Format: ORAL

Kamil E. Frankiewicz1,2,3,4, Carmen R. Marcati3, Anthony R. Magee1, Alexei A. Oskolski5,6

1 Compton Herbarium, South African National Biodiversity Institute, Private Bag X7, Claremont 7735, Cape Town, South Africa 2 Bolus Herbarium, Department of Biological Sciences, Faculty of Science, University of Cape Town, Private Bag X3, Rondebosch 7700, South Africa 3 Laboratório de Anatomia da Madeira, Departamento de Ciência Florestal, Solos e Ambiente, Faculdade de Ciências Agronômicas, Universidade Estadual Paulista (UNESP), Botucatu, 18603970 São Paulo, Brazil 4 Plant Cytogenetics and Molecular Biology Group, Institute of Biology, Biotechnology and Environmental Protection, Faculty of Natural Sciences, University of Silesia in Katowice, 40-032 Katowice, Poland 5 Department of Botany and Plant Biotechnology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006 Johannesburg, South Africa 6 Komarov Botanical Institute of the Russian Academy of Science, Prof. Popov Str. 2, 197376 St. Petersburg, Russia

Transitions between herbaceous and woody habits is a key to angiosperm ecological and evolutionary success. Some correlations between habits and climatic niches have been established, such as that the increase in herbaceous life forms since the Eocene was a strategy to evade seasonal frost, or that reversals to woodiness during the Miocene climate aridification enhanced resistance to drought-induced embolism. While these relationships are well-understood, intrinsic (organismal) traits governing angiosperm habit adaptability remain elusive. We take advantage of our extensive sampling to delve into the anatomical distinctions between woody and herbaceous plants. Do these differences extend beyond quantity of secondary xylem? Are there any qualitative traits universally distinguishing herbs from woody plants? What makes angiosperms so adaptable when their closest extant relatives (gymnosperms) are constrained to a narrow range of life forms? First, we observe that across diverse angiosperms, spanning magnoliids, asterids, and rosids, the rays of secondary woody may simply prolongate patterns from primary xylem, without qualitative changes. Then, we show that quantitative traits such as vessel area and vessel wall thickness develop at comparable rates in closely related herbaceous and secondarily woody species (i.e., shrubs derived from herbaceous ancestors). These underscore that woodiness may be a mere prolongation of patterns present in herbaceous ancestors. Finally, we propose a novel perspective: habit shifts are linked to changes in wood lignin content, influencing stem mechanical properties and, consequently, maximal height. We argue that this could foster morphological disparification contributing to angiosperms unparalleled diversity. In summary, our observations emphasize that angiosperm habit adaptability results from their ability to alter cambial activity and, possibly also, the amount of lignin produced and deposited in xylem. Simultaneously, while wood anatomical traits are crucial for adaptations to specific niches, they do not seem to be primarily related to life forms.