Water flux patterns and aquaporin dynamics: linking stomatal demand to root hydraulics in pearl millet hybrids

Water-saving traits are critical for the adaptation of pearl millet (Pennisetum glaucum) to arid environments. This study investigates how breeding history and specifically selection in High Rainfall (HR) versus Low Rainfall (LR) zones influences root hydraulics and the molecular regulation of aquaporins (AQP). The study utilized four contrasting hybrids to integrate physiological assays with anatomical and molecular phenotyping. Key methodologies included measuring transpiration responses to vapor pressure deficit (VPD), root pressurization, and pharmacological inhibition. These were coupled with anatomical characterization of root systems, aerial growth and transcriptional profiling of PIP and TIP (Plasma membrane/Tonoplast Intrinsic Protein) genes. LR hybrids exhibited an "opportunistic" strategy characterized by superior root and canopy growth supported by extensive metaxylem proliferation. However, these genotypes showed lower constitutive root hydraulic conductivity and a steeper transpiration increase during pressurization, suggesting a radial transport bottleneck. At the molecular level, HR hybrids significantly downregulated PIP2;3 in roots under high VPD. In contrast, LR hybrids maintained PIP2;3 expression, facilitating radial flow to match their high transpiration demand. The findings demonstrate that breeding for specific rainfall zones has shaped divergent hydraulic strategies. HR genotypes utilize a conservative "hydraulic brake" mechanism driven by AQP downregulation to conserve water. Conversely, LR genotypes employ an acquisitive strategy, relying on coordinated anatomical capacity and molecular gating to meet high transpiration demands. These distinct mechanisms provide a blueprint for selecting resilient crops based on specific environmental pressures.