Introduction
The
xylem system of vascular plants generally features a “tip-to-base”
widening with the maximal number of the narrowest conduits in the
terminal parts; the size of these terminal conduits should not vary with
plant size (or leaf size) (Lechthaler, Colangeli, Gazzabin & Anfodillo,
2019, Rosell & Olson, 2019, West, Brown & Enquist, 1999). With this
hierarchical and basipetally widening xylem architecture, the energy
cost of long-distance water transport is minimized (Anfodillo, Carraro,
Carrer, Fior & Rossi, 2006, Shinozaki, Yoda, Hozumi & Kira, 1964, Westet al. , 1999). Under the negative pressure created by stomatal
transpiration, water ascends from the soil, progressively through stem
and leaf xylem vessels, all the way up to the terminal stomata.
Covariation of stomatal and xylem traits in leaves is required to
maintain a balance in water exchange between the liquid (water delivery)
and the vapor (water loss) phase (Brodribb, McAdam & Carins Murphy,
2017, Carins Murphy, Jordan & Brodribb, 2014, Zhang, Carins Murphy,
Cardoso, Jordan & Brodribb, 2018).
There is mounting evidence that vein density is proportional to stomatal
density in leaves, and this pattern is applicable to diverse plants
within and across species (Brodribb et al. , 2017, Carins Murphy,
Jordan & Brodribb, 2016, Fiorin, Brodribb & Anfodillo, 2016). However,
the causality of this relationship is difficult to interpret for three
reasons. Firstly, leaf vein traits have been proposed to be proxies for
leaf xylem properties (Blonder, Violle, Bentley & Enquist, 2011, Sack,
Scoffoni, McKown, Frole, Rawls, Havran, Tran & Tran, 2012). Veins
consist of more than xylem (e.g. they also host phloem), so simply
considering vein density will ignore xylem vessel number and vessel
lumen diameter, which have been deemed the predictors of conductive path
length and leaf area respectively (Echeverría, Anfodillo, Soriano,
Rosell & Olson, 2019, Rosell & Olson, 2019). We are not aware of any
studies linking stomatal traits to xylem traits per se (i.e. size
covariation of stomata and xylem vessels) (but see Meinzer and Grantz
(1990) about xylem-stomatal conductance relationships) within and across
species. Secondly, stomatal and vein densities reflect leaf water
relations in terms of a leaf plane, while it is a system of conduits
within a three-dimensional system, obviously finely tuned by natural
selection in a way that directs water nearly optimally given carbon
costs, conductance, and embolism resistance (Enquist, 2002, Westet al. , 1999). Thirdly, these vein-stomatal density studies
(Brodribb et al. , 2017, Carins Murphy et al. , 2014, Sack,
Dietrich, Streeter, Sanchez-Gomez & Holbrook, 2008) use the water
balance of single leaves to implicate the whole-plant water balance.
This approach might be an oversimplification for understanding the
entire liquid phase and the vapour phase relation, even though leaf area
has been proved to predict photosynthetic productivity precisely, from
the single leaf, to the branch, to the whole-tree, to the forest level
(Li, Reich, Schmid, Shrestha, Feng, Lyu, Maitner, Xu, Li & Zou, 2020).
We address this knowledge gap with a laboratory growth experiment that
enabled us to obtain xylem and stomatal traits both at leaf and
whole-plant level. We grew seedlings of 53 diverse woody species from
cool-temperate and Mediterranean Europe in a standard growing
environment (Cornelissen, Castro-Díez & Hunt, 1996, Zhong, Castro-Diez,
Puyravaud, Sterck & Cornelissen, 2019).
We examined relations between xylem dimensions (Zhong et al. ,
2019) and stomatal dimensions of these seedlings both at whole-plant
level and at leaf level. Specifically, this study presents, for the
first time, the allometric scaling relationships at two scales: (i)
between stem xylem cross-sectional area (as well as stem xylem
conductance area) and total stomatal area at the whole-plant level, and
(ii) between leaf midvein xylem area and leaf total stomatal area at
single leaf level. We hereby introduce uniformity in the analyzed
pairwise traits as they are expressed in the same physical units, which
helps to represent more directly the selection effect on the water flux
and enlightens our understanding of the whole-plant water balance.
Furthermore, as was proposed by the West-Brown-Enquist (WBE) model, the
terminal vessels should be ‘invariant’ with plant size (or leaf size)
along with plant growth for a given individual (Roddy, Théroux-Rancourt,
Abbo, Benedetti, Brodersen, Castro, Castro, Gilbride, Jensen & Jiang,
2020, Simonin & Roddy, 2018, West et al. , 1999). We tested the
relations between terminal vessel traits (i.e. minor vessel number and
individual minor vessel area) and plant size (represented by stem xylem
area and/or total leaf area per plant) across these 53 species, which
were grown in a similar environment, in order to understand whether the
terminal vessels ‘depended’ on plant size across diverse species.
