ORIGINAL ARTICLE

Efficacy of spermine foliar application in delaying senescence of the fourth youngest leaf in wheat

The objective of this study is to elucidate the role of exogenously applied spermidine (Spm) in delaying both naturally and drought-induced senescence in the fourth youngest leaf of wheat, thereby helping it remain in a vegetative state for as long as possible. Wheat seedlings were grown until the emergence of the fourth youngest leaf (30 days after sowing, DAS) and then subjected to drought stress up to the 50^th DAS.

This leaf, which undergoes natural senescence due to aging, showed further signs of decline under drought conditions, including reductions in growth parameters, starch content, and the levels of Ca²⁺ and K⁺, along with increases in Na⁺ and soluble sugars content. Additionally, catalase isozyme expression varied significantly, while five distinct peroxidase isozymes were observed, depending on age and drought exposure. Concurrently, chloroplast ultrastructure exhibited notable alterations, such as reduced volume and a more spherical shape, diminished thylakoid membranes, increased diameter and number of plastoglobuli, disappearance of starch grains, stacking and bending of lamellae, thickening of the cell wall, and greater distance between chloroplasts and the cell wall. However, foliar application of Spm mitigated the adverse effects of senescence on growth parameters, carbohydrates, ion content, and isozyme expression of both catalase and peroxidase. It also improved chloroplast structure, as indicated by the chloroplasts’ restored elliptical shape, proximity to the cell wall, and reappearance of starch grains.

These findings suggest that Spm can counteract the negative impacts of leaf senescence in wheat, significantly delaying the onset of senescence and helping the fourth leaf maintain its vegetative status. This delay could contribute to enhanced productivity by modulating key metabolites, ions, and chloroplast structure.