Are Roots a Source of Abscisic Acid for the Shoots of Flooded Pea Plants?

Flooding the soil for 2–5 d decreased stomatal conductances of pea plants (Pisum sativum L., cv. Sprite) with six or seven leaves. This coincided with slower transpiration, increased leaf water potentials and increased concentrations of abscisic acid (ABA) in the leaves. No increase in ABA was found in the terminal 20 mm of roots of flooded plants over the same time period. Small stomatal conductances associated with increases in foliar ABA were also found in plants grown in nutrient solution when aeration was halted, causing the equilibrium partial pressures of dissolved oxygen to fall below 05 It Pa. No increase in ABA concentration in young secondary roots of the non-aerated plants was detected after 24, 48 or 72 h, even when the shoot, the presumed site of deposition for any ABA from the roots, was removed 5–6 h before analysis. Similarly, ABA concentrations in roots were not increased when the nutrient solution was de-oxygenated by continuous purging with nitrogen gas. The abscisic acid concentration in leaf epidermis, the tissue most likely to be the recipient of any ABA moving in the transpiration stream from oxygen-deficient roots, was lower than in the remaining parts of the leaf when examined in the mutant Argenteum which possesses easily removable epidermal layers. It is concluded that the leaves of plants subjected to flooding of the soil or oxygen shortage in the root environment are not enriched substantially with ABA from the roots. A more probable source of this growth regulator is the leaf itself.