From Photosynthesis to Polyphenols: Caffeine Enhances Fruit Quality, Ripening, and Secondary Metabolism in Olive (Olea europaea L.) cv. ‘Leccino’

Abstract

Despite growing interest in caffeine as a potential plant biostimulant, its effects on perennial crops such as olive remain poorly understood. This study investigated the effects of low-dose caffeine application on leaf gas exchange and photosynthetic activity, fruit ripening, and fruit quality in olive trees (Olea europaea L. cv. ‘Leccino’). Five-year-old potted plants were irrigated for 52 days with either water (control) or water supplemented with 13C-caffeine. Caffeine-treated plants exhibited enhanced photosynthetic performance, characterized by consistently higher stomatal conductance (+44%), net photosynthetic rate (+21%), and transpiration (95%) throughout the experimental period. Fruit ripening progressed more rapidly in treated plants, as reflected by a higher pigmentation index (+8.8%) and an increase in oil accumulation (+14%), while fruit size and water content remained unchanged. Polyphenolic profiling revealed a marked enrichment in several key compounds, including oleuropein, verbascoside, hydroxytyrosol, and ligstroside, indicating a broad stimulation of the phenylpropanoid and secoiridoid pathways. Caffeine was detected exclusively in roots, confirming minimal translocation to aerial tissues. Hormonal analysis showed elevated abscisic acid levels in treated fruits (+44.2%), consistent with the observed advancement in ripening. Although these results demonstrate the potential of caffeine as a biostimulant in olive, the use of potted plants under controlled greenhouse conditions represents a limitation, and further field trials are required to confirm these effects under commercial growing conditions.