Inter-Day Instability in Plant Sap Composition Undermines Single-Day Diagnostics

Although plant sap analysis is a practical, real-time approach to nutrient monitoring, current practice still relies on single-day diagnostics. Because management interventions and environmental drivers can rapidly modulate physiological processes that shape metabolite pools, we hypothesized that sap composition is not temporally stable but instead undergoes significant inter-day shifts under field conditions. To test this, we conducted a five-day field trial in broccoli, quantifying sap pH, EC, K+, NO3−, and °Bx daily under five extraction methods. Extraction method had a negligible effect on inter-day shifts (%Δ), whereas chemical parameters differed markedly, ranking pH (0.8%) < °Bx (3.7%) < K+ (5.9%) < EC (6.2%) < NO3− (8.2%). The largest observed changes across successive days were 0.13 pH units, 0.49 °Bx, 6.94 mmol/L K+ (271 mg/L), 1.17 dS/m EC, and 11.98 mmol/L NO3− (743 mg/L). These results show that inter-day instability can bias single-day diagnostics, motivating adoption of new sampling strategies. We urge researchers and advisors to account for inter-day variation and incorporate multi-day sampling into experimental design and decision-making until predictive frameworks are available. Addressing this dimension will improve sap diagnostic reliability and support more sustainable, science-based nutrient management.