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Plant Physiol, August 2001, Vol. 126, pp. 1716-1724
Transpiration Rate. An Important Factor Controlling the Sucrose
Content of the Guard Cell Apoplast of Broad Bean1
William H.
Outlaw Jr.* and
Xiaoyi
De Vlieghere-He
Department of Biological Science, Biology Unit I, Florida State
University, Tallahassee, Florida 32306-4370
Evaporation of water from the guard cell wall concentrates
apoplastic solutes. We hypothesize that this phenomenon provides two
mechanisms for responding to high transpiration rates. First, apoplastic abscisic acid is concentrated in the guard cell wall. Second, by accumulating in the guard cell wall, apoplastic sucrose (Suc) provides a direct osmotic feedback to guard cells. As a means of
testing this second hypothesized mechanism, the guard cell Suc contents
at a higher transpiration rate (60% relative humidity [RH]) were
compared with those at a lower transpiration rate (90% RH) in broad
bean (Vicia faba), an apoplastic phloem loader. In
control plants (constant 60% RH), the guard cell apoplast Suc content
increased from 97 ± 81 femtomol (fmol) guard cell pair 1 to 701 ± 142 fmol guard cell
pair 1 between daybreak and midday. This increase is
equivalent to approximately 150 mM external, which is
sufficient to decrease stomatal aperture size. In plants that were
shifted to 90% RH before daybreak, the guard cell apoplast Suc content
did not increase during the day. In accordance, in plants that were
shifted to 90% RH at midday, the guard cell apoplast Suc content
declined to the daybreak value. Under all conditions, the guard cell
symplast Suc content increased during the photoperiod, but the guard
cell symplast Suc content was higher (836 ± 33 fmol guard cell
pair 1) in plants that were shifted to 90% RH. These
results indicate that a high transpiration rate may result in a high
guard cell apoplast Suc concentration, which diminishes stomatal
aperture size.
1
This work was supported by the U.S. Department
of Energy (grant to W.H.O.).
*
Corresponding author; e-mail outlaw{at}bio.fsu.edu; fax
850-644-0481.
© 2001 American Society of Plant Physiologists
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