Differential responses of grapevine rootstocks to water stress are associated with adjustments in fine root hydraulic physiology and suberization

Abstract

Water deficits are known to alter fine root structure and function, but little is known about how these responses contribute to differences in drought resistance across grapevine rootstocks. The ways in which water deficit affects root anatomical and physiological characteristics were studied in two grapevine rootstocks considered as low-medium (101-14Mgt) and highly (110R) drought resistant. Rootstocks were grown under prolonged and repeated drying cycles or frequent watering ('dry' and 'wet' treatments, respectively), and the following parameters were evaluated: root osmotic and hydrostatic hydraulic conductivity (Lp os and Lp hyd, respectively), suberization, steady-state root pressure (P rs), sap exudation rates, sap osmotic potential, and exosmotic relaxation curves. For both rootstocks, the 'dry' treatment reduced fine root Lp, elicited earlier root suberization and higher sap osmotic potential, and generated greater P rs after rewatering, but the rootstocks responded differently under these conditions. Lp os, Lp hyd, and sap exudation rates were significantly higher in 110R than in 101-14Mgt, regardless of moisture treatment. Under 'dry' conditions, 110R maintained a similar Lp os and decreased the Lp hyd by 36% compared with 'wet' conditions, while both parameters were decreased by at least 50% for 101-14Mgt under 'dry' conditions. Interestingly, build-up of P rs in 110R was 34% lower on average than in 101-14Mgt, suggesting differences in the development of suberized apoplastic barriers between the rootstocks as visualized by analysis of suberization from fluorescence microscopy. Consistent with this pattern, 110R exhibited the greatest exosmotic Lp os (i.e. Lp os of water flowing from roots to the soil) as determined from relaxation curves under wet conditions, where backflow may have limited its capacity to generate positive xylem pressure. The traits studied here can be used in combination to provide new insights needed for screening drought resistance across grapevine rootstocks.

Keywords: Drought resistance; Lp; Vitis.; fine root conductivity; root hydraulics; traits.

Fig. 1.

Example of a timeline for root pressure (P _r) measurements of a whole-root system in grapevine rootstock. The figure shows the 101-14Mgt rootstock grown under ‘dry’ (top curve) and ‘wet’ (bottom curve) conditions. Shoots were excised and a pressure transducer connected to the cut end stem 200min before any measurement was taken. Soil was flushed with sucrose solution at time 0min. P _rs, steady-state root pressure; T _1/2, half-time of water exchange between root xylem and medium.

Fig. 2.

Patterns of root suberization in the maturation zone for two grapevine rootstocks (101-14Mgt and 110R) grown under ‘dry’ and ‘wet’ conditions. Root sections were taken 2–3cm from the root tip just before any lateral root had emerged. The exodermis is indicated by white arrows. A higher degree of suberization is observed in the exodermis on both rootstocks grown under ‘dry’ conditions (A, C) compared with ‘wet’ conditions (B, D). Scale bars=100 μm.

Fig. 3.

Patterns of suberization in the maturation zone for two grapevine rootstocks (101-14Mgt and 110R) grown under ‘dry’ and ‘wet’ conditions. Root sections were taken 4–6cm from the root tip, where lateral roots had emerged. Suberization of the exodermis (arrows; A–D) is more advanced on both rootstocks grown under ‘dry’ conditions (A and C), while under ‘wet’ conditions (B and D) areas not yet suberized are easily observed (arrowheads). Suberization of the endodermis (arrows; E–H) is more developed under ‘dry’ conditions (E and G), while under ‘wet’ conditions (F and H) areas less suberized are observed (arrowheads). Scale bars=100 μm.

Fig. 4.

Exudation experiments on whole-root systems of two grapevine rootstocks (110R and 101-14Mgt) grown under ‘dry’ and ‘wet’ conditions. Sap exudate rate (A), sap osmotic potential (B), and root osmotic conductivity (Lp _os) (C). Sap exudate was driven with an osmotic gradient established between the root xylem sap and the soil that was saturated with deionized water simulating an irrigation event after a period of deficit. Values are the mean ±standard error (n=8). Means followed by different letters are significantly different at P<0.05.

Fig. 5.

Hydrostatic measurements on whole-root systems of two grapevine rootstocks (110R and 101-14Mgt) grown under ‘dry’ and ‘wet’ conditions. Values are the mean ±standard error (n=7). Means followed by different letters are significantly different at P<0.05.

Fig. 6.

Exosmotic half-time (T _1/2, black bars) of water exchange obtained from whole-root systems of two grapevine rootstocks (110R and 101-14Mgt) grown under ‘dry’ and ‘wet’ conditions (see Fig. 1). Root osmotic hydraulic conductivity (Lp _os, white bars) calculated from root fresh weights and exosmotic T _1/2 events (see the Materials and methods and Fig. 1 for further details). Values are the mean ±standard error (n≥6). Means followed by different letters are significantly different at P<0.05.

Fig. 7.

Steady-state root pressure (P _rs) of whole-root systems of two grapevine rootstocks (110R and 101-14Mgt) grown under ‘dry’ and ‘wet’ conditions. Data shown are the composite of the two root pressure experiments. Values are the mean ±standard error (n=8). Means followed by different letters are significantly different at P<0.05.

Fig. 8.

Comparison of root physiological and anatomical responses for two grapevine rootstocks (110R and 101-14Mgt) grown under ‘dry’ and ‘wet’ conditions. The figure shows root traits associated with water uptake: sap osmotic potential (Sap ψ), root hydraulic conductivity (Lp), osmotic Lp (Lp _os), exosmotic Lp (Lp _os ^ex), and suberin barriers (continuous line, present; dashed line, absent or incomplete).