Deciphering Sugar Beet Responses to Phosphate Deficiency through Integrated Phenotyping and Molecular Analyses

Phosphorus (P) availability constrains crop production, but dose-dependent responses of sugar beet (Beta vulgaris L.) remain poorly characterized. This study descriptively examined sugar beet responses to five nominal external phosphate (Pi) concentrations (0-150 µM) by integrating root system architecture, biometric measurements, tissue hydration, leaf spectral phenotyping, and RT-qPCR analysis of twelve candidate genes selected from a previous RNA-seq screen. Under the tested hydroponic conditions, intermediate Pi supplies (5-20 µM) were associated with root architectural traits comparable to or greater than those observed under the Pi_70 reference treatment, whereas complete Pi deprivation was associated with strongly restricted root and shoot development. Candidate-gene expression showed non-monotonic and tissue-specific patterns, with several genes exhibiting stronger responses at Pi_5 than at Pi_0. These expression patterns are compatible with, but do not demonstrate, possible involvement of gsP8-SPX-PHR regulatory network. Tissue-specific expression patterns in Bv_ALMT and Bv_EXLA1 further identified these genes as candidates for future investigation of organic acid transport and cell-wall remodeling under Pi limitation. The observed descriptive patterns suggest three provisional response ranges: severe metabolic constraint below 5 µM Pi, enhanced acquisition-related development between 5 and 20 µM Pi, and increased shoot allocation at higher Pi availability. The Water Band Index and Anthocyanin Reflectance Index also emerged as candidate non-invasive indicators of Pi-stress responses before pronounced visible symptoms. Because each Pi treatment was applied to a single hydroponic container, plants within treatments represented subsamples rather than independent experimental replicates. The findings should therefore be interpreted as descriptive and hypothesis-generating. Independently replicated experiments across multiple genotypes and soil-based environments are required to confirm the proposed response ranges and evaluate the candidate molecular and spectral traits identified here.