An Improved Infiltration Series for Studying Stomatal Opening as Illustrated with Coffee

Abstract

Several liquids or series of liquids have been utilized for the purpose of estimating the degree of stom-atal opening by the so-called " infiltration " or " in-jection" method. Stahl (9) used a series consisting of paraffin oil, petroleum, and petroleum ether; MIolisch (5) employed absolute alcohol, benzene, and xylol; Dietrich (2) used paraffin oil, castor oil plus turpentine, petroleum, and xylol. Since these series included only three or four liquids of different surface tensions, they gave rather rough estimations of stom-atal aperture. Schorn (8) attempted to increase the accuracy of the infiltration method by using a larger series of liquids, which was prepared with isobutyl alcohol and ethylene glycol in eleven mixtures varying from pure isobutyl alcohol to pure ethylene glycol in steps of 10% by volume. As pointed out by Op-penheimer and Mendel (7), pure isobutyl alcohol does not penetrate stomata of small aperture and for that reason this series cannot 'be utilized for determining slight changes in the opening of such stomata. The authors have obtained very satisfactory results in estimating the degree of stomatal opening of several plant species by using infiltration series prepared with Nujol mineral oil diluted with xylol, kerosene , n-dodecane, n-tetradecane, or Bayol-D.2 All of these materials, except Nujol, infiltrate the leaves when the stomata are only slightly open. Nujol does not penetrate even wide open stomata, except for plants with particularly large stomata. Series of ten or eleven concentrations were used in steps of 10% by volume from pure xylol (or n-dode-cane, n-tetradecane, etc.) to pure Nujol. Steps of less than 10% concentration can be used for greater precision. This has proven especially suitable for plants with relatively small stomata, such as cacao, which is not penetrated by solutions containing more than 80% Nujol. All of the materials which have previously been recommended for the infiltration technique, including xylol and kerosene, are toxic and cause necrotic spots where the drops are applied. This becomes a distinct disadvantage when it is desirable to utilize the same leaves for periodic tests of stomatal aperture, or when the number of leaves is limited, as in nutrient culture work with small plants. The pure hydrocarbons , n-dodecane and n-tetradecane, and the highly refined white oil, Bayol-D, are not toxic to most plants (3). The authors found that the pure hydrocarbons were toxic to bean leaves, although Bayol-D caused no 1 Received January 19, 1953. 2 The three latter materials were provided through the courtesy of the Standard Oil Development Company, Linden, New Jersey. 97 injury. Neither the oil nor the hydrocarbons injured cacao, coffee, cassava, citrus and rubber. Nujol did not produce necrotic injury, but it evaporated very slowly from the leaf. It caused no injury to cacao and coffee, but concentrations above 80% caused some injury to beans. Figures 1 and 2 illustrate results obtained with the infiltration series proposed by the authors. These data represent the cycle of stomatal opening of coffee leaves both in sunlight and in shade during two consecutive days (Oct. 15 and 16, 1952). These data were obtained with the series prepared with n-dode-10