Release of Guttation Fluid from Passive Hydathodes of Intact Barley Plants. II. The Effects of Abscisic Acid and Cytokinins

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Abstract
Guttation was used as a non-destructive way to study the flow of water and mineral ions from the roots and compared with parallel measurements of root exudation. Guttation of the leaves of barley seedlings depends on age and on the culture solution. Best rates of guttation were obtained with the primary leaves of 6- to 7-day-old seedlings grown on full mineral nutrient solution. The growing leaf tissue becomes saturated with K+ below 1.5 mM K+ in the medium, whereas K+ concentration in the guttated fluid still increases further as K+ concentration in the medium is raised. At 3 mM K+ average values of guttation were 1.4–2.4 mm3 h−1 per plant with a K+ concentration of 10–20 mM; for exuding plants the flow was 4.2–7.6 mm3 h−1 per plant and K+ concentration 35–55 mM. Abscisic acid (ABA) at 10−6 to 10−4 M 0–2 h after addition to the root medium increased volume flow of guttation and exudation and the amount of K+ exported. Three h after addition of ABA both volume and amount of K+ were reduced. There was an ABA-dependent increase in water permeability (Lp) of exuding roots shortly after ABA addition. Later Lp was decreased by 35 per cent and salt export by 60 per cent suggesting an effect of ABA on salt transport to the xylem apart from its effect on Lp. Benzyladenine (5 × 10−8 to 10−5 M) and kinetin (5 × 10−6 M) progressively reduced volume flow and K+ export in guttation and exudation and reduced Lp. Guttation showed a qualitatively similar response to phytohormones as found here and elsewhere using exuding roots.