The Measurement of Stomatal Responses to Stimuli in Leaves and Leaf Discs

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Abstract
A comparison has been made of stomatal responses in intact leaves, leaf discs supplied with water via their cut edges and leaf discs floating on water. Xanthium pennsylvanicum leaf discs watered via their cut edges appeared to be more turgid than intact leaves; this considerably slowed down the rate of stomatal opening but it slightly increased the final steady-state stomatal opening. When the water potential of such leaf discs was lowered by pre-treatment with mannitol solutions rates of stomatal opening increased whereas maximum steady-state openings decreased. In tobacco leaf discs floating on water the stomata in contact with water were wider open than those in contact with normal air and they did not respond to treatment with carbon dioxide-free air. The rate of photosynthesis was severely reduced in tobacco leaf discs floating with the lower epidermis on water, most probably owing to the slow rate of diffusion of carbon dioxide in water. By floating such discs on osmotica the degree of stomatal opening was increased, however, a response to treatment with carbon dioxide-free air was still not measurable. It is postulated that, on account of the relative unavailability of carbon dioxide from the water, the carbon dioxide concentration in the substomatal cavities of the lower surface is abnormally low, irrespective of whether ordinary air or carbon dioxide-free air is available to the upper surface. A comparison between porometer readings and measurements of siliconerubber impressions of stomatal pores taken from inside the porometer cup confirmed that the silicone-rubber impression method of assessing stomatal responses to stimuli has severe limitations, especially at small stomatal apertures.

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