Experimental waterlogging of grape seeds, impact on seed shape and geometrical reversing for morphometric inference

Résumé En

While the impact of charring on grape seed morphology is well documented, waterlogged pips have up to now been considered in morphometric studies to undergo very limited changes and to be directly comparable to reference collections of fresh seeds. In this study we investigate the impact of waterlogging using different chemicals (H2O2, HNO3, NaOH, KOH) on pips of various cultivars and wild grapevines. Pips are soaked individually in a given chemical solution for increasing periods of time, photographed at each stage, and their dorsal and lateral outlines analysed using Elliptic Fourier Transforms to quantify shape changes. We observe that the different products have comparable effects. The seeds start to swell and then return to their original shapes. Deformation is limited and primarily affects the beak which is slightly blunt and somewhat more pointed than on the original pips. These changes are reminiscent of the deformations observed in waterlogged archaeological seeds. Our results show that they are not strong enough to affect significantly the identification of wild and domesticated morphotypes by seed outline analyses. The effect is greater on the identifications at cultivars level but the accuracy can still be considered acceptable. We experiment the application of an average reverse vector of shape changes to compensate for some of the deformation of artificially waterlogged pips. This correction appears to have a positive effect on the accuracy of identifications for some cultivars. Our conclusion is that outline analysis can be confidently performed on archaeological waterlogged grape pips to identify wild or domesticated status and that it is more appropriate to aim at identifying groups of varieties than individual cultivars. Considering that deformations due to charring and waterlogging have differentiated effects on the identifications it is recommended to take into account both waterlogged and charred seed assemblages.