Abstract
The xylem in plants is specialized to transport water, mechanically support the plant body, and store water and carbohydrates. Balancing these functions leads to trade-offs that are linked to xylem structure. We proposed a multivariate hypothesis regarding the main xylem functions and tested it using structural equation modeling. We sampled 29 native shrub species from field sites in semiarid Southern California. We quantified xylem water transport (embolism resistance and transport efficiency), mechanical strength, storage of water (capacitance) and starch, minimum hydrostatic pressures (Pmin), and proportions of fibers, vessels, and parenchyma, which were treated as a latent variable representing "cellular trade-offs." We found that xylem functions (transport, mechanical support, water storage, and starch storage) were independent, a result driven by PminPmin was strongly and directly or indirectly associated with all xylem functions as a hub trait. More negative Pmin was associated with increased embolism resistance and tissue strength and reduced capacitance and starch storage. We found strong support for a trade-off between embolism resistance and transport efficiency. Tissue strength was not directly associated with embolism resistance or transport efficiency, and any associations were indirect involving Pmin With Pmin removed from the model, cellular trade-offs were central and related to all other traits. We conclude that xylem traits are broadly governed by functional trade-offs and that the Pmin experienced by plants in the field exerts a strong influence over these relationships. Angiosperm xylem contains different cell types that contribute to different functions and that underpin trade-offs.
Highlights
We present relationships among xylem traits as a multivariate hypothesis in a path diagram (Fig. 1)
A positive development is that several recent studies that have applied structural equation modeling (SEM) to understanding some xylem functions and traits [7,8,9]
These vascular systems perform multiple functions that affect the fitness of plants, and trade-offs are present among these functions
Summary
We present relationships among xylem traits as a multivariate hypothesis in a path diagram (Fig. 1). The division of cellular labor mitigates some direct functional trade-offs found in species with tracheid-based vascular systems [13]; trade-offs may arise based on the amount of tissue volume allocated to different cells [4, 14] We examined this trade-off as a latent variable in our SEM model where “cellular trade-off” is represented by the proportions of different cell types in cross section (Fig. 1). The second centerpiece in our hypothesis is that the hydrostatic pressure potential experienced by plants during droughts or dry periods (Pmin) exerts a mechanical strain giving rise to direct and indirect effects on all other traits [11, 15, 16]. We show that xylem traits are broadly governed by trade-offs related to transport, mechanical support, and storage, which are rooted in cellular structure, and that the level of dehydration experienced by plants in the field exerts a strong influence over these relationships. Species have evolved broad differences in embolism resistance (xylem safety) that is under strong selection by drought when negative pressures in the xylem exceed safety thresholds, leading to dieback and mortality [17,18,19]; xylem safety is strongly associated with Pmin [20, 21]
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