Photosynthetic Response of Larix gmelinii var. japonica Saplings After Exogenous Glutathione Foliar Application

Sapling survival and growth depend on photosynthetic assimilates. Therefore, improving physiological performance during early stages may enhance subsequent performance and nursery production. This study evaluated whether exogenous oxidized glutathione (GSSG), reported to enhance photosynthesis, improves the photosynthetic, physiological, and growth-related traits of Larix gmelinii var. japonica saplings. Sixteen saplings were assigned to four treatments: GSSG, 5-aminolevulinic acid, Hyponex, and a water control. Photosynthetic, nitrogen-related, and growth traits were measured before treatment and at 3, 6, 13, and 31 days after treatment, and biomass was assessed after three months. The GSSG treatment showed no difference in the net CO2 assimilation rate (Amax) compared with the control, but exhibited a significantly earlier peak at 6 days than the other treatments. This response was supported by the stability of GSSG-treated saplings against photoinhibition (Fv/Fm) and a tendency toward greater resilience to midday light stress (ΦPSII). Enhanced photosynthetic performance was associated with reduced carbon and nitrogen fluctuations and was accompanied by numerically greater root and stem biomass in the 2024 terminal shoots. Although fertilization effects were generally weak and transient, GSSG elicited notable responses, suggesting that the immediate enhancement of photosynthesis underlies its impact. However, its antioxidant properties under stressful conditions warrant further investigation.