Xylem: Differentiation, Water Transport and Ecology

Abstract

The xylem constitutes the part of plant vascular system which is primarily concerned with the long‐distance transport of water, dissolved minerals and signalling molecules from root to shoot. The development of xylem during tissue differentiation and organ growth is governed in particular by auxins and cytokinins. The mode of transport in xylem is bulk flow, driven by hydrostatic pressure gradients between root and shoot. These gradients involve substantial negative pressures (xylem tensions) during daytime transpiration. As xylem tension increases, so does the possibility of embolism and xylem dysfunction, particularly when water supply is sparse, unless embolism is repaired. The evolution of xylem reflects a combination of demands: optimising the volume flow rate, optimising control over xylem sap composition and minimising the chance of xylem dysfunction through embolism. Strategies to meet these often conflicting demands are reflected through differences in xylem anatomy between species which differ in their ecology. Key Concepts A key innovation during the evolution of plants was the development of a long‐distance, ‘vascular’, transport system. The mode of long‐distance transport of substances is bulk flow. Bulk flow is driven by gradients in hydrostatic pressure. The xylem constitutes the part of vascular system which is primarily concerned with the transport of substances, including water, between root and shoot. The xylem is made up of cells which are dead at full maturity and fulfil a transport (xylem vessels, tracheids) and mechanical support (fibres) role and cells which are alive and provide metabolic activity (xylem parenchyma). Development of xylem starts from procambial and cambial cells and is governed by auxin and also cytokinins. Daytime transpiration is associated with significant negative pressures (tensions) in xylem; this increases the chance of embolism and (partial) loss of function of xylem. The structural design of xylem of plant species, which differ in their taxonomy and ecology, often reflects differences in the transpirational demand (water flux rates), water availability (and associated likeliness of embolism formation) and soil solution composition (and need to control xylem sap composition) encountered by these species.

Particle-Based Imaging Tools Revealing Water Flows in Maize Nodal Vascular Plexus

“…Plants are living hydrodynamic systems functioning by virtue of the movement of water, which is one of the fundamental factors of their homeostasis [1]. The water uptake is performed by the part of the plant vascular system named xylem, which provides a lowresistance pathway for water from soil to atmosphere through roots, stems, and leaves [2,3]. Plants mainly transport water and dissolved minerals through xylem vessels composed of dead lignified cells connected in a complex system [4].…”

Particle-based Imaging Tools Revealing Water Flows in Maize Nodal Vascular Plexus

“…Plants are living hydrodynamic systems functioning through the movement of water, which is one of the fundamental factors of their homeostasis [1]. The plant water uptake system, a xylem, constitutes the part of the vascular tissue which is concerned with the transport of substances between root and shoot [2]. The major function of the xylem is to provide a low-resistance pathway for water from soil to the atmosphere through the plant roots, stems, and leaves [3].…”

“…The water transport system in vascular plants is very diverse and most often structurally complex. In the xylem, dead cells fulfil a transport (xylem vessels, tracheids) and mechanical support (fibres) role and cells which are alive and provide metabolic activity (xylem parenchyma) [2]. In most grasses, the xylem of the stem consists of nodes and internodes.…”

Visualization of water transport pathways in various organs on the fruit-bearing shoot of walnut trees

“…With the developments in the study of transport physiology, it has been established that the long-distance transport of water from the roots to the leaves is carried out mainly through the xylem, a process in which xylem vessels play an important role (Fricke 2017). The driving force for conducting both water and dissolved mineral ions from the roots to the leaves is primarily composed of two forces: the transpiration pull from leaves and root pressure (Tyree et al 2002).…”