Horse chestnut (Aesculus hippocastanum L.) bud scales harbor photosynthetically active chloroplasts during their entire lifespan

Bud scales (cataphylls) protect latent shoot apices and preformed organs in the majority of deciduous trees, yet these modified structures of leaf origin and their potential other functions are rarely studied in detail. Here, we investigated seasonal and developmental variations in anatomy by bright-field microscopy; plastid ultrastructure by transmission electron microscopy; optical properties, photosynthetic pigment contents and organization by various spectroscopic methods; photosystem II (PSII) activity by Imaging PAM; and gross photosynthesis by infrared gas exchange analysis in outer brownish and inner greenish bud scales of horse chestnut (Aesculus hippocastanum L.) throughout their lifespan, i.e. from their initiation end of spring until their senescence after the bud break next year. The composition of the resin secreted on the outer bud scales was characterized by HPLC-UV-HRMS following acetonic extraction, revealing methoxylated flavonols as the dominant constituents and suggesting additional roles in photoprotection and chemical defense. Both outer and inner bud scales retained structurally intact, photosynthetically competent chloroplasts throughout all seasons, despite low chlorophyll content and limited photosynthetic capacity. Outer scales exhibited strong light-filtering, photoprotective properties, and pronounced seasonal downregulation of photosynthetic performance, besides their known roles in thermal insulation, and environmental shielding. In contrast, inner scales maintained higher chlorophyll content, more stable PSII efficiency during dormancy, and rapidly recovered photosynthetic activity during bud break, indicating a metabolically poised state supporting early spring development. Our findings underscore bud scales' multifunctional roles as protective barriers and active contributors to plant physiology by photosynthesis and the maintenance of chloroplast ultrastructure throughout their lifespan.