Interacting influences of global change factors on leaf stomatal response in prairie grasses

Gregory T. Nelson, Zoe Zehner ● Faculty Mentor: Dr. Tali D. Lee Department of Biology-University of Wisconsin Eau Claire

*nelsongt@uwec.edu

Introduction and Objective• Plants play an important role in the carbon cycle, fixing atmospheric carbon dioxide (CO2) into organic compounds.10 Pores in plant leaves called stomata are the gateway between the atmospheric and biological portions of the carbon cycle as they modulate gas exchange (CO2 and H2O).5,15

• Plants optimize CO2 uptake and minimize H2O loss through development by adjusting stomatal densities7,8 and physiology by controlling stomatal conductance.6,8

• The leaf is also the primary site of perception of many environmental variables and can relay information to guide new leaf development.11

• Previous work has shown that stomatal densities have declined since the industrial revolution as a result of rising CO2 concentrations.16,17,18 Stomatal density can also be affected by environmental factors such as H2O12,13 and nitrogen (N) availability.4

• Agropyron repens, Andropogon gerardii, and Bromus inermis are three grass species common to new world prairies.14

• Our main objective was to examine leaf stomatal responses when multiple environmental factors were manipulated, as part of a large scale ecosystem study, in an attempt to better predict the effects of changing global factors on plant function and the implications for carbon and water cycles.

Methods• Study site: Cedar Creek Ecosystem Science Reserve, east central Minnesota (

http://www.biocon.umn.edu/).

• Plants were grown in 2x2 m2 plots representative of grasslands communities.

• Experimental treatments were arranged in factorial combination of CO2 concentration (ambient, ambient + 180 ppm), H2O supply (ambient, ambient - 50% precipitation), and N availability (ambient, ambient + 4g N m-2yr-1).

• Upper fully expanded leaves, “newest”, and oldest functional leaves, “oldest”, were harvested per plant to make leaf surface impressions using clear acrylic to determine stomatal and epidermal cell densities visible through a microscope (400x).

• Number of stomata per total number of epidermal cells (stomatal index, SI) was calculated as a fair measure of stomata density to control for unrelated changes in leaf morphology.

• Stomatal conductance to H2O was measured using a LiCOR 6400 portable infrared gas exchange system in the pervious three years.

• Analysis of variance (ANOVA) statistical analyses were performed using JMP v7 (SAS Cary, NC). P <0.1 considered biologically significant.

Discussion• Leaves that develop later in the growth season had greater numbers of stomata suggesting an affect of ontogeny. In some cases, this response was dependent on environmental factors, suggesting that grasses are able to detect and subsequently respond developmentally to seasonal changes.

• Although there has been a historic trend of decreasing stomatal density with increasing CO2

1,2, some studies are finding no change or even an increase in stomatal index3, as we have in this study. There is likely a point at which further decreases in stomatal densities can compromise basic leaf function regardless of CO2 concentrations and that other adjustments such as in changes in physiology become relatively more effective.

• This idea is supported by our results as stomatal conductance was lower in plants grown under elevated compared to ambient CO2 due to changes in stomatal physiology.

• In contrast with findings for other plant groups, environmental factors had little effect on stomatal index in these grasses. Instead grasses seem to rely on their ability to physiologically control conductance.9

• To improve our understanding of how plants will respond to changing global environments, ontogeny should be considered. In addition, the physiological regulation of stomatal function in grasses may become relatively more important than stomatal density changes as plants adapt to future global conditions. Finally, as different plant groups have different stomatal evolutionary histories, responses must be evaluated per group.

AcknowledgementsFunding:

UWEC Office of Research and Sponsored ProgramsUWEC Differential Tuition

Cedar Creek Ecosystem Science Reserve

Field and technical support:Molly Kreiser, Adam Schneider, Susan Barrott, and Kally Worm

References1. Beerling DJ, Chaloner WG. 1993. Evolutionary responses of stomatal density to global carbon dioxide change. Biol. J. Linn. Soc. 48(4):343–532. Beerling DJ, Chaloner WG, Huntley B, Pearson JA, Tooley MJ. 1993. Stomatal density responds to the glacial cycle of environmental change.

