Unraveling the effects of plant hydraulics on stomatal closure during water stress in walnut

Abstract

The objectives of the study were to identify the relevant hydraulic parameters associated with stomatal regulation during water stress and to test the hypothesis of a stomatal control of xylem embolism in walnut (Juglans regia x nigra) trees. The hydraulic characteristics of the sap pathway were experimentally altered with different methods to alter plant transpiration (Eplant) and stomatal conductance (gs). Potted trees were exposed to a soil water depletion to alter soil water potential (Psisoil), soil resistance (Rsoil), and root hydraulic resistances (Rroot). Soil temperature was changed to alter Rroot alone. Embolism was created in the trunk to increase shoot resistance (Rshoot). Stomata closed in response to these stresses with the effect of maintaining the water pressure in the leaf rachis xylem (P(rachis)) above -1.4 MPa and the leaf water potential (Psileaf) above -1.6 MPa. The same dependence of Eplant and gs on P(rachis) or Psileaf was always observed. This suggested that stomata were not responding to changes in Psisoil, Rsoil, Rroot, or Rshoot per se but rather to their impact on P(rachis) and/or Psileaf. Leaf rachis was the most vulnerable organ, with a threshold P(rachis) for embolism induction of -1.4 MPa. The minimum Psileaf values corresponded to leaf turgor loss point. This suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P(rachis) might be the physiological parameter regulated by stomatal closure during water stress, which would have the effect of preventing extensive developments of cavitation during water stress.

Figure 1

VCs for walnut leaf midribs and rachis, and current year shoots and roots. Close symbols refer to measures on air-pressurized segments. White symbols refer to leaf midribs and rachis collected on trees during the different experiments. Error bars are ± se. Lines are logistic fits through the data. For shoots, the lines are logistic fits through the data published by Tyree et al. (1993) for current year (plain line) and previous year (dashed line) segments.

Figure 2

Typical changes of plant transpiration and xylem water pressure during the first stage of a soil water stress for one of the studied tree. The four different symbols correspond to 4 consecutive d. Four different light intensities were used on each day to vary E_plant. Lines are linear regressions through the data for each day. The slope of the lines represents the R_plant.

Figure 3

Typical time course of plant transpiration and xylem water pressure for a non irrigated tree exposed to a constant light intensity.

Figure 4

Typical time course of xylem water pressure and soil temperature (top panel), and plant transpiration and g_s (bottom panel) during a soil chilling experiment. Error bars are ± se (n = 5).

Figure 5

Changes in g_s (top panel) and xylem water pressure (bottom panel) of trees exposed to increasing pneumatic pressures around their trunk (x axis). The curve represents the change in embolism in the trunk versus the applied pressure.

Figure 6

Dependence of g_s (top panels) and transpiration (E_plant) on xylem water pressure (left panels) and Ψ_leaf. The different symbols represent the different experiments conducted in this study. Transpiration was normalized by the transpiration of each tree before treatment (E_plant/E_max).

Figure 7

Dependence of plant transpiration and xylem rachis embolism on rachis xylem pressure (top panel) and dependence of plant transpiration and leaf turgor pressure on Ψ_leaf (bottom panel). Error bars are ± se. The VC was replotted from Figure 1, and data for transpiration rate were averaged from Figure 7.

Figure 8

Plant transpiration versus whole plant hydraulic conductance. Different symbols represent different experiments. The plain, dashed, and dotted lines represent the critical transpiration rate provoking 100%, 10%, and 1% loss conductance in the leaf rachis, respectively.