1. Introduction
Vertical transport of non-structural carbohydrates (NSC) in phloem and
water in xylem of tree stems have been the subject of countless
investigations. In contrast, radial transport processes remain
understudied, and this limits our understanding of day-to-day growth
dynamics in trees (Cuny et al. 2015; De Swaef et al.2015). The prevailing view is that trees accumulate NSC during the day
in leaves and use them to support stem growth during the night when
tissues are highly turgid (e.g., Hölttä et al. 2010; Steppeet al. 2015). This view is supported, at least partly, by
time-series data documenting maximum stem expansion during the night in
several tree species of the temperate zone of the northern hemisphere.
However, examples from other biomes have now shown that the origin of
radial movement in tree stems can be far more complex, involving
irreversible (i.e., radial growth) and reversible (i.e., radial
expansion-contraction) processes that can occur independently or
simultaneously, during the day and/or night (e.g., Pfautsch et
al. 2015a; Mencuccini et al. 2017).
At the cellular level, irreversible radial growth
(D G) results from periclinal division of mother
cells located inside (sapwood) and outside (phloem) a meristematic
region of the secondary cambium (Schrader et al. 2004). A
combination of exogenous and endogenous factors triggers the formation
of primary cell walls. Subsequently, individual layers of new cell walls
undergo biosynthetic processes that lead to densification, dehydration
and lignification, which conclude the process ofD G. Not only do sapwood mother cells
differentiate more than phloem cells – typically at a ratio of 4:1 to
10:1 in trees (Fromm 2013) – they are also permanently retained as
wood, whereas phloem cells are eventually shed. It is widely accepted
that NSC are the primary carbon source supportingD G in trees (e.g., Hartmann and Trumbore 2016;
Martínez-Vilalta et al. 2016).
In contrast to D G, reversible stem expansion
and contraction is the result of differences in hydraulic pressure and
osmotic potential between xylem and capacitive water stores in sapwood
and bark (note that bark in this context is the living section of bark
that contains both phloem and cambium). The resulting bi-directional
transport of water and osmotic substances is mostly organized through
transport in rays that provide a functional link between bark and xylem
(e.g., Sevanto et al. 2011; Pfautsch et al. 2015b;
Pfautsch 2016). Contraction of stems during daytime and their expansion
during night-time, similar to a sine-wave curve (Scenario 1 in Fig. 1),
partly results from radial transport processes. It is widely accepted
that this fluctuation in radial direction is primarily a result of
mobilising and replenishing stored water in xylem, sapwood and bark,
following shifts in vertical gradients of hydraulic pressure (e.g.,
Zweifel et al. 2001; Steppe et al. 2006; Hölttä et
al. 2009; Mencuccini et al. 2013). Contraction and expansion of
sapwood and bark is commonly observed to occur in synchrony, although
movement of bark may lag slightly behind that of sapwood as a
consequence of the indirect coupling through rays with the transpiration
stream inside the xylem (e.g., Sevanto et al. 2003, 2011).
Studies of stem diameter changes in Eucalyptus species have
revealed different, asynchronous patterns. Sapwood of E. globuluscontracted and bark expanded during the day (Scenario 2 in Fig. 1),
which indicated asynchronous patterns driven by strong hydraulic
decoupling between xylem and bark (Zweifel et al. 2014). Solomonet al. (2010) observed a temporal separation of processes related
to biosynthesis and lignification of new cell wall materials in E.
globulus , as well as temporal variation in regulation of genes that
encode aquaporins, which facilitate influx of water in expanding,
growing cells. These findings provide additional evidence thatD G and reversible expansion and contraction of
stems are complex processes that can act at different time scales. An
additional asynchronous pattern was observed in stems of E.
saligna and E. tereticornis (Scenario 3 in Fig. 1), where
sapwood expanded, and bark contracted during the day (Pfautsch et
al. 2015a; Mencuccini et al. 2017). The two asynchronous
patterns do not match widely accepted plant hydraulic modelling of a
contraction of sapwood during morning hours as the result of
increasingly negative hydraulic pressure in xylem (e.g., Steppe et
al. 2015). A mechanistic basis for asynchronous radial expansion and
contraction of stem tissues has not yet been fully established.
It is technically challenging to simultaneously document dynamic changes
in multiple sinks to determine sink strength and preferential use of
available carbon resources. Systematic isolation of potential carbon
sinks from supply of NSC by using the phloem girdling technique can
circumvent this issue. Phloem girdling has been used successfully to
investigate carbon dynamics of trees (e.g., Hogberg et al. 2001;
Binkley et al. 2006; Appel et al. 2012), and regulation of
photosynthesis by turnover of sugars and starch in leaves (Nebaueret al. 2011). Importantly, measurements of girdling effects on
stems, leaves, NSC metabolism and transpiration markedly increased our
understanding of the interrelatedness of water and carbon economies in
plants (e.g., De Schepper et al. 2011; Sellin et al. 2013;
López et al. 2015).
Assessment of girdling effects on short-term dynamics of radial growth
in stems is complex because of the simultaneous appearance of cambial
cell development and reversible expansion and contraction of stems
caused by hydraulic and osmotic effects. For example, with the onset of
transpiration during morning hours, water stored in bark and sapwood can
be drawn into xylem, where it mitigates increasingly negative water
potentials in conducting vessels. The loss of capacitive water in the
bark or sapwood will result in contraction of the affected tissues
(e.g., Zweifel et al. 2001; Pfautsch et al. 2015a).
Similarly, osmotically-induced radial fluxes of water can lead to
expansion or contraction of bark. Thus, when recording the radial
movement of stems at high temporal resolution, several signals are
recorded concurrently (Chan et al. 2016).
Here, we use phloem girdling to manipulate source-sink relationships inE. tereticornis trees and employ point dendrometers with high
temporal and dimensional resolution to document changes inD G and reversable expansion and contraction of
bark and sapwood. However, deciphering the sequence of processes that
result in stem growth cannot be achieved by observing effects of
girdling alone because of the fundamental links between the carbon and
water economies inside trees. It is necessary to also quantify the
effects of girdling on carbon assimilation and related changes in the
use of NSC, and loss of carbon (C) through respiration (R ), but
also any effects on transpiration. Girdling has been shown to increase
respiratory C loss from tree stems by 45% (Yang et al. 2019) and
markedly reduce transpiration (Oberhuber et al. 2017). We
developed a mechanistic model that can disentangle most of these
underlying processes, including radial transport of water between bark
and xylem. The present work refines previous models (Mencuccini et
al. 2013, 2017) by including a term for the lateral flux of solutes and
the associated changes in osmotic potential between bark and xylem that
drive radial viscoelastic changes in tree stems. We hypothesise that
daytime increases in osmotic potential in rays lead to sapwood expansion
which significantly counter-balances tension-driven contraction in xylem
and fibres. The observed high rates of leakage and uptake of soluble
sugars by phloem (Epron et al. 2016; Furze et al. 2018),
and the pronounced radial transport of water in the symplast of rays
(Pfautsch et al. 2015b), support this concept. Use of a wide
range of experimental data and output from the improved mechanistic
model allowed us to generate empirical evidence of “pathway effects”
(sensu Sellier & Mammeri 2019) of phloem loading, its impact onD G at different positions along the vertical
stem axis and underlying asynchronous patterns of reversible expansion
and contraction of bark and sapwood tissues.