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The characteristics of ABA-induced changes in the fluxes of CO2 and water vapour from whole leaves of spring wheat (Triticum aestivum cv. Wembley) were examined. Aqueous solutions of ABA were supplied via the transpiration stream to intact leaves of different ages mounted within a gas exchange cuvette. ABA caused a reduction in stomatal conductance (g) that was proportional to the concentration in the solution fed to the leaf. For the maintenance of a reduction in g there was a requirement for a continual supply of ABA. At concentrations greater than 10−2 mol m−3 ABA reduced g by at least 50% of the control value, while 1.0 mol m−3 closed stomata within 2 h. Concentrations as low as 10−3 mol m−3 produced a 20% reduction in g. As leaves aged they became less responsive to applied ABA. The possibility that the stomatal response may change after a leaf has previously experienced a pulse of ABA was explored by repeating the exposure of a leaf to 10−2 mol m−3 ABA. The first pulse of ABA produced a greater reduction in g than a subsequent exposure the following day. This decline in response of g to ABA on repeated exposure was maintained with leaves of different ages. The characteristics of the stomatal response to ABA are discussed in the context of what is known about the location of receptors for the hormone. It seems likely that a failure to respond to ABA that has previously accumulated in the guard cells should be viewed by means of maximizing the sensitivity to the current supply of ABA. It is suggested that the smaller response of the stomata of older leaves to ABA makes them more susceptible to water stress, so that they can act as sensors for declining water potentials to give early protection to younger, metabolically active leaves.
Atkinson et al. (Sun,) studied this question.