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).