Root-Zone Engineering in Closed Soilless Horticulture: From Plant Physiology to Sensor-Guided Control

Soilless systems are increasingly important in protected horticulture because they improve water and nutrient-use efficiency, support year-round production, and reduce dependence on field conditions. However, the root zone is still commonly managed as a passive nutrient reservoir using mainly electrical conductivity and pH set points. This review presents the root zone as an actively engineered biological environment in which dissolved oxygen, root-zone temperature, nutrient-solution chemistry and hydraulics, microbiomes and biofilms, and sensor-guided control interact to determine crop performance. These domains converge on root respiration and ATP production, membrane transport, aquaporin activity, hydraulic conductance, calcium delivery, oxidative balance, and microbial or pathogen selection. Their combined effects influence fresh mass, tissue hydration, nutrient uptake, phytochemical composition, tipburn incidence, disease resilience, and overall system stability. Recent evidence indicates that active aeration, targeted root-zone heating or cooling, optimized flow scheduling, and calcium-focused interventions can improve the yield and quality of leafy vegetables, although responses vary with crop species, cultivar, developmental stage, and production-system architecture. Current evidence also indicates important uncertainties, including crop- and cultivar-specific response thresholds, architecture-dependent performance, energy and resource costs, and the still-limited predictability of microbiome manipulation. Emerging sensing, machine learning, digital-twin, and predictive-control approaches could enable a transition from threshold-based correction to physiology-informed root-zone state management. Nevertheless, wider commercial translation is constrained by inconsistent reporting of sensor location, hydraulic conditions, nutrient composition, microbial status, and resource use. We therefore propose a minimum reporting framework and research priorities for developing reproducible, energy-aware, microbiologically robust, and crop-specific root-zone management strategies for closed soilless horticulture.