Integrating Vegetative and Reproductive Critical Saturated Water Accumulation Curves for Robust Water Status Diagnosis in Winter Wheat

Context The increasing frequency of droughts and freshwater competition exacerbates the need for robust, plant-based crop water status diagnosis that functions across various phenological phases. Allometric relationships between plant saturated water accumulation (SWAP) and dry mass (PDM), and between ear saturated water accumulation (SWAE) and dry mass (EDM) exist for diagnosing winter wheat water status during the vegetative and reproductive growth phases, respectively. However, an integrated whole-season framework and clear post-anthesis diagnostic approach remain unresolved. Objective This study aimed to construct the critical SWAP curves based on accumulated growing degree day (AGDD) and PDM during the whole growth period of winter wheat under different nitrogen treatments, identify the time boundary of SWAP from accumulation to decline, and establish an integrated whole-season water status diagnostic framework for winter wheat. Methods: A four-year rainout-shelter experiment with two nitrogen and four irrigation levels was conducted to determine critical SWAP points and fit AGDD-based double logistic and PDM-based piecewise (power function and Weibull decay function) model. The water diagnostic index (WDI) was calculated as the ratio of the observed to the critical SWAP value, and its relationship with yield components was evaluated. Results SWAP accumulated first and then declined with time or PDM accumulation. Piecewise functions successfully described the SWAP accumulation and decline processes respectively under nitrogen-limited (N1) and non-nitrogen-limited (N2) conditions (N1: SWAP = 5.34PDM0.77, 23.71exp[-(PDM/12.78)10.20]; N2: SWAP = 7.37PDM0.83, 49.01exp[-(PDM/17.02)12.56]). N2 treatment had a higher SWAP vertex and steeper accumulation and decline segments. The AGDD-based critical SWAP curves showed that the time boundary of SWAP accumulation and decline was about AGDD = 1600 °C d (anthesis) regardless of nitrogen status. The decrease in plant WDI under water stress may be associated with multiple processes, including drought-related soil indirect nitrogen deficiency and premature senescence. However, during the reproductive growth period, the correlation between plant WDI and 1000-grain weight was lower than the ear WDI constructed in previous study. Conclusions With AGDD = 1600 °C d or anthesis as boundary, the plant WDI based on the critical SWAP curve and the ear WDI based on the critical SWAE curve can provide the best water status diagnosis for winter wheat during the vegetative and reproductive growth periods, respectively. Significance/Implications This integrated method can provide time continuous, size-independent water status diagnosis under different water and nitrogen conditions, which will help to provide an effective tool for precise irrigation decision-making and improve wheat productivity.