Estimation of stomatal density is often done when studying photosynthesis (at GCSE and higher levels), and can offer a way of illustrating use of the graticule with post-16 students. There are a number of ways to measure stomatal density. Because of the size of stomata, you will need a reasonably good microscope for this. Your choice of magnification will depend on the leaf material that you are using, and the size of the stomata.
One popular method has been to use clear nail varnish to make an impression of the epidermis. Making the impression and viewing it under a microscope can be completed in one lesson. However, some leaves are prone to damage from the solvent in the nail varnish. The leaves absorb it, turn brown, and fail to produce any impression. Pupils lose interest and get frustrated because their leaves ‘aren’t working’. Also, for a GCSE class, several pots of nail varnish are needed so that no one is left waiting, thus adding to expense. Other methods include using Germolene New Skin and using a water-based varnish from DIY shops.
Selecting your plants
One of the best plants for doing epidermal peels is the red hot poker plant Kniphofia. Being a monocot its stomata are highly ordered in rows, but they are big and great for stomatal opening and closing using solutions of different concentrations.
Almost as good is the Elephants Ear Saxifrage Bergenia. This also peels very easily, but the stomata are smaller although clearly visible at x100 magnification. This is a dicot so the distribution is more random.
Many labs have a Pelargonium, and these can also be used for leaf peels.
Spider plants (Chlorophytum comosum variegatum) make excellent leaf peels, with particularly interesting and regular patterns of stomata along the green leaf areas only.
I’ve been putting some freshly-cut Aloe-leaf into fixative this evening and thought I’d make a quick & dirty epidermal-peel. Stained briefly with Toluidine-blue (aq) then water-mounted with coverslip, very colourful.
Had a quick try of a cardboard ‘Matthias-arrow’ I ‘made’ when reading the great Walter Dioni’s articles when I started-out – it’s really not very good compared to the simply fantastic oblique-illumination often seen of this forum (to say the least! ) but gives a little relief at least.
Here are a few pictures, apologies for the poor quality, it was a very quick foray as I was actually fixing tissue rather than meaning to do this, but though it’d be nice to have a peek. I’ll definitely go back and do some ‘proper’ ones I think, maybe mounted in glycerin or perhaps an alcohol-based mountant, anyway here are a few pictures to peruse!
When stomata are open, the air spaces in the spongy mesophylllayer of a leaf become continuous with the atmosphere. This means that photosynthetic gases are free to diffuse in and out of the plant. In general, carbon dioxide diffuses in through stomatawhile oxygen and water diffuse out.
Stomata are generally open during the day to allow the free exchange of photosynthetic gases, and closed at night to prevent water loss when photosynthesis is not taking place.
As a consequence, water loss is highest during the day. This is not a problem for well-watered mesophytes. However, plants that live arid conditions with saline soils – xerophytes – have special adaptations to reduce water loss by transpiration. These include:
a thick cuticle, giving the leaf a waxy or leathery appearance, or leaves covered in small hairs to prevent water loss through evaporation
stomata concentrated on lower surfaces or in deep pits protected from the wind
fleshy stems that store water – in the case of cacti, stems are photosynthetic and leaves are reduced to short spines
There are two major methodical approaches with which changes of status in stomatal pores are addressed: indirectly by measurement of leaf transpiration, and directly by measurement of stomatal apertures.
Application of the former method requires special equipment, whereas microscopic images are utilized for the direct measurements. Due to obscure visualization of cell boundaries in intact leaves, a certain degree of invasive leaf manipulation is often required.
Our aim was to develop a protocol based on the minimization of leaf manipulation and the reduction of analysis completion time, while still producing consistent results. We applied rhodamine 6G staining of Arabidopsis thaliana leaves for stomata visualization, which greatly simplifies the measurement of stomatal apertures.
By using this staining protocol, we successfully conducted analyses of stomatal responses in Arabidopsis leaves to both closure and opening stimuli. We performed long-term monitoring of living stomata and were able to document the same leaf before and after treatment.
Moreover, we developed a protocol for rapid-fixation of epidermal peels, which enables high throughput data analysis. The described method allows analysis of stomatal apertures with minimal leaf manipulation and usage of the same leaf for sequential measurements, and will facilitate the analysis of several lines in parallel.
Stomatal function can be used effectively to monitor plant hydraulics, photosensitivity, and gas exchange. Current approaches to measure single stomatal aperture, such as mold casting or fluorometric techniques, do not allow real time or persistent monitoring of the stomatal function over timescales relevant for long term plant physiological processes, including vegetative growth and abiotic stress.
Herein, we utilize a nanoparticle-based conducting ink that preserves stomatal function to print a highly stable, electrical conductometric sensor actuated by the stomata pore itself, repeatedly and reversibly for over 1 week.
This stomatal electro-mechanical pore size sensor (SEMPSS) allows for real-time tracking of the latency of single stomatal opening and closing times in planta, which we show vary from 7.0 ± 0.5 to 25.0 ± 0.5 min for the former and from 53.0 ± 0.5 to 45.0 ± 0.5 min for the latter in Spathiphyllum wallisii. These values are shown to correlate with the soil water potential and the onset of the wilting response, in quantitative agreement with a dynamic mathematical model of stomatal function. A single stoma of Spathiphyllum wallisii is shown to distinguish between incident light intensities (up to 12 mW cm−2) with temporal latency slow as 7.0 ± 0.5 min. Over a seven day period, the latency in opening and closing times are stable throughout the plant diurnal cycle and increase gradually with the onset of drought. The monitoring of stomatal function over long term timescales at single stoma level will improve our understanding of plant physiological responses to environmental factors.