Plants navigate fluctuating fluid environments in both soil and hydroponic systems, yet the physical principles by which roots transduce directional hydrodynamic forcing into developmental responses remain poorly quantified. By combining a flow-configurable microfluidic platform (P-ROOT) with fluid-structure interaction modeling and multiscale phenotyping, we establish a dimensionless, direction-dependent hydraulic stress index, defined as the ratio of flow-induced stress to root stiffness, that predicts growth-permissive and stress-adaptive hydraulic loading regimes in Brassica juncea roots. Our results reveal a pronounced directional asymmetry: Under parallel flow, root elongation is sustained across a broad hydraulic loading range, whereas counterflow concentrates stress near the root apex, restricting the growth-permissive range and reducing elongation. Transitions across these hydraulic loading regimes coincide with coordinated cortical remodeling, auxin redistribution, reduced nitrogen uptake, and enhanced nitro-oxidative signatures near the root tip. These findings indicate that root developmental plasticity is governed more strongly by localized stress amplification than by the overall magnitude of hydraulic loading. By establishing hydraulic stress indices as predictive metrics linking mechanical loading to multiscale biological responses, this work provides a quantitative framework for understanding plant-fluid interactions and for designing flow-managed cultivation systems that optimize nutrient delivery.