Plant physiology research project during the fall semester of 2007

Swarthout Research Lab during the Fall of 2007. Sarah Herington (lab technical assistant, left), Debbie Swarthout, Daniel Hills (plant physiology research student), Cindy Reimink (plant physiology research student, right)

Description of Plant Physiology Research Class

Cindy Reimink and Dan Hills are both seniors who have taken Plant physiology because of their inherent interest in knowing more about how plant function. Cindy is thinking about working in a local commercial greenhouse for a year before going to graduate school and Dan is interviewing at medical schools across the country. The small class size has allowed them the opportunity to perform an intensive investigative study throughout most of the semester instead of participating in regular classic plant physiology labs. Their investigative project entailed learning how to use gas exchange principles to estimate the biochemical limitations to photosynthesis in cottonwood, quack grass, big bluestem, and tall fescue grasses with different levels of endophyte infection when growing under high and low light levels. Much of the class lecture time was utilized to analyze chapters 7,8 and 9 of the 4th Edition of Taiz and Zeiger (2006) to provide the students with the theoretical biochemical background needed to understand photosynthesis in C3 and C4 plants at its most fundamental level. They also read and discussed five journal articles to help them gain a connection to what relevant questions research scientists are addressing.

Theme of plant physiology research class: The use of the principles of photosynthetic gas exchange when exposing plants to different CO2 and light levels (A:Ci and A:I curves)

Cindy and Dan were introduced to using the principles of photosynthetic gas exchange by collecting empirical data for the photosynthetic response to CO2 (A:Ci curves) and light (A:I curves) on a single cottonwood tree one beautiful, warm day in September at Holland State Park. The remaining lab times during the first half of the semester was spent analyzing their data and comparing it with two grass species that were studied this past summer by Beth Weidenhaft and Emily Schuiteman at the Pierce Cedar Creek Institute near Hastings, MI. Cindy and Dan took apart a nonlinear curve fitting model that was published by Sharkey et al (2007) recently in Plant, Cell and Environment which provided an online Excel program called SOLVER to determine the different biochemical limitations of photosynthesis using a LiCor 6400 gas exchange system shown in Figure 1. They were then able to observe how changes in Rubisco kinetics (Vcmax), electron tranport usage to regenerate RuBP (J), triose phospate utilization (TPU), dark respiration (Rd) and mesophyll conductance (gm) contributed to the given photosynthetic rate of the different species. They were then instructed on how to calculate oxygenation to carboxylation ratios by Rubisco at the chloroplast concentrations measured at ambient CO2 concentrations in the atmosphere in order to determine whether there was a species difference in photorespiration rates. Cindy and Dan were required to write a full scientific lab report on this preliminary investigation and were asked why the C4 species did not fit the model correctly. They were examined on their knowledge in the form of one midterm exam in October.

Figure 1. The use of a Li-Cor 6400 gas exchange system to determine the limitations of photosynthesis.

Description of independent research project

The last six weeks of the semester were dedicated to their independent project where they were required to use their knowledge to design an experiment to investigate whether photorespiration changed with infection status of an endophyte infection in tall fescue grasses. Students used an immunoblot detection method to estimate level of infection of the fungus in the grasses (Figure 2). They then designed their experiment so that they would have each of two replicates with low, intermediate and high infection rates growing at low light levels (six individual pots, each containing a single tall fescue grass). The same level of distinction was used for the six grasses growing under high light conditions. They collected CO2 and light response curves and analyzed the data using the SOLVER model (Sharkey et al., 2007). Their final evaluation was based on an oral presentation and a scientific writeup of their project during the final exam week.

References used in this class

Golan, T., Mouler-Muller, P., Niyogi, K.K. (2006) Photoprotection mutants of Arabidopsis thaliana acclimate to high light by increasing photosynthesis and specific antioxidants. Plant, Cell and Environment 27:979-987.

Huxman, T.E. and Monson, R.K. (2003) Stomatal responses of C3, C3-C4 and C4 Flaveria species to light and intercellular CO2 concentration: implications for the evolution of stomatal behaviour. Plant, Cell and Environment 26: 313-322.

Malinowski, D.P. and Beleskey, D.P. (2006) Ecological importance of Neotyphodium spp. grass endophytes in agroecosystems. Grassland Science 52:1-14.

Sharkey, T., Bernacchi, C.J., Farquhar, G.D. and Singaas, E.L. (2007) Fitting photosynthetic carbon dioxide response curves for C3 leaves. Plant, Cell and Environment, doi. 10.1111/j.1365-3040.2007.01710.x

Smart, D.R.and Bloom, A. J. (2000) Wheat leaves emit nitrous oxide during nitrate assimilation. Proceedings of the National Academy of Science, 98:7875-7878.

Taiz, L.and Zeiger, E. (2006) Plant Physilogy. (Fourth Edition). Sinauer Associates, Inc., Publishers, MA, USA.