Ineffectiveness of Abscisic Acid in Stomatal Closure of Yellow Lupin,Lupinus luteusvar. Weiko III

Abstract

Stomata of yellow lupin leaves are remarkably insensitive to abscisic acid (ABA). Stomatal resistance was monitored using both a viscous now porometer and a diffusion porometer. Results were confirmed with scanning electron microscopy. When exogenous ABA solutions were supplied via petioles, 10⁻⁶ M solutions had no effect on stomatal resistance. Upper (adaxial) stomata were not affected by 10⁻⁵ M ABA but lower stomata showed 3-fold more resistance after 2 h. Stomata of both surfaces closed after 30 min in 10⁻⁴ M ABA. Isolated epidermal peels of lupin leaves were floated on ABA solutions yet upper surface peels showed no stomatal closing in 10⁻⁴ M ABA, while lower surface stomata closed to a barely significant extent. Stomata of intact leaves were not very sensitive to darkness, showing at most a doubling in resistance after 6 h darkness. Complete stomatal closure, however, was readily produced by wilting leaves. Hence, lupin stomata are physically capable of closing. Endogenous ABA levels of water-stressed leaves increased approximately 10-fold, which corresponds to concentrations below 10 μM ABA. It is concluded that ABA is unlikely to play a role in controlling short-term stomatal response of lupins.