Hydraulic conductance and xylem vessel diameter of young maize roots subjected to sustained water‐deficit

Abstract

Water deficit can have large impacts on plants, including likely alteration of root hydraulic conductance and root xylem vessel diameter, which can decrease crop productivity. No results, however, exist to assess possible linkages between these two variables as critical components contributing to plant water status. This linkage was investigated in three maize (Zea mays L.) cultivars. A stable water‐deficit treatment was established and maintained in pots by allowing soil drying to the point where transpiration rate was held constant at about 0.5 of well‐watered pots. Initially, the root hydraulic conductance of the water‐deficit plants was equivalent to that of well‐watered plants. Subsequently, however, hydraulic conductance decreased substantially. The results for xylem vessel diameter at 5 cm from the root tip exhibited a pattern similar to the decrease in root hydraulic conductance. A graph of root hydraulic conductance versus xylem vessel diameter at 5 cm showed a curvilinear response with lessening in the increase in hydraulic conductance with increasing xylem vessel diameter. The results indicate a possible link between root conductance and xylem diameter but the conductance is much less sensitive to vessel diameter than the fourth power of the radius predicted by Poiseuille's law. The association between conductance and xylem vessel diameter may reflect interaction of radial and axial water flux through the root system as indexed by vessel radius in the zone near the root tip.

Recovery of root hydraulic conductance and xylem vessel diameter following prolonged water deficit of maize

“…Interestingly, the changes in root hydraulic conductance during recovery from water deficit among the three cultivars aligned with differences in their timing of decreases in hydraulic conductance during pre-exposure to water deficit (Jafarikouhini & Sinclair, 2023). In the previous study, CML 538 had the most delayed expression of decrease in hydraulic conductance when a water deficit was imposed among the three cultivars.…”

“…The three cultivars were selected to represent a range of responses in soil water content at which decreases in plant transpiration with soil drying were initiated. Jafarikouhini and Sinclair (2023) previously reported changes in root hydraulic conductance and xylem vessel diameter for these three cultivars when subjected to a prolonged water-deficit treatment in which pot watering was regulated to maintain transpiration at half the rate of well-watered plants.…”

“…The precursor water-deficit treatment in the current study was the 9-day water-deficit treatment in which pot watering was done to maintain a transpiration rate at approximately half the rate of the well-watered plants (Jafarikouhini & Sinclair, 2023). The transpiration rate was determined by weighing pots twice daily and calculating the transpiration rate from successive weights.…”

“…An important aspect of changes in root hydraulic conductance values in recovery from water deficit might be associated with possible changes in xylem vessel diameter. Previously, Jafarikouhini and Sinclair (2023) found that the water-deficit treatment of maize cultivars resulted in progressive decreases in xylem vessel diameters at 5 cm from the root tip during a 9-day water-deficit treatment. Since axial water flow in the roots is hypothesized to be defined by Poiseuille's law, which describes volumetric laminar flow in tubes as a function of the fourth power of the tube radius, decreasing xylem vessel diameters could result in major decreases in root conductance.…”

“…Ten pots continued to be maintained in a well-watered condition, and 20 pots were subjected to a water-deficit treatment. As applied previously by Jafarikouhini and Sinclair (2023), the water-deficit pots were allowed to dry until the plants reached a normalized transpiration rate (NTR) of 0.5 relative to the well-watered pots. Nine days after maintaining NTR at 0.5, the water-deficit pots were watered to return them to well-watered status.…”