Intraspecific trait variation and soil compaction
Differences in ‘Chardonnay’ leaf traits along the intragenotype LES
found here, which were correlated with soil compaction, were largely
attributable to variability in A mass. This trait
expressed the highest CV (43.2%), was the most strongly and negatively
correlated to soil bulk density (marginalr 2=0.403), and in turn, centrally defined
multivariate trait differences (i.e., r =0.831 along Axis 1 in our
PCA) and bivariate LES trait relationships in ‘Chardonnay’. Soil
compaction may reduce photosynthesis through both stomatal limitations
and N limitations (e.g. Morales et al., 2018), both of which therefore
likely play a role in structuring the compaction-induced intragenotype
LES in ‘Chardonnay’ observed here.
First, in our dataset, at saturating irradiance (where PPFD=2000μ mol m-2 s-1),
log-transformed stomatal conductance (g s, mol
H2O m-2 s-1)
predicts 83.6% of the variation in log-A max(simple linear regression p <0.001, n =45), and
declines significantly with bulk density (mixed model slope=-0.46±0.12
(s.e.), p <0.001, marginalr 2=0.407; data not shown). This would indicate
that stomatal limitations are at least partially driving differences in
‘Chardonnay’ leaves and plants along our intragenotype LES. Research has
shown that when water is limited, cavitation in the petioles of grape
leaves prevents embolisms from propagating to other parts of the plant,
which in turn acts as a signal for reduced g s via
stomatal closure (reviewed by Gambetta et al., 2020). So in our study
reductions in A max and g sin relation to increased bulk density, and in turn differentiation of
leaves along our intragenotype LES, are likely partially related to
reduced ability to access soil water.
Second, we also found 1) a statistically significant positive
correlation between leaf N and A max (Table S5),
and 2) a statistically significant decline in leaf N as a function of
soil bulk density (Figure 1). This would indicate that differences in
plant N availability and assimilation—i.e., conversion of inorganic
nitrate (NO3-) and ammonium
(NO4+) into amino acids and
proteins—across our site also drives intragenotype LES trait
variation. Across a soil compaction gradient, N uptake is often reduced
as plant roots are less able to forage N via root elongation (Colombi &
Keller, 2019). At our site, where fertilizers are applied uniformly,
reduced ability of roots to penetrate into areas of high soil N likely
contributes to differences in leaf N across planting rows (Table S1),
and in relation to soil bulk density (Figure 1).
These processes though are unlikely to be independent, and ultimately
our intragenotype LES in ‘Chardonnay’—particularly the strong
relationships between A mass and leaf N in
bivariate and multivariate trait space—likely owes to complex
covariation, feedbacks, and pathways among soil N availability, N and C
assimilation, and translocation of photosynthates (i.e., sucrose and
starch). In short, N uptake and assimilation is often reduced when
grapevine C status declines, since both are energy-dependent processes
that require a supply of C through the Krebs cycle (Keller, 2020). So
stomatal limitations to photosynthesis may also contribute to reduced N
assimilation and leaf N concentrations. Path analyses would help uncover
the causal pathways structuring LES trait covariation in ‘Chardonnay’
(e.g., see Shipley et al., 2006). Yet ultimately, literature suggests
the intragenotype LES in ‘Chardonnay’ found here has likely arisen as a
function of plant-, leaf-, and/or root-scale responses to micro-site
variation in water or inorganic soil N availability, both of which are
in turn influenced by soil compaction. Notably though, previous studies
on intraspecific or intragenotypic variation in crops have not uncovered
strong relationships between LES traits and soil N or moisture content,
likely due to limitations of static point sampling these environmental
variables (Isaac et al., 2017; Martin et al., 2019).