Stem xylem traits in poplar genotypes (Populus L.): additional parameters for selection and breeding

Selection programs on Populus species are primarily aimed at developing fast-growing genotypes. This study aimed to examine the anatomical features of stem xylem and vessel traits in Populus clones, providing valuable parameters to guide future selection and breeding efforts. To achieve this objective, cross-sections of one-year-old shoots of two-year-old trees were analyzed using light microscopy. Measurements of xylem parameters were performed using automated image analysis, and theoretical hydraulic conductance (Kh), vulnerability index (Vi), and vessel grouping index were calculated. Based on our findings, we propose that stem anatomical characteristics related to xylem and vessel traits can serve as valuable tools for selecting poplar clones with enhanced productivity, water-transport efficiency, drought resistance, or cavitation resistance. A significant correlation was found between shoot anatomical properties and Kh and Vi. From an anatomical perspective, breeding efforts aimed at increased growth should prioritize developing clones with a higher xylem proportion (xylem/phloem ratio), larger-lumen vessels, a greater proportion of solitary vessels, and a lower percentage of grouped vessels. Conversely, breeding for drought-tolerant clones and those with enhanced resistance to cavitation should prioritize clones with more small-lumen vessels, a higher percentage of grouped vessels, more vessel groups, and vessel groups with more vessels. The anatomical approach described here is an efficient and accessible method for assessing the growth potential, cavitation tolerance, and drought resistance of poplar trees.