Movement and compartmentation of abscisic acid in guard cells of Valerianella locusta: Effects of osmotic stress, external H+-concentration and fusicoccin

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Abstract
Epidermal peels of Valerianella locusta were acid-treated for 1 h at pH 3.9 to kill all cells other than guard cells. These guard-cell preparations were used to explore the steady-state one-way fluxes and the cytoplasmic and vacuolar contents of abscisic acid (ABA). The method of compartmental analysis has been applied. The intracellular ABA concentrations were surprisingly high. At an external pH of 5.8 the cytoplasm contained 1.28 mmol·dm-3 of ABA, twice of the amount which accumulated in the vacuoles (0.57 mmol·dm-3). The fluxes of ABA at the plasmalemma (ϕoc=ϕoc=0.43 fmol · cell −1 · h −1) were higher than those at the tonoplast (ϕcv=ϕvc=0.12 fmol · cell −1 · h −1). Moderate stress (0.1 and 0.3 mol·dm-3 sorbitol in the medium) caused a change in the kinetics of ABA movement. The rate constants of the fluxes from the cytoplasm into the vacuole (ϕcv) and into the apoplast (ϕco) were increased while the rate constant of the flux from the vacuoles into the cytoplasm (ϕvc) was decreased. As a consequence the amount of ABA sequestered in the vacuole remained unchanged; the cytoplasmic ABA content, however, was reduced to only 20% of that found in the control treatments (no sorbitol in the medium). Under moderate stress, one Valerianella guard cell released rapidly about 0.36 fmol·cell-1 to its direct cell-wall space. This surprising result is discussed in regard to rapid stomatal closure under reduced water supply.