Multi-trait selection defines a plant-architecture ideotype for soybean in the Russian Far East

Soybean production in the Russian Far East is constrained by a short frost-free growing period, making early-maturing, productive cultivars a major regional breeding objective. Plant architecture influences the number of pod-bearing sites and the efficiency of mechanical harvesting, yet no architectural ideotype has been defined for this region and the genotypes closest to such a target are unknown. Architectural traits are correlated with one another and with maturity, and must therefore be selected simultaneously. In this study, 63 soybean genotypes were evaluated over three years (2018-2020) at a single site in Khabarovsk Krai, Russia (48°N). Plant height, first pod height, number of branches, productive nodes on the main stem and on lateral branches, and days to maturity were recorded. All six traits showed significant genotypic variation, moderate to high broad-sense heritability (0.65-0.91), and a significant genotype by year interaction. The two branching traits combined high heritability with the highest genetic advance (67.3% and 78.2% of the mean). Correlation and principal component analyses revealed two independent dimensions, one combining plant size with later maturity and one representing branching. An ideotype was defined according to regional priorities, combining earlier maturity with a taller, more productive plant structure suited to mechanical harvesting. The multi-trait genotype-ideotype distance index (MGIDI) selected the nine genotypes closest to this target, with selection differentials in the desired direction for five of the six traits. The number of productive nodes on the main stem was the only trait to move against its desired direction, because it is genetically associated with later maturity. The multi-trait stability index (MTSI) selected nine genotypes on mean performance and year-to-year consistency, with selection differentials in the desired direction for all six traits. Gaterslebener stamm, VNIIS 1, ISZ 16, and No. 700 were selected by both indices, with Gaterslebener stamm ranking first under both. The stability estimated here is temporal, reflecting year-to-year consistency rather than adaptation across locations. These four genotypes are recommended for multi-location evaluation and as parents for breeding early-maturing cultivars suited to mechanical harvesting.