Taxonomy & Evolution

Leaf composites better than single leaves at capturing species differences

Accurate species prediction depended on adequate sampling and straightforward genetic background.

Leaf morphology within a given plant species can change with leaf age, along the length of a shoot, and between seasons due to differences in growing conditions, making its quantification challenging. Various approaches to quantifying leaf shape have been taken, including those focussing on venation, length to width ratios, and blade contours. In viticulture, the ability to discern different types of Vitis leaves took on greater importance in the late 19th and early 20th centuries as new and unfamiliar North American rootstocks were introduced to Europe. Genotyping wasn’t yet possible, but growers needed to be able to distinguish among varieties.

In a new article published in Applications in Plant Sciences, lead author Abigail E. Bryson and colleagues attempted to determine a better method of distinguishing Vitis species based on leaf morphology. The researchers digitally measured over 8000 leaves from more than 200 grapevines over the course of four years. They assembled a composite leaf shape for each species and tested whether the composite could better predict species identity versus individual leaves.

Examples of changes in leaf traits between different developmental stages in different grapevine species. Image: Bryson et al. 2020.

The measurements showed both significant differences in leaf shapes among species and developmental trends as leaves aged. Younger leaves tended to be thinner with deeper lobes, for example. Composite leaves were better able to discriminate plants of known species identity than were individual leaves.

When vines of unassigned species identity were tested using composite leaves, correct identification depended on both sampling and ancestry. Plants which had not been adequately sampled or that had complex genetic backgrounds were more difficult to predict correctly. “[M]easuring the landmarks in so many leaves requires a significant amount of time; however, landmarks are a powerful way to capture the intricate details of leaf shape and the comparisons in this work (individual vs. composite leaves) rely on the same data sets,” write the authors. “[O]ur assessment of leaf morphology is limited with individual leaves, which only allow us to observe facets of the comprehensive phenotype. Thus, composite leaves can better help to identify and define species by allowing us to capture dynamic morphological data from developmental and environmental conditions compared with individual leaves.”