Exploring the interplay of shoot to root hydraulic conductance in varying soil water contents

Understanding how plants regulate their hydraulic systems in response to varying soil conditions is crucial for comprehending water fluxes in the soil-plant-atmosphere continuum. This study examines hydraulic conductance in maize plants grown in loamy soil under different soil water contents. We hypothesize that plants actively adjust both aboveground and belowground hydraulic conductance in response to soil texture and moisture to balance their water loss with water uptake.

Maize plants were grown in loamy soil with varying moisture levels, simulating optimal and water-stressed conditions. Pre-germinated seeds were planted in PVC pots (10 cm diameter, 24 cm height). At different growth stages (2, 3, 4 and 6 weeks), shoots were delicately separated from the roots. We assessed both total aboveground (K_ab) and belowground hydraulic conductance (K_be). The K_be was determined by subjecting soil and roots to incremental pressure increases in a pressure pot, collecting the sap to derive water flow at a given pressure. To calculate the total aboveground hydraulic conductance (K_ab,tot), effective internal aboveground xylem hydraulic conductance (K_ab,xyl), and stomatal conductance (K_ab,sto), we measured transpiration, leaf water potential, temperature, and vapor pressure.

In optimal conditions (OC), our initial findings show a linear increase during the initial growth stage in both above- and belowground conductance, followed by deceleration at the late developmental stage. Significantly,  K_be surpassed  K_ab,tot by over two orders of magnitude. It's worth noting that although K_ab,xyl displayed a higher magnitude in our measurements, exceeding K_{be ,} the K_ab,sto took precedence as the primary controlling factor when considering the overall soil-plant hydraulics. Under water-stressed conditions, plants exhibited an overall increase in hydraulic conductance with growth, where K_be once again surpassed K_ab,tot by over two orders of magnitude. However,  K_ab,tot values were approximately half of those obtained in OC. Notably,  K_ab,xyl decreased with plant age but remained greater than  K_be. These results provide valuable insights into the intricate interplay between root and shoot hydraulic conductance. This research contributes to our understanding of how plants dynamically regulate their hydraulic systems under varying soil conditions, contributing to the broader knowledge of the soil-plant-atmosphere continuum.