Plant genomes present a continuous range of variation in nucleotide composition (G+C content). In coding regions, G+C-poor species tend to have unimodal distributions of G+C content among genes within genomes and slight 50-30 gradients along genes. In contrast, G+C-rich species display bimodal distributions of G+C content among genes and steep 50-30 decreasing gradients along genes. The causes of these peculiar patterns are still poorly understood. Within two species (Arabidopsis thaliana and rice), each representative of one side of the continuum, we studied the consequences of intron presence on coding region and intron G+C content at different scales. By properly taking intron structure into account, we showed that, in both species, intron presence is associated with step changes in nucleotide, codon, and amino acid composition. This suggests that introns have a barrier effect structuring G+C content along genes and that previous continuous characterizations of the 50-30 gradients were artifactual. In external gene regions (located upstream first or downstream last introns), species-specific factors, such as GC-biased gene conversion, are shaping G+C contentwhereas in internal gene regions (surrounded by introns), G+C content is likely constrained to remain within a range common to both species.