Exploring evolutionary adaptations of leaf heteroblasty in subspecific taxa of Hawaiian Chenopodium oahuense

Premise Chenopodium oahuense is a polymorphic Hawaiian endemic plant inhabiting several xeric habitats. Considerable variability in leaf morphology has made comprehensively describing its diversity difficult. Chenopodium oahuense subsp. ilioense is differentiated from C. oahuense subsp. oahuense by smaller, less-lobed, succulent leaves, smaller seeds, and prostrate to scandent habit. The lacking quantification of leaf shape, succulence, and previously unknown heteroblastic leaf transition in C. oahuense subsp. ilioense complicates the morphological boundaries separating subspecies. Methods This study used landmark analyses, elliptical Fourier descriptors (EFDs), and traditional shape descriptors measured from 1585 greenhouse-grown plant leaves collected over 18 weeks. Principal component analyses visualized correlations in leaf shape, and linear discriminant analyses predicted classifications, either subspecific or heteroblastic. Results Identity determination and heteroblastic change visualization were limited in landmark analyses. On the basis of EFDs and shape descriptors, C. oahuense subsp. ilioense was determined to be morphologically differentiated from C. oahuense subsp. oahuense with the Pu'u Ka Pele population as intermediate. The EFDs depicted heteroblastic change, predominantly in lobing. All analyses were restricted in correctly attributing a leaf to the week collected. Shape descriptors generally represented significant heteroblastic change over the growth period. Conclusions These analyses support significant differentiation between the subspecies, particularly from shape descriptors. Furthermore, we quantified the morphological intermediacy of the Pu'u Ka Pele population. Results suggest this population could be the result of incomplete lineage sorting or a recent hybridization of the two subspecies. Hawaiian Chenopodium is a polymorphic lineage notable for future research in adaptive radiations, phenotypic plasticity, and heteroblasty.

Automated summary

Chenopodium oahuense is a polymorphic Hawaiian endemic plant inhabiting several xeric habitats. This study used landmark analyses, elliptical Fourier descriptors (EFDs), and traditional shape descriptors to analyze leaf morphology of 1585 greenhouse-grown plant leaves. Results showed significant differentiation between the subspecies, particularly from shape descriptors. The Pu'u Ka Pele population was found to have intermediate characteristics. However, all analyses were limited in determining the exact collection week of each leaf.