The Effect of Exogenous Abscisic Acid on Stomatal Development, Stomatal Mechanics, and Leaf Gas Exchange inTradescantia virginiana

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Abstract
Gas exchange parameters and stomatal physical properties were measured in Tradescantia virginiana plants grown under well-watered conditions and treated daily with either distilled water (control) or 3.0 mm abscisic acid (ABA). Photosynthetic capacity (CO2 assimilation rate for any given leaf intercellular CO2 concentration [c i]) and relative stomatal sensitivity to leaf-to-air vapor-pressure difference were unaffected by the ABA treatment. However, at an ambient CO2 concentration (c a) of 350 μmol mol−1, ABA-treated plants operated with significantly lowerc i. ABA-treated plants had significantly smaller stomata and higher stomatal density in their lower epidermis. Stomatal aperture versus guard cell pressure (P g) characteristics measured with a cell pressure probe showed that although the form of the relationship was similar in control and ABA-treated plants, stomata of ABA-treated plants exhibited more complete closure atP g = 0 MPa and less than half the aperture of stomata in control plants at any givenP g. Scaling from stomatal aperture versusP g to stomatal conductance versusP g showed that plants grown under ABA treatment would have had significantly lower maximum stomatal conductance and would have operated with lower stomatal conductance for any given guard cell turgor. This is consistent with the observation of lower c i/c a in ABA-treated plants with a c a of 350 μmol mol−1. It is proposed that the ABA-induced changes in stomatal mechanics and stomatal conductance versusP g characteristics constitute an improvement in water-use efficiency that may be invoked under prolonged drought conditions.