Introduction
The Leaf Economics Spectrum (LES) is a framework for understanding the
causes and consequences of differences in the comparative ecophysiology,
morphology, and biochemistry of plants (I. J. Wright et al., 2004). On
one end of the LES, are species expressing “resource acquiring” trait
syndromes that include high maximum leaf-level photosynthesis (A )
and dark respiration (R ) rates, high leaf nitrogen (N)
concentrations, and low leaf mass per unit area (LMA). The other end of
the LES is defined by plants expressing the opposite suite of trait
values, which represent “resource conserving” trait syndromes (I. J.
Wright et al., 2004). Variability in traits along the LES underpin
differences in how plant species respond to environmental conditions and
change (e.g. I.J. Wright et al., 2005), and are central in driving
relationships between plant species composition and ecosystem
functioning (e.g. P.B. Reich, Rich, Lu, Wang, & Oleksyn, 2014).
Correlations and trade-offs among LES traits detected across thousands
of plant species, both within and across biomes (P.B. Reich et al.,
1999; P. B. Reich, Walters, & Ellsworth, 1997; Thomas et al., 2020),
have also informed our understanding of the evolutionary and
environmental factors that constrain leaf form and function (Donovan,
Maherali, Caruso, Huber, & de Kroon, 2011; P.B. Reich et al., 2003;
Shipley, Lechowicz, Wright, & Reich, 2006).
The original formulation and early research on the LES, focused trait
differences across plant species or communities (P. B. Reich et al.,
1997; I. J. Wright et al., 2004). However more recently, meta-analyses
have argued and shown that variation within species constitutes a
considerable proportion (e.g., ~29% of LMA and leaf N)
of total LES trait variation within plant communities (Albert et al.,
2010; Fajardo & Siefert, 2018; Siefert et al., 2015). Extending from
this work, studies have now begun focusing on evaluating how plants of
the same species differ in their LES traits, with conspecific plants
commonly differing from one another along an intraspecific LES (Hayes et
al., 2019; Martin et al., 2017; Niinemets, 2015). Moreover, in unmanaged
systems, the within-species variation that exists in certain LES traits
has also been found to be a significant correlate of ecosystem
structure, function, and responses to environmental change
(Laforest-Lapointe, Martínez-Vilalta, & Retana, 2014; Mitchell, Ames,
& Wright, 2021; Siefert & Ritchie, 2016; see also Westerband, Funk, &
Barton, 2021 and references therein).
Research on LES trait variation and relationships within species also
informs an understanding of how and why the functional ecology of crops
varies in managed agroecosystems. Specifically, studies have shown that
individuals of the same crop species or genotype express wide variation
in their LES traits, often along an intraspecific or intragenotypic Leaf
Economics Spectrum. This includes studies detecting within species or
genotype LESs that exists in several of the world’s most common crops
including soy (Hayes et al., 2019), rice (Xiong & Flexas, 2018), coffee
(Gagliardi, Martin, Virginio Filho, Rapidel, & Isaac, 2015; Martin et
al., 2017), wheat (Roucou et al., 2018), and maize (Martin et al.,
2018). Across these studies, intraspecific or intragenotypic LES trait
variation in crops was a statistical correlate of agroecosystem
functions including yield (Gagliardi et al., 2015; Hayes et al., 2019),
photosynthetic N-use efficiency (Xiong & Flexas, 2018), tissue
decomposition (Coleman, Martin, Thevathasan, Gordon, & Isaac, 2020),
N2-fixing structures (Martin et al., 2019), and soil
microbial diversity (Fulthorpe, Martin, & Isaac, 2019).
Studies on crops have also helped elucidate the factors that cause
plants to differentiate along a given intraspecific or intragenotypic
LES, which to date includes temperature and precipitation regimes
(Martin et al., 2018), soil nutrient availability (Buchanan, Isaac, Van
den Meersche, & Martin, 2019), plant ontogenetic stages (Hayes et al.,
2019) or size (Martin & Isaac, 2021), or light (Gagliardi et al.,
2015). Results differ across crops and spatial scales, though generally
studies have found plants of the same crop move towards the resource
conserving end of a within-species- or -genotype LES (i.e., plants
expressing low A , low leaf N, high LMA) under the following: 1)
hot and dry environments (Martin et al., 2017); 2) shaded conditions,
such as those in agroforestry systems (Gagliardi et al., 2015); and 3)
following reproductive onset (Hayes et al., 2019; Martin & Isaac,
2021). While these studies are instructive, there remain important
factors that may also lead to differences in crop traits along an
intraspecific or intragenotypic LES that have yet to be explored.
