<p>Celery collenchyma can be considered a convenient model for studying the development and functioning of thickened, pectin-rich primary cell walls. A considerable amount of information on collenchyma cell wall ultrastructure, polysaccharide composition, and gene expression has already been acquired, and the changes in these parameters during petiole growth have been discovered. However, so far, little attention has been paid to the possible differences in collenchyma of the first true leaf petioles and petioles that emerge much later in the celery life cycle. To investigate these potential differences, we examined the morphological and biochemical features of collenchyma in celery petioles at different positions within the rosette. It was found that, depending on their location, petioles exhibit two distinct development strategies and contain different cell wall polysaccharides in their collenchyma strands. Peripheral petioles supporting the first true leaves have smaller cross-sections, fewer collenchyma cells, and slower growth; during development, their collenchyma cell walls mainly involve xyloglucan and pectins, with a high proportion of galactose. Later emerging inner petioles have more collenchyma cells, grow faster, and show a sharp increase in arabinan and homogalacturonan levels, with reduced xyloglucan. Maturation of all petioles results in increased collenchyma cell wall thickness and involves carbohydrate redistribution: homogalacturonan increases, while rhamnogalacturonan I and xyloglucan, accessible by cell wall hydrolysis with trifluoroacetic acid, decrease, leading to tighter microfibril packing and increased cell wall strength. The established differences in development of celery collenchyma depending on petiole position within the rosette add to the fundamental understanding of plant mechanical tissue functioning and provide a basis for more appropriate selection of plant material for further study.</p> Graphical abstract <p></p>