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Adaxial and Abaxial Stomatal Resistance of Cotton in the Field¹

An experiment was conducted near. Sydney, Australia to determine the adaxial and abaxial stomatal responses of cotton (Gossypium hirsutum L.) to environmental factors under noncycling field conditions. Cotton plants were grown in small lysimeter pans and transpiration was measured by strain guage lysimeters. Leaf temperature, ambient temperature and other micrometeorological parameters were monitored continuously, Stomatal resistances were regularly measured for both surfaces with a diffusion resistance porometer developed by the author.

It was found that under field conditions, the adaxial and abaxial stomata differ in their response to light, water stress, and ambient temperature. Under most conditions, including darkness and. drought, the adaxial surface was considerably higher in resistance than the abaxial surface. However, the differences in resistance between the two surfaces disappeared under a combination of high light and temperature conditions. High transpiration rates and low leaf resistance were associated with high ambient temperatures. With sufficient soil moisture there was no evidence of midday stomatal closure even under sew;re light and temperature regimes.

It was found that sampling the resistance of one surface alone (particularly the adaxial) did not give an accurate estimate of the overall leaf resistance. In addition, it was also found that similar .resistances on both surfaces under any one set of environmental conditions does not necessarily imply that this will be so under all conditions.

Key Words: Diffusion resistance porometer • Water stress • Light • Temperature

² Former Graduate Student, University of New South Wales (now Assistant Professor, Department of Bioengineering, Texas A&M University, College Station, Texas 77843).