stimulation increases availability of
photosynthates for the plant and its symbionts
§  Under light limiting conditions, photosynthesis
cannot be increased
§  Under light limitation, do symbionts turn into
parasites and reduce plant fitness?
Mycorrhizal fungi © Nathan Brandt
Discussion
Methods
Introduction
Shoot:Root Ratio
Mycorrhizal fungi © Mark Brundrett
Plants respond with a sink stimulation of photosynthesis when colonized by fungal mutualists, which compensates for costs of carbohydrate allocation to the microbes. Problems may
arise when light is limited and plants cannot increase photosynthesis. We hypothesize that under such conditions the costs for maintaining the mutualism outweigh the benefits,
which ultimately turns the beneficial microbes into parasites exploiting resources and reducing host fitness. We study these plant-microbe interactions under different light
availabilities using lima bean plants and mycorrhizal fungi. In our study, we apply two levels of light (full light and light intensity reduced by 75%) and two levels of microbial
inoculation (sterile soil and mycorrhiza). Fitness-relevant plant parameters are measured including plant vegetative growth as well as flower and seed production. Preliminary results
indicate a reduction of flower and seed production in full light mycorrhizal colonized plants, with no difference between shaded treatments. Our study will promote our current
understanding of costs and benefits of plant-microbe interactions and will provide insights into effects of these interactions on plant fitness under variable ambient conditions.
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Control
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No Shade
Shade
© Homekitchengarden.com
J. A. Millar
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Shade
© Joseph De Leo
Literature Cited
1.  Kaschuk, G., Kuyper, T.W., Leffelaar, P.A., Hungria, M.,
Giller, K.E. 2009. Are the rates of photosynthesis
stimulated by the carbon sink strength of rhizobial and
arbuscular mycorrhizal symbioses? Soil Biology &
Biochemistry 41, 1233–1244.
2.  Mortimer, P.E., Pérez-Fernández, M.A., Valentine, A.J.
2008. The role of arbuscular mycorrhizal colonization in
the carbon and nutrient economy of the tripartite
symbiosis with nodulated Phaseolus vulgaris. Soil Biology
& Biochemistry 40, 1019–1027.
*For correspondence: jmillar@pdx.edu