Parenchyma underlies the interspecific variation of xylem hydraulics and carbon storage across 15 woody species on a subtropical island in Japan.

Parenchyma is an important component of the secondary xylem. It has multiple functions and its fraction is known to vary substantially across angiosperm species. However, the physiological significance of this variation is not yet fully understood. Here, we examined how different types of parenchyma (ray and axial parenchyma, and axial parenchyma in direct contact with vessels [APV]) are coordinated with three essential xylem functions: water conduction, storage of non-structural carbohydrate (NSC), and mechanical support. Using branch sapwood of 15 co-occurring drought-adapted woody species from the subtropical Bonin Islands, Japan, we quantified ten xylem anatomical traits and examined their linkages to hydraulic properties, storage of soluble sugars and starch, and sapwood density. The fractions of APV and axial parenchyma in the xylem transverse sections were positively correlated with the percentage loss of conductivity in the native condition, whereas that of ray parenchyma was negatively correlated with the maximum conductivity across species. Axial and ray parenchyma fractions were positively associated with concentrations of starch and NSC. The fraction of parenchyma was independent of sapwood density, regardless of parenchyma type. We also identified a negative relationship between hydraulic conductivity and NSC storage and sapwood density, mirroring the negative relationship between the fractions of parenchyma and vessels. These results suggest that parenchyma fraction underlies species variation in xylem hydraulic and carbon use strategies, wherein xylem with a high fraction of axial parenchyma may adopt an embolism repair strategy through an increased starch storage with low cavitation resistance.