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The genus Malus has anywhere between 25 and 33 species along with several subspecies. Malus species as well as clones within the same species have varying ploidy levels, as these are more than likely collected from different trees and (or) from different locations. In recent years, large numbers of Malus germplasm accessions have been collected and maintained at the United States National Germplasm Clonal Repository; however, genome sizes of this material have not yet been determined. In this study, leaf tissues from young grafted trees of 100 Malus species and hybrids growing in a nursery at the University of Illinois were collected and immediately used for extracting nuclei. Leaf tissues from apple and maize line W-22, used as an internal standard, were co-chopped and prepared for flow cytometric analysis. Apple nuclei were stained with propidium iodide, an intercalating dye, and a minimum of 8000 nuclei per sample were analyzed. Mean fluorescence of apple nuclei was then determined. A total of four replications per sample was used. Among 100 Malus accessions analyzed, one tetraploid, three triploid, and 96 diploid genotypes were identified. Significant differences in genome size were identified among the three ploidy types observed and also within diploid genotypes. The 2C mean value for tetraploids was 3.13 pg and ranged from 2.27 to 2.41 pg for triploids, whereas 2C values for diploids ranged between 1.44 and 1.72 pg. In addition, leaf impressions of young, fully expanded leaves were collected from young trees of 10 selected genotypes based on their ploidy and flow cytometric analysis and used to measure the nucleotypic parameter stomatal length. Ten stomata were measured per slide, three slides were analyzed per leaf, and three leaves were analyzed per accession. Overall, mean length of stomata ranged between 19.47 Rm (diploid) and 27.6 [tm (tetraploid), indicating that stomatal length in a tetraploid Malus genotype was 1.4-fold higher than that of a diploid genotype. A positive correlation between genome size and the nucleotypic parameter stomatal length was observed. Key words: apple, Malus species, nuclear DNA content, nucleotypic effects. Le genre Malus contient entre 25 et 33 especes, ainsi que plusieurs sous-especes. Les especes du genre Malus ainsi que les clones au sein d'une meme espece presentent des niveaux variables de ploidie puisque ceux-ci sont le plus vraisemblablement echantillonnes a partir de differents arbres ou localites. Dernierement, de grandes quantites de ressources genetiques pour le genre Malus ont ete collectees et maintenues au sein du United States National Germplasm Clonal Repository bien que les tailles des genomes au sein de ce materiel n' aint pas encore ete determinees. Dans ce travail, des tissus foliaires de jeunes greffons appartenant a 100 especes ou hybrides cultives dans une pepiniere a la University of Illinois ont ete preleves et utilises immediatement pour extraire des noyaux. Les tissus foliaires de pommiers et de la lignee W-22 du mais, cette derniere etant utilisee comme temoin, ont ete haches ensemble et prepares en vue d'une analyse de cytometrie en flux. Les noyaux de pommiers ont ete colores au propidium iodure, un colorant intercalaire, et un minimum de 8 000 noyaux par echantillon ont ete analyses. La fluorescence moyenne des noyaux de pommiers a ensuite ete determinee. Un total de quatre replications par echantillon a ete realise. Parmi les 100 accessions examinees, un tetraploide, trois triploides et 96 diploides ont ete identifies. Des differences significatives en ce qui a trait a la taille du genome ont ete observees entre les trois niveaux de ploidie ainsi qu'au sein des genotypes diploides. La valeur 2C moyenne chez le tetraploide etait de 3,13 pg alors que celle-ci variait entre 2,27 et 2,41 pg chez les triploides et entre 1,44 et 1,72 pg chez les diploides. De plus, des empreintes de jeunes feuilles pleinement developpees ont ete prelevees sur de jeunes arbres de 10 genotypes, choisis en fonction de leur ploidie et l'analyse de cytometrie en flux, afin de mesurer un parametre nucleotypique : la longueur des stomates. Dix stomates ont ete mesures sur chaque lame, trois lames ont ete examinees par feuille et trois feuilles ont ete analysees par accession. Globalement, la longueur moyenne des stomates variait entre 19,47 [micro]m (diploide) et 27,6 [micro]m (tetraploide) montrant que la longueur des stomates etait 1,4 fois superieure chez le tetraploide. Une correlation positive entre la taille du genome et la longueur des stomates a ete observee. Mots-cles : pommier, especes du genre Malus, contenu en ADN nucleaire, effets nucleotypiques., Introduction The genus Malus belongs to the family Rosaceae, and subfamily Maloideae, which includes several important genera and accounts for most deciduous fruit crops including apple and pears. It has [...]