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2.01.10 - Xylem physiology


Melvin Tyree, China


Jing Cai, China

About Unit

The Working Party on Xylem Physiology was formed in the mid-1970s and was active during the subsequent decade under the leadership of Patricia Denne of the UK. It is composed of scientists interested in the ecological and physiological aspects of xylem formation in woody plants.

Specific fields of interest include the effects of environmental stresses on xylogenesis, hydraulic architecture of trees, hormonal regulation of xylogenesis, control of vessel and xylem differentiation (Figure 1), regulation of early- and latewood formation, physiological aspects of anomalous wood formation and metabolite dynamics in woody tissues. The scope includes both coniferous and hardwood xylem physiology.


Figure 1: Photographs demonstrating the regulatory role of the auxin hormone (produced in young leaves) in controlling vessel width along the stem axis. In this experiment, the auxin was applied to a decapitated stem immediately after the leaves and buds above it were excised. Both cross sections were taken from the same internode (same magnification) and they show the downward increase in vessel diameter and decrease in vessel density with increasing distance from the auxin source: (A) xylem produced 5 mm, and (B) xylem produced 40 mm below the site of auxin application.

For more information on the gradual increase in vessel diameter from the leaves downwards to the roots, see: Tyree MT, Zimmermann MH (2002) Xylem Structure and the Ascent of Sap, 2nd edn. Springer-Verlag, Berlin; Aloni R, Zimmermann MH (1983) The control of vessel size and density along the plant axis - a new hypothesis. Differentiation 24: 203-208.


Melvin Tyree, a Marcus Wallenberg Prize Winner

In 2002, Mel was awarded the prestigious prize with the following citation: “Dr. Melvin Tyree is awarded the nineteenth Marcus Wallenberg Prize for his pioneering scientific discoveries leading to basic understanding of water transport in trees. His path breaking research has elucidated the hydraulic architecture of a wide range of temperate and tropical trees. Dr. Tyree's scientific work has been fundamental to the understanding of stress induced malfunction of water transport in trees. His theoretical contribution as well as the development of methods has contributed to the understanding of the importance of cavitation in ecology and physiology of trees. Tyree's work gives us perspective on the evolution of wood structure and on the distribution of trees in forests around the world.