Exine morphometric analysis as a new tool for Citrus species identification: a case study from Oplontis (Vesuvius area, Italy)

In the Mediterranean area, the occurrence of citrus plants in ancient times is clearly attested in Sardinia and along the southern Tyrrhenian coasts through the presence of pollen grains and organic residues dating back to the sixth century bc. The presence of Citrus pollen was also detected in soil sediments from Pompeii (ad 79), in marine sediments from the ancient harbour of Naples (first century ad), in Rome (27 bc–ad 14) as well as in a garden soil at the Villa di Poppea in Oplontis (first century bc). The first citrus tree to be identified was citron (Citrus medica L.), but even if the presence of other citrus species was supposed, identification was limited by the difficulty of discriminating the exine characteristics through optical microscopy. Exine micromorphology of eight citrus species, including primitive to cultivated, was studied by SEM and optical microscope observations. By means of the image analysis method, the morphometry of lumina and brochi was evaluated. Results indicate that the size of lumina and brochi increases from primitive, wild to cultivated citrus up to lemon. In addition, there is an overall correspondence between brochi dimension and hybridization history. The morphometric analysis on modern pollen material and comparison with the fossil grains found at Oplontis allowed attesting for the first time the presence of C. limon and C. reticulata in the Roman Age. The statistical methods used for the analysis of exine ornamentations proved to be a useful tool for interpreting the complex citrus hybridization history and for shedding light on the scenario of citrus fruits introduction in the Roman period. Keywords Pollen · Image analysis · Roman