Dehydration of isolated roots of seven <Emphasis Type="Italic">Lupinus</Emphasis> species induces synthesis of different amounts of free,but not conjugated,abscisic acid

Abscisic acid (ABA) is recognised as an important hormone involved in root-to-shoot communication of drought stress in plants. This study aimed to determine whether isolated roots can produce both free and conjugated ABA (ABA–glucose ester) and whether Lupinus species vary in the synthesis of ABA in the roots when dehydrated. The concentration of free and conjugated ABA at 100 and 50% root water content was measured in the distal 10 mm of the roots of 3- to 5-day-old seedlings of seven Lupinus species with and without 10⁻⁵ M tetcyclacis, an inhibitor of the oxidative breakdown of ABA. When the root tips were exposed to tetcyclacis, the concentration of free ABA increased by 20% on average, suggesting that oxidative breakdown of free ABA was limited in the isolated Lupinus roots. The concentration of free ABA of the fully hydrated plants varied significantly among genotypes and more than doubled on average across genotypes with dehydration of the root tips to 50% water content. The concentration of conjugated ABA also varied significantly with species, but was only one-tenth the concentration of free ABA in the roots and did not change significantly with root dehydration or the inhibition of oxidative metabolism. The production of free ABA in response to the water deficit varied with species from +470% in L. digitatus to +33% in L. angustifolius. The small concentration and lack of increase of conjugated ABA with water deficit suggests that it is unlikely to have an important role as a root signal in response to soil drying in Lupinus species.