An ecophysiological and developmental perspective on variation in vessel diameter

Abstract

Variation in xylem vessel diameter is one of the most important parameters when evaluating plant water relations. This review provides a synthesis of the ecophysiological implications of variation in lumen diameter together with a summary of our current understanding of vessel development and its endogenous regulation. We analyzed inter-specific variation of the mean hydraulic vessel diameter (D ) across biomes, intra-specific variation of D under natural and controlled conditions, and intra-plant variation. We found that the D measured in young branches tends to stay below 30 µm in regions experiencing winter frost, whereas it is highly variable in the tropical rainforest. Within a plant, the widest vessels are often found in the trunk and in large roots; smaller diameters have been reported for leaves and small lateral roots. D varies in response to environmental factors and is not only a function of plant size. Despite the wealth of data on vessel diameter variation, the regulation of diameter is poorly understood. Polar auxin transport through the vascular cambium is a key regulator linking foliar and xylem development. Limited evidence suggests that auxin transport is also a determinant of vessel diameter. The role of auxin in cell expansion and in establishing longitudinal continuity during secondary growth deserve further study.

Influence of Root Diameter and Soil Depth on the Xylem Anatomy of Fine- to Medium-Sized Roots of Mature Beech Trees in the Top- and Subsoil

“…This observation may suggest that maximum vessel diameter is restricted already in medium-sized beech roots (∅ 5–10 mm), perhaps for avoiding drought- or frost-induced cavitation. Larger conduit sizes greatly increase the risk of freeze-thaw (Mayr et al, 2006; Pittermann and Sperry, 2006; Christensen-Dalsgaard and Tyree, 2014) and drought-induced embolism (Hargrave et al, 1994; Tyree et al, 1994a,b; Hajek et al, 2014), presumably because wider vessels may have thinner and more porous pit membranes compared to narrower ones (Hacke et al, 2016). Already 30 years ago, Tyree and Sperry (1989) speculated that the development of frost- or drought-induced embolism is not directly influenced by conduit size but rather indirectly by pit properties, a hypothesis recently confirmed by Li et al (2016).…”

“…Apart from such radial gradients of root hydraulic architecture, most studies recognized a successive increase in the diameter of xylem conduits from the terminal branches to the stem, and further to the roots (Aloni, 1987; Tyree and Zimmermann, 2002; Hacke et al, 2016). …”

The Parenchyma of Secondary Xylem and Its Critical Role in Tree Defense against Fungal Decay in Relation to the CODIT Model

“…Species with fewer AP cells might benefit from having smaller conduit sizes, which is the case in trees of temperate origin (Zanne et al, 2014; Morris et al, 2016; Hacke et al, 2016). Smaller conduits can maintain hydraulic integrity with greater ease, as there is less opportunity for emboli formation due to having fewer and smaller pits (Sperry and Saliendra, 1994; Wheeler et al, 2005), and therefore, a reduced chance for fungal development (Martín et al, 2009; Pouzoulet et al, 2014).…”

Hydraulic Anatomy and Function of Trees—Basics and Critical Developments

“…Auxin, a plant hormone, plays a central role in the development of xylem [17][18][19]. The interested reader is referred to Hacke et al [20] for a review on the role of auxin in primary and secondary growth, including formation of xylem. Auxin is synthesized in developing leaves before being transported downwards into the shoot and stem where it is involved in regulating cambial cell differentiation.…”

“…Besides immediate responses to water availability, trees can adjust their hydraulic system in response to long-term availability of water [20,56]. Selection processes have resulted in specialization of anatomical structures involved in water transport within species and along environmental gradients.…”