Ontogeny of the multiple cambial variants and secondary growth in some species of Merremia and related genera (Convolvulaceae)

ABSTRACT

The present study investigates the stem anatomy and development of cambial variants in 10 species of Merremia and related genera (Convolvulaceae) were compared histologically. Among them, Camonea kingii, C. umbellata, Distimake aegyptius, D. quinquefolius, D. vitifolius and M. hederacea increased their stem diameter by initiating successive cambia. In contrast, remaining species showed regular secondary growth. These species also demonstrated inversely oriented vascular elements, phloem wedges, intraxylary cambia, parenchyma proliferation and development of radial vascular cylinders in the large rays of xylem. Phloem wedges developed by the differential activity of regular cambium in small segments by depositing only unlignified parenchyma internally whereas nearby sectors of the cambium behaved regularly. After the development of intraxylary protophloem from procambial derivatives, additional phloem was formed from the pith cells and by initiating intraxylary cambium. Initially, intraxylary cambium was unifacial in all species, but eventually became bifacial (except M. emarginata) and deposited both xylem and phloem. In conclusion, Merremia species use all feasible methods to generate new vascular elements to compensate for the supply of water, nutrients and photosynthate. Inter-and intraxylary phloem development appears to provide an additional and safe conduit for long-distance vertical conduction, whereas radial vascular cylinders facilitate radial conduction.

Keywords:

• Cambial variant
• interxylary cambium
• ray cambium
• internal (intraxylary) cambium
• climbing species

Acknowledgements

One of the authors (KSR) is thankful for the Science and Engineering Research Board (SERB, No. CRG/2022/002299), Government of India for the financial support to study the systematic anatomy of the Convolvulaceae. SBP is thankful to the University Grant Commission (UGC) New Delhi for financial assistance through the Senior Research Fellowship [Ref. no.: 895/(CSIR-UGC NET DEC. 2018)]. The authors are thankful to Prof. Carlo Blasi and anonymous reviewers for their valuable suggestions on the earlier version of the manuscript. The authors also thank the Gujarat Biodiversity Board (GBB) and Maharashtra Biodiversity Board (MBB) for the necessary permission to collect the samples of different species of Merremia.

Author’s contribution

Rajput K.S.: Conceptualization (lead), sample collection and investigation (lead), writing original draft and figure preparations (lead), review and editing of the manuscript. Ramoliya D.G.: Collection of samples, laboratory processing and histological preparations (lead). Gondaliya A.D.: laboratory processing and histological preparation (supporting), micro-photography, figure preparations (supporting). Shimpale V.B.: Financial support to execute field trips, administrative support in Maharashtra state, seeking necessary permission for sample collection and authentication of the identity. Patil S.B.: sample collection (supporting), preparation of vouchers, identity authentication (supporting). Raole V.M.: Administrative support, editing of the final version of the MS (supporting).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

All data supporting the findings of this study are available within the paper and if any query remains, please contact the author for correspondence.