The Composition of Phloem Exudate and Xylem Sap from Tree Tobacco (Nicotiana glauca Grah.)

Abstract

The composition of xylem sap and exudate from stem incisions of Nicotiana glauca Grah. was compared in detail. Exudation from stem incisions occurred over a 5 min period in certain plants, enabling collection of 5–30 μl of sap. The rate of exudation showed an exponential decline. Exudate had a high dry matter content (170–196 mg ml⁻¹) and high sugar (sucrose) levels. Xylem sap had a low pH (5.8) and exudate a pH of 7.9. Glutamine dominated the amino compounds in xylem sap and exudate, and K⁺ was the major cation. Total amino compounds in stem exudate reached 10.8 mg ml⁻¹ whereas xylem sap contained much lower levels (0.28 mg ml⁻¹). All mineral elements and amino compounds with the exception of calcium were more concentrated in stem exudate than in xylem sap. Sucrose was labelled heavily in stem exudate following pulsing of an adjacent leaf with ¹⁴CO₂. A concentration gradient of sugar (2.1 bar m⁻¹) was recorded for stems. Levels of sucrose, amino compounds and K⁺ ions in stem exudate showed a diurnal periodicity. Each commodity reached maximum concentration at or near noon and minimum concentration about dawn. The evidence suggests that exudate from stem incisions of N. glauca is a representative sample of solutes translocated in the phloem.