A trade-off between leaf hydraulic efficiency and safety across three xerophytic species in response to increased rock fragment content

Abstract

Limited information is available on the variation of plant leaf hydraulic traits in relation to soil rock fragment content (RFC), particularly for xerophytes native to rocky mountain areas. In this study, we conducted a field experiment with four gradients of RFC (0, 25, 50 and 75% ν ν-1) on three different xerophytic species (Sophora davidii, Cotinus szechuanensis and Bauhinia brachycarpa). We measured predawn and midday leaf water potential (Ψleaf), leaf hydraulic conductance (Kleaf), Ψleaf induced 50% loss of Kleaf (P50), pressure-volume curve traits and leaf structure. A consistent response of hydraulic traits to increased RFC was observed in three species. Kleaf showed a decrease, whereas P50 and turgor loss point (Ψtlp) became increasingly negative with increasing RFC. Thus, a clear trade-off between hydraulic efficiency and safety was observed in the xerophytic species. In all three species, the reduction in Kleaf was associated with an increase in leaf mass per area. In S. davidii, alterations in Kleaf and P50 were driven by leaf vein density (VLA) and Ψtlp. In C. szechuanensis, Ψtlp and VLA drove the changes in Kleaf and P50, respectively. In B. brachycarpa, changes in P50 were driven by VLA, whereas changes in both Kleaf and P50 were simultaneously influenced by Ψtlp. Our findings suggest that adaptation to increased rockiness necessarily implies a trade-off between leaf hydraulic efficiency and safety in xerophytic species. Additionally, the trade-off between leaf hydraulic efficiency and safety among xerophytic species is likely to result from processes occurring in the xylem and the outside-xylem hydraulic pathways. These findings contribute to a better understanding of the survival strategies and mechanisms of xerophytes in rocky soils, and provide a theoretical basis for the persistence of xerophytic species in areas with stony substrates.

Keywords: drought acclimation; leaf hydraulic traits; leaf structure; rock fragments.

Figure 1

Response of leaf water potential (Ψ_leaf) to RFC in three xerophytic species. (a) Predawn Ψ_leaf, (b) midday Ψ_leaf. Mean ± SE (n = 5). Different lowercase letters indicate significant differences (LSD test, P < 0.05) among RFC treatments.

Figure 2

Maximum leaf hydraulic conductance on a leaf area (K_area) (a) and leaf dry mass (K_mass) (b) basis, as measured in three species varied with soil RFC. Mean ± SE (n = 5). Different lowercase letters indicate significant differences (LSD test, P < 0.05) among RFC treatments.

Figure 3

Changes of leaf hydraulic conductance on a leaf area basis (K_area) as a function of leaf water potential (Ψ_leaf) in S. davidii (a–d), C. szechuanensis (e–h) and B. brachycarpa (i–l), varied with soil RFC. Dotted lines, respectively, indicate the Ψ_leaf values inducing 12% (P₁₂), 50% (P₅₀) and 88% (P₈₈) of K_area decline.

Figure 4

Relationships between: leaf water potential inducing 50% loss of leaf hydraulic conductance (P₅₀) and maximum leaf hydraulic conductance on area (K_area) (a) and mass basis (K_mass) (b); turgor loss point (Ψ_tlp) and maximum K_area (c), maximum K_mass (d). Lines were fitted using a linear model and reported together with r² and P values. ^*P < 0.05, ^**P < 0.01, ^***P < 0.001.

Figure 5

Relationships between LMA and leaf water potential inducing 50% loss of leaf hydraulic conductance (P₅₀) (a), turgor loss point (Ψ_tlp) (b) and maximum leaf hydraulic conductance (K_area) (c), as well as relationships with VLA and P₅₀ (d), Ψ_tlp (e), K_area (f).

Figure 6

Spearman correlation coefficients between leaf hydraulic-related traits on S. davidii (a), C. szechuanensis (b) and B. brachycarpa (c). K_area: maximum area-based leaf hydraulic conductance; K_mass: maximum mass-based leaf hydraulic conductance; π₀ (−MPa): leaf osmotic potential at full turgor; Ψ_tlp (−MPa): turgor loss point; C_area: area-based bulk leaf capacitance before turgor loss; C_mass: mass-based bulk leaf capacitance before turgor loss; ɛ: modulus of elasticity; P₁₂: Ψ_leaf values inducing 12% loss of leaf hydraulic conductance; P₅₀: Ψ_leaf values inducing 50% loss of leaf hydraulic conductance; P₈₈: Ψ_leaf values inducing 88% loss of leaf hydraulic conductance; A_leaf: leaf area; VLA: total leaf vein density; VLA_major: major leaf vein density; VLA_minor: minor leaf vein density. ^*P < 0.05, ^**P < 0.01, ^***P < 0.001.

Figure 7

The PLS-PM describing the direct and indirect effects on leaf hydraulic efficiency (K_leaf) and safety in S. davidii (a), C. szechuanensis (b) and B. brachycarpa (c). Solid line and dotted line indicated the relationships are significant (P < 0.05) and insignificant (P > 0.05), respectively. Numbers adjacent to the arrows are standardized path coefficients. The proportion of variance explained (R²) appears in each response variable in the model.