Multilevel adaptation strategy of the Cupressus gigantea fine root system between the growing and non-growing seasons

Background Fine root functional differentiation is important for plant adaptation to environmental stress. Clarifying the adaptive strategies employed by the endangered conifer Cupressus gigantea is essential for understanding its survival mechanisms under seasonal drought, we compared absorptive fine roots and transport fine roots from Bayi District (BD), Lilong Township (LT), and Lang County (LC) during the growing and non-growing seasons. Twenty-seven morphological, nutritional, anatomical, physiological, and hormonal traits were measured, together with mitochondrial ultrastructure. Results Functional fine root type significantly affected 22 of the 27 traits. Transport fine roots showed greater structural investment, including higher biomass, root volume, root surface area, and periderm development, whereas absorptive fine roots had higher specific root length and stronger physiological, hormonal, and mitochondrial responses. Site specific patterns were also evident. BD had the highest growing season soil moisture and the largest seasonal decline, LT had the lowest growing season soil moisture but greater root length, volume, and surface area during the growing season, and LC showed broader morphological and anatomical trait variation. During the growing season, the dominant trait associations were mainly related to fine root morphology and anatomy, and mitochondria were predominantly elongated and interconnected. During the non-growing season, antioxidant, oxidative, osmotic, and hormonal traits became more prominent, while mitochondrial swelling and fragmentation increased, particularly in absorptive fine roots. The first two principal components explained 54.0% and 57.0% of total trait variation in the growing and non-growing seasons, respectively. The structural equation models showed a good fit and distinct regulatory networks for both seasonal datasets. Conclusions These findings show that C. gigantea emphasizes root growth and resource acquisition during the growing season, but shifts toward antioxidant defense, osmotic adjustment, hormonal regulation, and cellular maintenance during the non-growing season. Complementary specialization between absorptive and transport fine roots supports fine root function under seasonal water limitation.