Temporal Dynamics of Nitrogen Accumulation and Root–Shoot Nitrogen Metabolic Responses in Rice During Recovery from PEG-Induced Osmotic Stress

Recovery from PEG-induced osmotic stress involves changes in photosynthesis and nitrogen metabolism, but the associated root and shoot responses remain incompletely understood. Nipponbare rice seedlings were subjected to a PEG-free control (CK), continuous 10% (w/v) PEG-6000 treatment (CD), or PEG removal after 3 d of stress (CE). Net photosynthetic rate, tissue nitrogen-pool estimates, shoot nitrogen-metabolic enzyme activities, and paired root–shoot transcriptomes were assessed, with qRT-PCR corroboration of selected shoot genes. Under CE, the mean net photosynthetic rate approached the CK mean by R1. Descriptive shoot nitrogen estimates were higher at R4 and R6 than at D3, whereas root estimates changed relatively little. These unreplicated nitrogen estimates and the absence of R1 and R2 measurements preclude establishing a delay relative to photosynthetic recovery. More CE–CD differentially expressed genes were detected in roots than in shoots at every recovery stage, with 86 genes shared at R6. The nitrogen metabolism pathway was significantly enriched among root DEGs throughout recovery, and nitrogen transport, reduction, and assimilation transcripts showed stage-dependent treatment contrasts. Shoot enzyme activities and transcript abundances also exhibited distinct response patterns. Together, these findings characterize organ-dependent nitrogen-related responses during recovery and motivate a root–shoot temporal complementarity hypothesis that requires direct testing.