DOI: 10.5281/zenodo.17185682
title: Dune grass physiological responses

description: We characterized how the temperature and light environments affected the physiology of dominant species differing in photosynthetic pathways and compared to field values. We quantified physiological responses (i.e., photosynthesis, stomatal conductance, electron transport rate, water use efficiency, etc.) to light (0 – 2000 PAR µmol CO2 m-2 s-1) and temperature (20 - 40°C) for Ammophila breviligulata, Carex kobomugi, Panicum amarum, and Uniola paniculata in growth chamber experiments and through field sampling. Plants were potted with two individuals in a 3.8 L pot with a 50:50 mix of sand collected from Hog Island, Virginia with commercially available play sand that was sifted to remove larger grain sizes that were not found in the Hog Island sand. Each pot was placed in plastic watering trays which were filled three times a week throughout the experiment to mimic access to groundwater. Plants were stored in the Virginia Commonwealth University greenhouse until the experiments began. Plants were grown in a Conviron growth chamber (Model E15, Conviron, Winnipeg, Manitoba) under a photosynthetically active radiation (PAR) of approximately 1000 mmol m-2 s-1, 48% relative humidity, a photoperiod of 12 h. During the day, temperatures were set to the experimental temperature, starting at 20°C. At night, the temperature was 10°C lower than the experimental temperature. Experimental temperatures were increased five degrees every two weeks, starting at 20°C and ending at 40°C.

After plants had acclimated for 2 weeks at the target temperature, each plant (n=10) was measured at that temperature using a Li-6400XT portable gas exchange system (Licor, Lincoln, NE). Light and fluorescence curves were measured for each plant at 25°C and 35°C. For these curves, the CO2 was set to 400 ppm, and the flow rate to 1000 µmol s-1. Photosynthetic light curves were completed from a range of 0 to 2000 µmol m-2 s-1 (n=10) using the Li-6400 (Licor; Lincoln, NE) at two temperatures (25 and 35 ℃) for each species. For every measurement the Licor records photosynthetic rate (A), stomatal conductance rate (GSW), electron transport rate (ETR), and transpiration rate (E). Fluorescence curves were used to collect the quantum yield of photosystem II (ΦPSII also known as ΔF/Fm'). Panicum was unable to photosynthesize at 25℃ and was removed from further analysis. Using photosynthetic data WUE (water use efficiency), AQY (apparent quantum yield), LCP (light compensation point), LSP (light saturation point), and Rd (dark respiration) were calculated. 

Equation 1. WUE = Anet / E

WUE - Water Use Efficiency

Anet - Net photosynthetic rate (µmol CO2 m-2 s-1)

E - Transpiration rate  (µmol H2O m-2 s-1)

 

 

Equation 2. AQY = (y2 - y1) / (x2 - x1)

 

AQY - Apparent Quantum Yield, the slope of the light limiting portion of the light curve. Here PAR 0 - 500 was used. 

x2 - 500 (µmol m-2 s-1)

x1 - 0 (µmol m-2 s-1)

 

Equation 3. LCP = - (b/m)

 

LCP - Light Compensation Point, the light level when net photosynthesis is 0 

b - y-intercept (dark respiration point)

m - slope of the curve where it meets the x-axis (y=0)

 

Equation 4. LSP  = (y - b) / m

 

LSP - Light Saturation point, the light intensity that maximizes photosynthesis

y (Anet) = 97% of Amax 

b - y-intercept (dark respiration point)

m - slope of the curve where it meets the x-axis (y=0)

 

The field comparison measurements were completed at Back Bay National Wildlife Refuge in Virginia Beach, VA. The northernmost mile of the refuge is not open for public access, and was the focus of field data collected in this study. All species were measured on the face of the foredune. Measurements were completed on days where cloud cover was low, during full sun, 11am - 2pm. To capture seasonal differences, measurements were performed in April, June, August, and October 2023. 

Using a Li-600 (Licor, Lincoln, NE) porometer and fluorometer, measurements were taken at midday on sunny days with minimal to no cloud cover. All four dune species (Ammophila breviligulata, Uniola paniculata, Carex kobomugi, and Panicum amarum) were included (n=15), replications included one plant from each species, which was repeated 15 times. This method was used to account for time of day on the plant. Individual plants were not repeated on a single day and were chosen at random. This instrument measures stomatal conductance and fluorescence of the leaf, allowing for comparison to the growth chamber.