Roles of morphology, anatomy, and aquaporins in determining contrasting hydraulic behavior of roots

Abstract

The contrasting hydraulic properties of wheat (Triticum aestivum), narrow-leafed lupin (Lupinus angustifolius), and yellow lupin (Lupinus luteus) roots were identified by integrating measurements of water flow across different structural levels of organization with anatomy and modeling. Anatomy played a major role in root hydraulics, influencing axial conductance (L(ax)) and the distribution of water uptake along the root, with a more localized role for aquaporins (AQPs). Lupin roots had greater L(ax) than wheat roots, due to greater xylem development. L(ax) and root hydraulic conductance (L(r)) were related to each other, such that both variables increased with distance from the root tip in lupin roots. L(ax) and L(r) were constant with distance from the tip in wheat roots. Despite these contrasting behaviors, the hydraulic conductivity of root cells (Lp(c)) was similar for all species and increased from the root surface toward the endodermis. Lp(c) was largely controlled by AQPs, as demonstrated by dramatic reductions in Lp(c) by the AQP blocker mercury. Modeling the root as a series of concentric, cylindrical membranes, and the inhibition of AQP activity at the root level, indicated that water flow in lupin roots occurred primarily through the apoplast, without crossing membranes and without the involvement of AQPs. In contrast, water flow across wheat roots crossed mercury-sensitive AQPs in the endodermis, which significantly influenced L(r). This study demonstrates the importance of examining root morphology and anatomy in assessing the role of AQPs in root hydraulics.

Figure 1.

The root systems of wheat and narrow-leafed lupin at 14 DAS. The root systems were stained for 1 h with 0.5% methylene blue to enhance the image contrast. The insets show freehand, transverse sections of the root (A and F) or stele (B–E and G–K). Sections A and F were stained with 0.05% toluidine blue. Sections stained with Sudan Red and viewed under white light (B and D) or phase contrast (G and J) show suberin lamellae in the walls of the endodermis (en). Fluorescence in sections stained with the berberine-aniline blue procedure and viewed under UV light (C, E, H, and K) show Casparian bands (E and K; indicated by white arrows) in the endodermis and lignified cell walls, phloem, and xylem vessels (C, E, H, and K). Bars = 50 μm.

Figure 2.

The relationship between root length and L_r (A–C) or Lp_r (D–F) of detached root segments (black symbols) and whole root systems (white symbols) of narrow-leafed lupin (A and D), yellow lupin (B and E), and wheat (C and F). Note the log scale of the root length and L_r axes. Comparison of linear regression for root segments and whole root systems resulted in the same linear fit (P > 0.05) between L_r and root length for narrow-leafed lupin (A) and yellow lupin (B).

Figure 3.

The relationship between distance from the root tip and the cross-sectional area of lignified xylem as indicated by fluorescence (A_x; A) or L_ax (B) for narrow-leafed lupin (black circles), yellow lupin (white circles), and wheat (inverted black triangles). L_ax was measured on 10-mm-long root segments after measuring L_r with the root pressure probe. The curves represent best statistical fits. The error bars in A represent se (n = 9), and each point in B represents a segment from an individual root.

Figure 4.

The correlation between the L_r of a detached root segment and its L_ax. L_ax values are shown in Figure 3B, where distance from the root tip denotes the length of the original root that L_r was measured on. The correlation is significant for narrow-leafed lupin (black circles; P < 0.0001, r² = 0.81) and yellow lupin (white circles; P = 0.002, r² = 0.56) but not for wheat (inverted black triangles; P = 0.712). Each point represents an individual root segment.

Figure 5.

Radial profiles of the mean Lp_c (A) and adjusted Lp_c (Lp_ca; B) for each cortical cell layer, 30 to 50 mm from the tips of narrow-leafed lupin (black circles, solid lines), yellow lupin (white circles, dotted lines), and wheat (inverted black triangles, dashed lines) roots. Lp_ca was calculated from Equation 1 to account for the decrease in area of each cell layer toward the root axis. The cell layers are numbered from the outside of the root (epidermis = 1) toward the root axis. Error bars represent se (n = 4–16 cells per layer, except for the epidermis of both lupin species, where Lp_c was not measured).

Figure 6.

L_membrane of each cell layer predicted by the concentric membrane model, for a 120-mm-long root of narrow-leafed lupin (black circles, solid line), yellow lupin (white circles, dotted line), and wheat (inverted triangles, dashed lines). For wheat, the predictions are shown for all of the root length involved in water uptake (inverted black triangles) or if water uptake predominantly occurs between 5 and 40 mm from the root tip (inverted white triangles).

Figure 7.

L_r predicted by the concentric membrane model in relation to root length for narrow-leafed lupin (black circles, solid line), yellow lupin (white circles, dotted line), and wheat (inverted triangles, dashed lines). For wheat, the predictions are shown for all of the root length involved in water uptake (inverted black triangles) or if water uptake predominantly occurs between 5 and 40 mm from the root tip (inverted white triangles). The regression coefficients for the linear regressions that were significantly greater than zero (P < 0.01) are given in Table IV.

Figure 8.

The effect of mercury on Lp_r and Lp_c of whole root systems (A), individual roots (B), and root cortical cells (C) of narrow-leafed lupin (NL), yellow lupin (YL), and wheat (W). Measurements were conducted before (black bars) and after (white bars) treatment of roots with 50 μm HgCl₂. Asterisks denote significant differences due to treatment at the 5% (*), 1% (**), and 0.1% (***) levels of significance. Error bars represent se (n = 4–7 for roots and n = 10–14 for cells).

Figure 9.

Diagram of the concentric membrane model used to calculate L_r and Lp_r from radial profiles of Lp_c. The diagram shows a cylindrical root segment consisting of five cell layers, from the epidermis (Ep) to the endodermis (En). Each cell layer consists of two membranes, inner and outer, the radii of which were determined from anatomical studies and measurements of cell diameters. Water crosses each cell layer in series to reach the vascular tissue in the stele. The model assumes that radial water flow occurs by the cell-to-cell flow path, except in the stele. The cell layers where cellular flow occurs and the length of the root segment can be varied to examine the effect on L_r.