Anatomical drivers of organ size and trait coordinated association between pseudobulbs and roots in Dendrobium.

Abstract

Background: Organ size is a key phenotypic trait linking plant structure, physiological function, and ecological adaptation. The epiphytic orchid genus Dendrobium comprises highly diversified species adapted to heterogeneous canopy microhabitats, where water and nutrients are scarce and unpredictable. It has evolved two specialized organs, water-storing pseudobulbs and absorptive succulent roots that are critical for epiphytic survival. However, their central role in drought adaptation, interspecific anatomical variation and the drivers of organ size remain poorly understood. Here, we quantified 7 pseudobulb and 13 root traits across 37 Dendrobium species, using phylogenetic independent contrasts to explore the relationships among traits.

Results: All traits exhibited considerable interspecific variation, with pseudobulbs showing greater trait lability than roots, reflecting diversified water-use strategies under epiphytic pressure. Weak phylogenetic signals across traits indicate environmental selection dominates anatomical diversification. Pseudobulb radius correlated strongly with parenchyma area and vascular bundle traits, regardless of phylogenetic correction, but not with epidermis thickness after phylogenetic correction. Root radius was associated closely with cortex, velamen, and vascular bundle traits across analyses, but not with exodermis thickness after phylogenetic correction. Functionally analogous traits (pseudobulb radius vs. root radius; pseudobulb vascular bundle area vs. root vascular bundle area and root exodermis area; pseudobulb parenchyma area vs. root cortex thickness, velamen area, and velamen cell area) between pseudobulbs and roots were consistently and positively correlated, revealing coordinated water-use strategies across organs.

Conclusion: Dendrobium organ size is determined by tissue-level adaptive architecture. Pseudobulb size depends primarily on water-storage parenchyma, while root size is tightly linked to absorptive velamen. The species-level variation, driven mainly by environmental selection, underscores diversified hydraulic strategies in epiphytic orchids. Coordinated trait association between pseudobulb and root traits reflects adaptive integration of storage and absorption organs, a key evolutionary strategy for epiphytic survival. These findings highlight the fundamental role of cell and tissue dimensions in shaping orchid organ morphology.