Preventive and Concurrent Foliar Application of a Rare Fatty Acid-Based Biostimulant Alleviates Early Photosystem II Disruption Under Acute NaCl Shock in Welsh Onion (Allium fistulosum L.)

Acute salt shock can perturb photosynthetic function before severe visual injury becomes pronounced, creating a short interval in which early photochemical responses can be resolved. This study compared preventive and concurrent foliar application of LEAFENERGY® (LE), a proprietary commercial biostimulant whose active components are described by the manufacturer as naturally derived rare fatty acids produced from plant oils, in Welsh onion (Allium fistulosum L.) seedlings subjected to a single-step 200 mmol L−1 NaCl shock. Four treatments were established with 10 independent plant/vessel biological replicates per treatment: normal nutrient solution (CK), 200 mmol L−1 NaCl (SALT), 1% (v/v) LE applied 24 h before NaCl (T1), and 1% (v/v) LE applied concurrently with NaCl (T2). Fast chlorophyll fluorescence was repeatedly measured on the same plants at −1, 2, 4, 8, 23, 28, and 47 h. Repeated measurements were analyzed using generalized estimating equations (GEEs), with pre-specified contrasts adjusted by the Holm procedure. Significant treatment × time interactions were detected for Fv/Fm, PIABS, and the principal JIP-test parameters. At 4 h, PIABS declined to approximately 0.58-fold of baseline in SALT but remained at 0.95- and 0.91-fold in T1 and T2, respectively. Both LE treatments significantly moderated salt-shock-induced increases in VJ and M0 and decreases in ψ0 and φE0; T1 also significantly reduced ABS/RC and DI0/RC. No robust T1–T2 difference remained after multiplicity correction. At 47 h, neither LE treatment significantly restored leaf relative water content relative to SALT. Thus, under this single-run controlled acute-shock experiment, preventive and concurrent LE application transiently attenuated several components of the early PSII disturbance, particularly at 4 h. These findings do not demonstrate sustained salinity acclimation or long-term salt tolerance; longer-term, dose–response, timing, and production-scale studies are required before agronomic recommendations can be made.