Integrated physiological, transcriptomic and metabolomic analyses characterize treatment-associated autumn shoot self-pruning and maturation in Wogan mandarin

Autumn shoot maturation is important for citrus canopy management and subsequent reproductive development. We compared five foliar treatments but interpret their effects as treatment-specific because the salts were not equimolar and KH₂PO₄ supplied both K and P. Water (CK), 0.3% (w/v) KNO₃, 0.3% KH₂PO₄, 0.3% K₂SO₄ and a seaweed-polysaccharide product were applied twice to 15 three-year-old Wogan mandarin trees (three trees per treatment). Under an exploratory equal-weight framework, KH₂PO₄ had the highest integrated score (0.858) and remained first in all 11 leave-one-indicator-out analyses. Longitudinal mixed models detected treatment-by-linear-time interactions for shoot length, shoot diameter, internode length, leaf number, leaf width and leaf thickness, but not for self-pruning rate or SPAD. RNA-seq and untargeted LC-MS profiling of CK and KH₂PO₄ leaves and stems showed tissue- and stage-dependent responses. The broad VIP > 1 and P < 0.05 metabolite set was largest at 3 d (554 leaf and 357 stem features), whereas only 31 leaf and one stem feature at 3 d met VIP > 1 and BH q < 0.05; the remaining six comparisons had none. Co-expression, putative PPI and pathway analyses prioritized lignin-, hormone- and redox-related candidates. qRT-PCR of nine genes detected 19 FDR-significant tissue–time responses and supported several key RNA-seq expression patterns. Foliar KH₂PO₄ was associated with high leaf K, low Na, carbon–osmotic adjustment, redox stabilization and lignin-related transcription. These data identify treatment-associated candidate pathways and genes rather than a K-specific, P-specific or causal mechanism.