Xylem wall collapse in water-stressed pine needles

Abstract

Wall reinforcement in xylem conduits is thought to prevent wall implosion by negative pressures, but direct observations of xylem geometry during water stress are still largely lacking. In this study, we have analyzed the changes in xylem geometry during water stress in needles of four pine species (Pinus spp.). Dehydrated needles were frozen with liquid nitrogen, and xylem cross sections were observed, still frozen, with a cryo-scanning electron microscope and an epifluorescent microscope. Decrease in xylem pressure during drought provoked a progressive collapse of tracheids below a specific threshold pressure (P(collapse)) that correlates with the onset of cavitation in the stems. P(collapse) was more negative for species with smaller tracheid diameter and thicker walls, suggesting a tradeoff between xylem efficiency, xylem vulnerability to collapse, and the cost of wall stiffening. Upon severe dehydration, tracheid walls were completely collapsed, but lumens still appeared filled with sap. When dehydration proceeded further, tracheids embolized and walls relaxed. Wall collapse in dehydrated needles was rapidly reversed upon rehydration. We discuss the implications of this novel hydraulic trait on the xylem function and on the understanding of pine water relations.

Figure 1.

Representative pictures of vascular bundles in pine needles exposed to different xylem pressures. a to f, Vascular bundles in P. cembra observed with the Cryo-SEM technique on frozen samples coated with gold. g and h, Bundles in P. nigra observed with the epifluorescent technique on frozen samples. The xylem is oriented upwards and the phloem downwards. Xylem pressures were: a, 0 MPa; b, -3.5 MPa; c, -4 MPa; d, -4.6 MPa; e, -5 MPa; f, -5.1 MPa; g, 0 MPa; h, -3.5 MPa. The arrows in c indicate early collapses of tracheid walls in contact with living cells. The white bars = 100 μm except for c and d (10 μm).

Figure 2.

Tracheid Q versus Ψ_leaf in bench dehydrated shoots of four pine species. Q equals 1 for a circle and decreases with tracheid eccentricity. White symbols are for embolized (air-filled) tracheids. Errors bars are se (n = 39 on average).

Figure 3.

Change in tracheid Q during needle rehydration. Q was measured for needles of P. nigra dehydrated to -3.5 MPa and frozen intact for 2 or 30 min after rehydration. Q values for needles at -0.15 MPa are given as control values for well-hydrated needles. Treatments having a letter in common are not statistically different at P = 0.05. Error bars represent one se.

Figure 4.

Predicted loss of needle xylem conductance due to wall collapse versus Ψ_leaf for bench dehydrated and pot dehydrated (P. nigra squares) pine shoots. White symbols are for embolized tracheids. Lines represent stems vulnerability to cavitation (data from previous works; see refs. in the text).

Figure 5.

Time courses of Ψ_leaf (top), transpiration (middle), and tracheid Q (bottom) during a drought cycle for a potted P. nigra sapling. Irrigation was stopped between d 5 and 40. Black and white circles in the top panel represent Ψ_leaf measured in dark and full-light conditions, respectively. Means ± se.

Figure 6.

Dependence of transpiration (black circles), percentage of loss conductance in the needles (white circles) and the stems (dashed line) on Ψ_leaf during a drought cycle (data similar to Fig. 4). The arrow on the x axis indicates the onset of tracheid cavitation in the needles. Stem vulnerability is replotted from Froux et al. (2003).