Additionally, the relations between stomatal traits (i.e. stomatal
number and individual stomatal area) and leaf size (represented by
midvein xylem area and/or average leaf area) were tested empirically, in
order to link with the large body of studies on the allocation mechanism
of leaf surface to stomata (Boer, Price, Wagner-Cremer, Dekker, Franks
& Veneklaas, 2016, Franks & Farquhar, 2007, Parlange & Waggoner,
1970). Additionally, as stomatal allocation at the leaf surface tends to
simultaneously minimize water loss (e.g. water exchange from the minor
vessels to stomata) while maximizing gas exchange to maintain a constant
photosynthetic productivity per unit leaf area (Boer et al. ,
2016), we expect that the number of stomata should scale linearly with
the number of minor vessels (Fig. 1).
Fig.1 Conceptual framework of this research concerning allometric
relations of plant hydraulics across woody species. Natural selection
acts on heritable variation between individuals within the same species.
Individuals with vessels that do not widen with height growth, or widen
little, will experience continual declines in leaf-specific conductance
with height growth and therefore declining growth and reproductive
output per unit leaf area. Individuals with vessels that widen very
markedly would have conduits of low resistance, in contrast to the
high-resistance variants with conduits that are ‘too narrow’, but they
would have their own set of disadvantages. For example, for a given leaf
area and transpirational demand, the wider conduits cost more for the
same service provided. Each unit of carbon invested in excessively wide
vessels is a unit that is not invested in further growth or
reproduction, and so these variants should be at a selective
disadvantage (Banavar, Cooke, Rinaldo & Maritan, 2014). Moreover, wider
vessels are more vulnerable to gas embolisms that obstruct conductance,
both from freezing ((Sevanto, Holbrook & Ball, 2012, Zanne, Tank,
Cornwell, Eastman, Smith, FitzJohn, McGlinn, O’Meara, Moles, Reich,
Royer, Soltis, Stevens, Westoby, Wright, Aarssen, Bertin, Calaminus,
Govaerts, Hemmings, Leishman, Oleksyn, Soltis, Swenson, Warman &
Beaulieu, 2014) and likely drought as well (Cai & Tyree, 2010,
Jacobsen, Pratt, Venturas & Hacke, 2019, Liu, Gleason, Hao, Hua, He,
Goldstein & Ye, 2019). As a result, plants with vessels that are ‘too
wide’ would also be at a selective disadvantage (Zhong et al.(2019)). The variants that should have the largest amounts of surplus
carbon to devote to growth and reproduction are those in the
intermediate zone, in which conduits widen just enough that conductance
remains constant per unit leaf area, but not so much as to incur
excessive carbon costs and embolism vulnerability.
In our previous study, based on the same woody seedling populations, we
found that, at the whole-plant level, the stem xylem cross-sectional
area (Xstem ) of stem medium (a) closely scales
with stem height (H ) and total leaf area per plant (LA ) asXstem ∝ H 1.52 andXstem ∝ LA 0.75 across
all the studied species. For individual leaves, vessel diameter
(D leaf) in the medium of leaf midvein (b) closely
scales with average leaf area (MLA ) asD leaf ∝ MLA 0.21 (Zhonget al. (2019). In this study, we test the poorly understood
xylem-stomata covariation from the size perspective We ask: are there
scaling relationships between total stomatal area and xylem
cross-sectional area across species, at the entire plant and at
individual leaf level? Specifically, we zoom in on the terminal part of
water exchange (from minor vessels to stomata), and ask: does the minor
vessel number (which scales with leaf area; see (Lechthaler et
al. , 2019, Rosell & Olson, 2019)) scale with stomatal number per leaf
and per plant across these woody seedlings? The conceptual picture
should deepen our understanding of plants’ water transport system and
have broad implications for integrating xylem architecture and stomatal
distribution.
Based on the expectations above, we test the hypothesis that, despite
large interspecific differences in leaf-habit, growth-form and relative
growth rate (RGR), similar scaling should exist between total stomatal
area and xylem area across woody seedlings, both at the entire plant and
at individual leaf level, to ensure the balance between liquid- and
vapour-phase water conductance. We also expect that the hypothesized
scaling of total stomatal area to xylem area should be driven by the
number covariation of stomatal and minor vessel elements; we also expect
a scaling relation between mean stomatal area and mean minor vessel area
as we presumed the distal element size for a given plant to be limited
under long-term nature selection.