Limited hydraulic adjustments drive the acclimation response of Pteridium aquilinum to variable light

Abstract

Background and aims: The success of invasive plants can be attributed to many traits including the ability to adapt to variable environmental conditions. Whether by adaptation, acclimation or phenotypic plasticity, these plants often increase their resource-use efficiency and, consequently, their fitness. The goal of this study was to examine the hydraulic and eco-physiological attributes of sun and shade populations of Pteridium aquilinum, a weedy fern, to determine whether the presence of vessels and other hydraulic attributes affects its success under a variety of light conditions.

Methods: Hydraulic traits such as cavitation resistance, hydraulic conductivity, photosynthesis and water potential at turgor loss point were measured on fronds from sun and shade populations. Anatomical and structural traits such as conduit diameter and length, stomatal density and vein density were also recorded. Diurnal measures of leaf water potential and stomatal conductance complement these data.

Key results: Gas exchange was nearly double in the sun plants, as was water-use efficiency, leaf-specific conductivity, and stomatal and vein density. This was largely achieved by a decrease in leaf area, coupled with higher xylem content. There was no significant difference in petiole cavitation resistance between the sun and shade leaves, nor in xylem-specific conductivity. Hydraulic conduit diameters were nearly equivalent in the two leaf types.

Conclusions: Shifts in leaf area and xylem content allow P. aquilinum to occupy habitats with full sun, and to adjust its physiology accordingly. High rates of photosynthesis explain in part the success of this fern in disturbed habitats, although no change was observed in intrinsic xylem qualities such as cavitation resistance or conduit length. This suggests that P. aquilinum is constrained by its fundamental body plan, in contrast to seed plants, which show greater capacity for hydraulic adjustment.

Keywords: Plasticity; embolism; modulus of elasticity; photosynthesis; stomatal conductance; stomatal density; turgor loss point; xylem.

Fig. 1.

Examples of sun and shade populations of P. aquilinum on the University of California Santa Cruz campus. Sun populations (top panels) are vertically oriented, and have thicker leaves with more pronounced venation than the shade-grown fronds (bottom panels). Sun fronds have a greater petiolar fraction of vascular tissue (top right) than shade plants (bottom left).

Fig. 2.

Diurnal measures of (A) photosynthetically active radiation (PAR), (B) vapour pressure deficit (VPD), (C) stomatal conductance (g_S) and (D) leaf water potential (ψ) in sun and shade populations of P. aquilinum (open and closed circles, respectively).

Fig. 3.

Hydraulic and anatomical trait comparisons in sun and shade fronds of P. aquilinum (white and grey boxplots, respectively): (A, B) cross-petiolar xylem and phloem fraction, (C, D) xylem-specific (K_S) and leaf-specific conductivity (K_LEAF), (E, F) mean conduit diameter (D) and hydraulic conduit diameter (D_H), (G) mean conduit length (L_C), (H) the ratio of leaf area to xylem area (A_L: A_X), and (I, J) stomatal density (SD) and vein density (VD).

Fig. 4.

Percentage loss of petiole conductivity in response to xylem pressure in sun and shade fronds of P. aquilinum.

Fig. 5.

(A) Water potential at turgor loss point (Ψ _TLP) and (B) maximum osmotic potential (Ψ _ΠMAX) in sun and shade fronds of P. aquilinum.

Fig. 6.

Gas-exchange response of sun and shade fronds of P. aquilinum (open and shaded boxplots, respectively). (A) The response of assimilation (A_n) to increasing leaf CO₂ levels (C_i). (B, C) The intrinsic leaf water-use efficiency (WUE) and maximum carboxylation rate (V_cmax).