Proc. R. Soc. London Ser. B 251:133–383. Casson S, Gray JE. 2008. Influence of environmental factors on stomatal development. New Phytologist 178(1): 9-23.4. Cen YP, Sage RF. 2005. The regulation of rubisco activity in response to variation in temperature and atmospheric CO2 partial pressure in sweet

potato. Plant Physiology 139(2): 979-990.5. Cowan IR, Troughto.Jh. 1971. Relative role of stomata in traspiration and assimilation. Planta 97(4): 325-&.6. Cowan IR. 1977. Stomatal behavior and environment. Advances in Botanical Research 4:117–2287. Drake BG, GonzalezMeler MA, Long SP. 1997. More efficient plants: A consequence of rising atmospheric CO2? Annual Review of Plant

Physiology and Plant Molecular Biology 48: 609-639.8. Drake PL, Froend RH, Franks PJ. 2013. Smaller, faster stomata: scaling of stomatal size, rate of response, and stomatal conductance. Journal

of Experimental Botany 64(2): 495-505.9. Haworth M, Elliott-Kingston C, McElwain JC. 2013. Co-ordination of physiological and morphological responses of stomata to elevated CO2 in

vascular plants. Oecologia 171(1): 71-82. 10. Hetherington AM, Woodward FI. 2003. The role of stomata in sensing and driving environmental change. Nature 424(6951): 901-908.11. Lake JA, Quick WP, Beerling DJ, Woodward FI. 2001. Plant development - Signals from mature to new leaves. Nature 411(6834): 154-154.12. McDowell N, Pockman WT, Allen CD, Breshears DD, Cobb N, Kolb T, Plaut J, Sperry J, West A, Williams DG, Yepez EA. 2008.

Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? New Phytologist 178(4): 719-739.

13. Reich PB, Hinckley TM. 1980. Water relations, soil fertility, and plant nutrient composition of a pigmy oak ecosystem. Ecology 61(2): 400-416.14. Reich PB, Tilman D, Craine J, Ellsworth D, Tjoelker MG, Knops J, Wedin D, Naeem S, Bahauddin D, Goth J, Bengtson W, Lee TD. 2001.

Do species and functional groups differ in acquisition and use of C, N and water under varying atmospheric CO2 and N availability regimes? A field test with 16 grassland species. New Phytologist 150(2): 435-448.

15. Sage RF. 1994. Acclimation of photosynthesis to increasing atmosheric CO2- the gas exchange perspective. Photosynthesis Research 39(3): 351-368.

16. Woodward FI. 1987.Stomatal numbers are sensitive to increases in CO2 from preindustrial levels. Nature 327:617–61817. Woodward FI. 1993. Plant-responses to past concentrations of CO2. Vegetatio 104: 145-155.18. Woodward FI, Kelly CK. 1995. The influence of CO2 concentration on stomatal density. New Phytologist 131:311–327

(A)

Figure 2. Interaction plots of stomatal index (SI = , mean ± SE) for three grass species showing age by (A) CO2, (B) N, and (C) H2O treatment interactions. Each point represents 32 plants pooled across other treatments. P values for environmental treatment and age main effects and treatment x age interactions are given for each species.

Results: Newest developed leaves had significantly higher SI than oldest leaves across species. Otherwise, the CO2, H2O, and N treatments had little effect on SI. Elevated CO2 grown leaves had slightly higher SI than ambient CO2 grown leaves in 2 of the 3 species and in 1 species, leaves grown at reduced H2O availability had more stomata than those grown at ambient H2O. Additionally, responses to N and H2O availability depended on leaf age in a few cases.

http://michaelforsberg.photoshelter.com/image/I00007mBBjm3c2P8

http://www.uniquelyminnesota.com/weekend-away/prairie-waters-mn.htm

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CO2 main effect: P = 0.0507 CO2 x age: P = 0.1162

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(C)(B) H2O main effect: P = 0.3562 H2O x age: P = 0.0615

N main effect: P = 0.9642 N x age: P = 0.1144

ambient CO2 elevated CO20.1

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CO2 main effect: P = 0.5625 CO2 x age: P = 0.2361

(A) N main effect: P = 0.2275 N x age: P = 0.0188

(B) H2O main effect: P = 0.9879 H2O x age: P = 0.4959

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CO2 main effect: P = 0.0413 CO2 x age: P = 0.7490

H2O main effect: P = 0.0454 H2O x age: P = 0.6893

N main effect: P = 0.6043 N x age: P = 0.1160

Andropogon gerardii:

Agropyron repens:

Bromus inermis:

ambient H2O reduced H2O0.1

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Age main effect: P < 0.001

Age main effect: P = 0.0933

Age main effect: P = 0.0248

Results

Figure 1. Photograph of a dumb-bell shaped stomata typical of grasses (left) and a kidney bean shaped stomata of other plants (right).10 Plants regulate CO2 uptake and H2O loss by controlling the number of stomata formed during growth as well as the aperture of the stomata as conditions change.

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A. repens

A. gerardii

B. inermis

Figure 4. Percent reductions in stomatal conductance rates due to reduced H2O availability.

Results: The magnitude of reduction due to reduced water availability was dependent on the CO2 level for A. gerardii.

Figure 3. Stomatal conductance (mean ± SE) for each species. Asterisks denote a significant difference between ambient and altered treatment levels (P < 0.05); data was natural log transformed for analysis.

Results: Conductance rates were lower in plants grown under elevated than ambient CO2 and in plants under reduced compared to ambient H2O. N did not affect conductance.

ambient CO2

elevated CO2