Modification of the CO2Responses of Maize Stomata by Abscisic Acid and by Naturally-Occurring and Synthetic Cytokinins

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Abstract
Fragments of maize leaves were incubated at controlled temperature and irradiance either on distilled water or on one of three concentrations of cytokinin (10−1, 10−2 and 10−3 mol m−3). The effects of zeatin or kinetin on stomatal aperture were determined by stripping abaxial epidermis from the fragments after incubation and immediately measuring stomatal apertures under the microscope. At each cytokinin concentration leaf pieces were incubated at 5 or 350 μmol mol−1 CO2 with or without ABA (10−1 mol m−3). At 5.0 μmol mol−1 CO2 increasing the concentrations of zeatin had a negligible effect upon stomatal aperture. When air containing 350 umol mol−1 CO2 was bubbled through the incubation solutions, apertures of stomata incubated on water were more than halved. Increasing cytokinin concentrations reduced the effect of CO2 on stomata and incubation on 10−1 mol m−3 zeatin completely removed any CO2 response. The addition of ABA restored the effect of CO2, even at the highest cytokinin concentration.