Researchers at ELTE Eötvös Loránd University and Semmelweis University have revealed that the sticky, brownish bud scales of the horse chestnut (Aesculus hippocastanum) are far more than just passive shields. In a study published in the Journal of Plant Physiology, the Hungarian research team demonstrated that these scales, traditionally viewed as inert "winter coats" for the developing shoot, actually harbor photosynthetically active chloroplasts throughout their entire lifespan.
While it was known that bud scales are modified leaves that surround and protect the delicate preformed leaf primordia of the next year from the frost, desiccation, and hungry herbivores, their internal physiology and further functions remained a mystery. The new research followed bud scales throughout their lifespan, from their formation at the end of May until their senescence and shedding after the following year’s bud break in early May.
The results showed that these modified leaves maintain a functional photosynthetic apparatus even in the depths of winter, acting as a "metabolically poised" reservoir that supports the tree's rapid transition to growth in early spring upon bud break.
Naked or Covered: That is the Question
The dividing tissues producing next year’s leaves and shoots are often located at the end of the shoot tip and need to be protected from the harsh weather, especially winter. Trees evolved various strategies for this: some have so-called naked buds where no specific closed structures and organs protect the shoot tip, while others have bud scales, also called cataphylls, which encircle and protect these precious tissues. Some trees hide their buds beneath the tree bark.
Horse chestnut (Aesculus hippocastanum) is a popular ornamental tree across Europe with high medicinal value of its active ingredients. It has large, sticky brown buds covered by several overlapping bud scale layers.
“I first started to decipher the secrets of buds and bud scales as a PhD student in the research group led by Professor Béla Böddi, my supervisor. I explored the light filtering and spectral properties of the bud scales of 37 woody plant species” - recalls Katalin Solymosi, the study’s last author. “However, by then we only took a snapshot from winter and spring without analyzing bud scale development, cellular and plastid differentiation, pigment synthesis and photosynthetic activity during their entire life cycle.”
Figure 1. The study species, horse chestnut (Aesculus hippocastanum L.), and the bud structures examined in this research. *
A Tale of Two Layers: Shields and Reservoirs
The researchers discovered a sophisticated "division of labor" between the different layers of the bud. The outer brownish scales serve as the primary defensive line, providing among others thermal insulation and strong light-filtering properties to protect the sensitive tissues inside. In contrast, the inner, thinner and greenish scales maintain higher chlorophyll content and stable although low photosynthetic efficiency also during winter dormancy.
"Bud scales are often considered inactive and non-photosynthetic, but our findings underscore their multifunctional roles as active contributors to plant physiology" - says Enkhjin Enkhbileg, a PhD candidate at ELTE and the first author of the study. “By keeping their internal "solar panels" (chloroplasts) intact and ready, these inner scales can rapidly recover their activity during bud break, providing a vital energy boost just when the tree needs it most for new shoot development. Also, these plastids might contribute to other metabolic processes during winter.”
The Sticky Science of Chemical Defense
Beyond their solar capabilities, the horse chestnut buds are famous for their sticky resin which might also inspire the development of biomimetic pressure-sensitive adhesives. Using high-resolution liquid chromatography-mass spectrometry (HPLC-UV-HRMS), the team characterized this resin in detail for the first time, identifying 15 distinct chemical constituents. The resin is dominated by methoxylated flavonols, which act as a sophisticated chemical shield against UV radiation and pathogens.
The study found that these compounds, including rhamnocitrin and various methylated quercetins, provide an antioxidant and antimicrobial barrier that complements the physical protection of the scales. This multilayered defense system (physical, thermal, light-shielding and chemical) ensures the survival of the tree’s future leaves and flowers through the most unfavorable conditions.
Figure 2. Resin secretion, bud scale anatomy and chloroplasts of developing summer horse chestnut buds. *
What Else Can Bud Scales Teach Us?
The research used a combination of bright-field microscopy, transmission electron microscopy (TEM), analyses of the pigments and their organization, as well as chlorophyll fluorescence imaging to track these changes across all four seasons. They found that while the chloroplasts in the outer scales eventually undergo senescence after bud break, the inner scales remain active longer, supporting the early growth of the emerging shoot.
Understanding these fundamental survival strategies is not just about appreciating the secret life of ornamental trees and their leaves. The mechanisms that allow horse chestnut buds to maintain their photosynthetic machinery during dormancy could offer key insights for agricultural science. As climate change increases the frequency of extreme winter and spring weather, understanding how plants "metabolically hibernate" could contribute to breeding more resilient and climate-adapted crop species.
Figure 3. Schematic model of horse chestnut bud scale function across seasons. *
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* Figure 1. The study species, horse chestnut (Aesculus hippocastanum L.), and the bud structures examined in this research. Horse chestnut tree near the university campus (left, upper panel). Closed dormant winter buds (top center). Slightly swollen bud on a twig at the beginning of bud break (top right). Breaking bud showing its dimensions next to a ruler (center right). Autumn bud scale layers dissected and arranged in order next to a ruler, illustrating their number, size, and organization (bottom).
Figure 2. Resin secretion, bud scale anatomy and chloroplasts of developing summer horse chestnut buds. Young bud fully covered with sticky protective resin (left). Light microscopic image of the trans section of a bud scale showing resin-producing multicellular secretory structures called colleters (black arrowhead) responsible for secreting the resin that coats and protects the developing bud (center), chloroplast present in the central region of the bud scale (right) with typical photosynthetic membrane structure, a granum (white asterisk). Scale bars: 5 mm (left), 100 µm (center) and 1 µm (right).
Figure 3. Schematic model of horse chestnut bud scale function across seasons. Outer scales primarily act as protective, light-filtering, thermal, chemical, and mechanical barriers, whereas inner scales retain greater photosynthetic competence and more stable chloroplast function. Seasonal shifts in pigment content, chloroplast structure, and photochemical performance indicate complementary roles of the two scale types in bud protection and recovery during bud break. This annual cycle begins in summer, progresses through autumn and winter, and leads into spring bud break, after which bud scales are shedding, and the cycle restarts at the end of May with the formation of the new buds of the next season.
Data of the published paper: Enkhjin Enkhbileg, Anna Skribanek, Imre Boldizsár, Katalin Solymosi (2026) Horse chestnut (Aesculus hippocastanum L.) bud scales harbor photosynthetically active chloroplasts during their entire lifespan. Journal of Plant Physiology 154819.
This work was financed by the OTKA grant K-135712 of the National Research, Development and Innovation Office (to I.B.). Authors are grateful for the support of the Bolyai János Research Scholarship of the Hungarian Academy of Sciences (to K.S.), the Stipendium Hungaricum Ph.D. Scholarship of Tempus Public Foundation and the EKÖP-24-3-II university doctoral research scholarship program of the Ministry for Culture and Innovation from the source of the National Research, Development and Innovation Fund (to E.E.).