Lawren Sack and Christine Scoffoni
This protocol explains how to make basic gmin measurements using simple equipment.
This protocol outlines how to obtain gmin for leaves (and can easily be modified for stems). The gmin is the conductance to vapor diffusion across the epidermis once stomata are closed, i.e., through the cuticle and any leaky stomata. The gmin varies many-fold across species, and can be a potentially strong determinant of drought tolerance, since a lower gmin better enables maintenance of hydration based on stored water (Kerstiens, 1996, Sack et al., 2003).
- Two lab stands
- Lab tape
- Analytical balance (to 0.001 g)
- Leaf area meter (or flatbed scanner)
- Candle and lighter
- Razor blade
- Thermometer and humidity sensor (options include a weather station with these sensors, or sensors with a datalogger, or using the cuvette of a porometer or photosynthesis system).
- Measurements should be made in a closed room with stable temperature.
Units, terms, definitions
gmin = minimum epidermal conductance
mfVPD = mole fraction vapor pressure deficit
RH = relative humidity (%)
T = air temperature (oC)
- Let shoots rehydrate overnight (optional)
- Set up lab stands up to 1 m apart, and run lab tape around them, sticking to itself between them, such that a “clothesline” runs between the stands, about 0.5 m above the bench.
- In the morning, remove leaves from shoot by cutting at base of petiole with a razor blade
- Seal the cut petiole ends with melted candlewax (optional)
- Number the leaves with a Sharpie and measure leaf areas
- Stick individual leaves to the lab tape “clothesline” with pieces of tape
- Place a fan under the clothesline or beside, such that the leaves sway gently in the breeze.
- Allow the leaves to dehydrate for at least 1 hour so that stomata are closed. This may take up to 4 h, depending on the species. You will be able to check this when you have your data.
- Set up temperature and humidity sensors next to the setup.
- Start timer
- Take the leaves off the clothesline, place in a plastic bag previously inhaled in (to minimize water loss). Stop the timer and record the time (hour, minutes, seconds). This will be your time 0.
- Weigh leaves, removing from the bag one at a time and replacing in the bag after weighing
- Tape leaves back onto the “clothesline”.
- Repeat steps 11-14 at 15-20 min intervals
- Aim for at least 8 points.
- At the end of the experiment, take leaves off the fan, measure leaf area. (See alternate protocol Measuring leaf perimeter and leaf area )
- Input in the gmin spreadsheet tool the weighing times, the leaf mass values, the initial and final leaf area values, temperature and relative humidity values.
- Input the VPsat for the given temperature values, using a Table (e.g., Table 7 in Pearcy et al., 1989), or computed from the Goff-Gratch or Arden Buck Equations.
- Determine the points to use for gmin calculation (the linear part of the leaf mass vs time plot).
- For those points, calculate the gmin based on intervals (From gmin analysis spreadsheet below: cell K16; select the points that were linear on the graph, and at least 3-4 points).
- Also calculate the gmin based on the slope (cell K17; for the formula, select the points that were linear on the graph, and at least 3-4 points). This value should be close to that calculated in step 20.
gmin Analysis Spreadsheet Tool created by Lawren Sack
Notes and troubleshooting tips
- Be sure to choose leaves that do not have cracks in the cuticle (unless this is what you wish to measure)
- Note that gmin can show plasticity within species, and within canopies (Sack et al., 2003, Cavender-Bares, Sack & Savage, 2007), and care should be taken to sample the right leaves for one’s questions.
- Ensure that your lab provides a realistic temperature and relative humidity. The gmin can be sensitive to these conditions (especially when these are extreme; Schreiber et al., 2001), and so the average conditions during measurement should be reported with the gmin. Some applications might require measurement in a chamber of given temperature and humidity.
- Typically, one person with a setup as described above can measure gmin for 10-20 leaves in the course of a day of measurement.
Cavender-Bares J., Sack L. & Savage J. (2007) Atmospheric and soil drought reduce nocturnal conductance in live oaks.
Kerstiens G. (1996) Cuticular water permeability and its physiological significance. Journal of Experimental Botany, 47, 1813-1832.
Pearcy R.W., Ehleringer J., Mooney H.A. & Rundel P.W. (1989) Plant Physiological Ecology: Field Methods and Instrumentation. Dordrecht, the Netherlands, Kluwer.
Sack L., Cowan P.D., Jaikumar N. & Holbrook N.M. (2003) The ‘hydrology’ of leaves: co-ordination of structure and function in temperate woody species. Plant, Cell and Environment, 26, 1343-1356.
Schreiber L., Skrabs M., Hartmann K.D., Diamantopoulos P., Simanova E. & Santrucek J. (2001) Effect of humidity on cuticular water permeability of isolated cuticular membranes and leaf disks. Planta, 214, 274-282.