Qualitative vs. quantitative morphology of seeds of an Italian critically endangered Hawkweed as predictor of germination capacity

Seed phenotyping through non-destructive imaging techniques may be vital for obtaining seed quality and viability information, aiding in plant species conservation and de-extinction. We used X-ray microtomography followed by a germination test on a seed sample of the rare and threatened Hieracium australe subsp. australe to compare qualitative and quantitative morphological traits as predictors of germination capacity. Five qualitative classes based on embryo structure integrity were assigned; eighteen morphometric parameters were defined via 3D image analysis examining outer seed shape and internal embryo structure; “germinated,” “ungerminated” and “dead” outcomes were recorded. The multinomial logistic model applied to the qualitative classes allowed to correctly classify 72.7% of “germinated” seeds but failed to predict “ungerminated” and “dead” seeds. Univariate analysis of morphometric parameters showed no significant differences in mean values among the three germination outcomes. However, unsuccessful germination was associated with smaller seed size, increased pericarp thickness, and higher radicle-to-seed length ratio. Multivariate discriminant analysis of eleven uncorrelated morphometric parameters highlighted "mean embryo-pericarp distance" as indicative of germination capacity specific to Hieracium, while "embryo “porosity" and “embryo surface/volume ratio” were associated with “dead” seeds obtaining an overall correct classification of 72.7% of the “germinated” seeds and 70.0% of the “dead” ones.