Soil compaction is a major characteristic of land degradation worldwide,
and a primary contributor to reductions in agricultural productivity and
sustainability (Colombi & Keller, 2019; Hamza & Anderson, 2005; Nawaz,
Bourrie, & Trolard, 2013). In some instances, increased soil compaction
results in higher rates of A , growth, and yield (Morales,
Pavlovič, Abadía, & Abadía, 2018). Though more often, growth and yield
reductions in plants under compaction occur as the cumulative
consequence of reductions in root growth, which in turn limit water and
nutrient uptake; compaction also triggers complex plant signalling
pathways, which ultimately reduce leaf-level A via stomatal and
non-stomatal factors (Colombi & Keller, 2019; Kozlowski, 1999; Lipiec
& Stępniewski, 1995; Morales et al., 2018; Sadras, O’Leary, & Roget,
2005). Existing literature therefore supports the untested hypothesis
that soil compaction drives trait covariation and/ or trade-offs along
an intraspecific or intragenotypic LES. Specifically, when soil
compaction gradients exist within a site, plants in high compaction
should express resource conserving LES traits (i.e., low A , leaf
N, and R , along with high LMA), while those in low compaction
areas should express the opposite suite of traits.
Existing work on crops has also focused only on a subset of the six
traits included in the original LES formulation. Specifically, studies
on coffee (Gagliardi et al., 2015; Martin et al., 2019), soy (Hayes et
al., 2019), and rice (Xiong & Flexas, 2018) have largely analyzed how
three LES traits—A , LMA or SLA, and leaf N—covary or
trade-off within crop species or genotypes. For instance, Martin et al.
(2017) found lower A for a given leaf N in coffee vs. wild
plants, and based on this finding hypothesized that either artificial
selection for caffeine, or luxury consumption of N-based compounds from
soil amendments, has altered LES trait relationships in that crop.
Conversely, Xiong and Flexas (2018) found that rice expressed a higherA for a given leaf N vs. wild rice plants, supporting the
hypothesis that artificial selection has resulted in higher
photosynthetic nitrogen-use efficiency in that crop. Other studies have
found that while crops such as soy, wheat, and maize occupy the extreme
resource-acquiring end of the LES (Martin et al., 2018; Milla, Osborne,
Turcotte, & Violle, 2015), domestication has not necessarily altered
the slope or strength of bivariate trait relationships among A ,
LMA, or leaf N (Hayes et al., 2019).
While these and other findings have informed our understanding of how
artificial selection influences plant trait syndromes, certain LES
traits—namely leaf R —have largely been omitted from these and
other analyses on crop trait syndromes. Leaf R is among the six
core traits forming the LES, which exists among plant species globally,
being significantly correlated (r 2=0.34-0.60)
to all other LES traits (I. J. Wright et al., 2004). The relationship
between R and other traits along the global LES, reflect evolved
physiological, biochemical, and structural trade-offs in plants: the
physiological cost of R , in terms of plant carbon (C) metabolism,
increases with greater leaf N and A and declines with increasing
LMA (P.B. Reich et al., 1998; I.J. Wright et al., 2006; I. J. Wright et
al., 2004). The incorporation of R into any LES is therefore
central, as it reflects a quantifiable physiological cost of resource
acquisition.
In crops, reducing R while maintaining plant growth and yield is
one of several goals of selection programs, with research on tomato
(Nunes-Nesi et al., 2005), canola (Hauben et al., 2009), cucumber
(Juszczuk et al., 2007), and rye grass (Wilson & Jones, 1982) showing
that reductions in plant C losses via R , due to artificial
selection were related to higher yields. Therefore, one might expect
that artificial selection may have altered the shape (i.e., the
intercept and slope) and strength of the relationship between leafR and other LES traits in crops vs. wild plants. Moreover,
changes in crop leaf R have been evaluated in responses to soil
nutrient amendments, irrigation, and growing temperatures, though
relationships between leaf R and soil compaction are less
commonly assessed (Amthor, 2012). Since, 1) croplands now cover at least
~12.2-17.1 million km2 of Earth’s
ice-free land (Ramankutty, Evan, Monfreda, & Foley, 2008), and 2)
compaction is a central feature on an estimated 68 million ha of soils
on the world’s arable lands (Colombi & Keller, 2019; Hamza & Anderson,
2005), then 3) understanding how R , and its relationship to other
LES traits in crops, is influenced by compaction is particularly
important for refining Earth System models (Atkin et al., 2015).
Here, we explored how LES traits vary in ‘Chardonnay’ (Vitis
vinifera var. ‘Chardonnay’), one of the world’s most commercially
important, widespread, and rapidly expanding winegrape varieties (Aryal
& Anderson, 2013). We evaluated LES and related traits on individual
‘Chardonnay’ vines that exist across a soil compaction gradient, to
address the following questions: 1) Is intra-genotype variation in
Chardonnay LES traits related to soil compaction? If so, then 2) does
soil compaction lead to ‘Chardonnay’ leaves and vines differentiating
from one another along an intragenotype LES? Finally, we assess 3)
whether or not the shape of a potential intra-genotype LES in Chardonnay
differs from the LES detected across plants globally?