Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer

Abstract

Just as a soggy paper straw is prone to yielding under the applied suction of a thirsty drinker, the xylem tracheids in leaves seem prone to collapse as water potential declines, impeding their function. Here we describe the collapse, under tension, of lignified cells peripheral to the leaf vein of a broad-leaved rainforest conifer, Podocarpus grayi de Laub. Leaves of Podocarpus are characterized by an array of cylindrical tracheids aligned perpendicular to the leaf vein, apparently involved in the distribution of water radially through the mesophyll. During leaf desiccation the majority of these tracheids collapsed from circular to flat over the water potential range -1.5 to -2.8 MPa. An increase in the percentage of tracheids collapsed during imposed water stress was mirrored by declining leaf hydraulic conductivity (K(leaf)), implying a direct effect on water transport efficiency. Stomata responded to water stress by closing at -2.0 MPa when 45% of cells were collapsed and K(leaf) had declined by 25%. This was still substantially before the initial indications of cavitation-induced loss of hydraulic conductance in the leaf vein, at -3 MPa. Plants droughted until 49% of tracheids had collapsed were found to fully recover tracheid shape and leaf function 1 week after rewatering. A simple mechanical model of tracheid collapse, derived from the theoretical buckling pressure for pipes, accurately predicted the collapse dynamics observed in P. grayi, substantiating estimates of cell wall elasticity and measured leaf water potential. The possible adaptive advantages of collapsible vascular tissue are discussed.

Figure 1.

A, Distal half of a 12-mm wide P. grayi leaf showing single vein. B, Fluorescence image of a paradermal section of leaf after infusion with Texas red. Tissues from left to right are leaf vein metaxylem (MX; red), transfusion tissue (TT; red), and ATT (yellow). Those ATT tracheids not containing dye appear blue due to lignin autofluorescence. C and D, Frozen cross sections of the leaf cut along the axis marked in B, showing ATT tracheids in blue (due to autofluorescence) and chlorophyll as red. Tracheids under small tensions appeared round in cross section (C) while those in water stressed leaves (D) became highly flattened. Scale bars in B = 200 μm and in C and D = 100 μm).

Figure 2.

Distributions of ATT tracheid cross-sectional geometry using a circularity index (C) that describes perfectly round cells as 1 and approaches zero as cells become flattened. At low tension (−0.7 MPa) nearly all cells were round while at high tension (−3.2 MPa) cells were mostly flattened. At intermediate tensions (−2.3 MPa) the frequency distribution of tracheid circularity was always strongly bimodal with 75% of cells either round (C ≥ 0.8) or strongly flattened (C ≤ 0.4). The three distributions shown are from representative leaves at the pressures shown (n = 140–150 tracheids/leaf).

Figure 3.

A, Mean values (±sd; n = 140) of ATT tracheid C for 40 leaves desiccated to a range of water potentials. A polynomial curve is fitted to the data indicating the decline in C at water potentials below −1 MPa to a minimum at −2.8 MPa followed by a small increase in C below −2.8 MPa, probably indicating initial cavitation. Very large sds indicate the bimodal distribution of tracheid shape. Values from whole plants droughted to −2.3 MPa and then rewatered are shown as black circles. B, Increasing percentage of collapsed ATT tracheids (C ≤ 0.4; squares) and decreasing percentage of uncollapsed tracheids (C ≥ 0.8; circles) in the same leaves as A. Leaves from droughted trees are shown as black squares. Ellipses at the left end of the graph show the range of shapes within the uncollapsed (top) and collapsed (bottom) catagories.

Figure 4.

A, The response of leaf hydraulic conductance: K_leaf (black diamonds) and percentage loss of leaf-vein hydraulic conductance (white triangles) to leaf water potential. Mean water potential (±sd; n = 8) at stomatal closure is indicated by a vertical line (sd by dotted lines) showing that stomata closed after a 25% loss of K_leaf and significantly in advance of any cavitation in the vein xylem. B, Comparison of the response of K_leaf and percentage collapse of ATT tracheids (white squares) to water potential during leaf desiccation. Strong correspondence between hydraulic conductance and cell collapse suggest a causal relationship. Stomatal closure corresponded with a 45% collapse rate of tracheids.

Figure 5.

Relationship between the cell radius and cell wall thickness of ATT tracheids (n = 80) from five leaves. Wall thickness and cell radius varied in proportion over the range of cell sizes. A highly significant linear regression is fitted (r² = 0.61; P < 0.001).

Figure 6.

Observed versus modeled cell collapse of ATT tracheids in P. grayi. Observed collapse data are the same as shown in Figure 3B (squares) while modeled values are shown as curves. Cell wall elastic modulus is unknown for tracheids, and hence two values for the radial modulus of Picea abies green wood are used for comparison. The larger of these estimates appears to be close to the real value for the tracheids observed here.