The Contribution of Anatomy and Physiology to Differential Root Hydraulic Responses to Water Stress in Grapevine Rootstocks That Differ in Drought Resistance.

In grapevine, young, elongating not-fully mature roots play a crucial role on water uptake, but little is known on their response to varying soil moisture conditions and how it affects rootstock performance under drought conditions. Water deficits are known to alter root anatomy, physiology, and developmental state, which affect how water moves from the soil to the xylem. We studied how changes in the development of suberization barriers, root hydraulic conductivity (Lp_r) and osmotic adjustment in young roots are associated with water deficits of two grapevine rootstocks with low-medium (101-14Mgt) and high (110R) drought resistance. Rootstocks were grown under prolonged drying cycles or frequent watering (‘dry’ and ‘wet’ treatments, respectively). Several experiments were conducted to determine sap flow, steady-state root pressure (P_rs), sap osmotic potential, and osmotic relaxation curves to determine the half time of water exchange between root xylem and soil medium (T_{rw 1/2}) and osmotic hydraulic conductivity (Lp_os). Staining of young root sections for analysis of suberization and whole plant biomass allocation were performed as well. Regardless of rootstock, the moisture treatment elicited earlier root suberization and higher root biomass, sap osmotic potential and P_rs. Interestingly, Lp_os and sap flow rates were higher in 110R than 101-14Mgt, regardless of moisture treatment. In addition, 110R ‘dry’ maintained similar Lp_os than the 110R ‘wet’, while 101-14Mgt ‘dry’ decreased >50% than 101-14Mgt ‘wet’. However, the P_rs in 110R was in average 34% lower than in 101-14Mgt. The exosmotic and endosmotic Lp_os (i.e., Lp_os of water flowing from soil to root or vice versa) responded differently among rootstocks and moisture, and appeared to affect the capacity of 110R to generate a positive xylem pressure. Our study highlights how different trait assemblies in young roots of two distinct rootstocks affect Lp_r and water uptake capacity under drought stress, and emphasizes the need to address drought resistance as a suite of interrelated traits for breeding and selection of rootstocks and field water management.