The Structure of Phenotypic Variation in Seven Segregating F2 Soybean Populations in an Extremely Hot and Dry Season in Southern Russia

Variation in segregating soybean populations has rarely been decomposed into between- and within-population components, and not yet for the southern Russian ecotype. We phenotyped 1062 individually harvested plants from seven F2 populations for thirteen quantitative traits in Krasnodar Krai, 2024. The season was extreme on the following four measures: 4415 °C as the sum of active temperatures, 93 days above 30 °C, 30 days above 35 °C (of which 26 were during heat waves lasting 19 and 7 days each), and 215 mm of rainfall. All four lie outside the six preceding years’ ranges. On average, 76.4% of variation resided within populations, ranging from 52.3% for plant height to 93.6% for productive nodes. The negative protein–oil correlation held in all seven populations (ρ −0.36 to −0.73) but differed in strength. Protein was unrelated to seed mass per plant, and the F2 median exceeded the parental values measured in another nursery in one of six crosses. The growing regime was completely confounded with the parental pairs, so the two groups of crosses are not comparable as treatments, making pseudoreplication measurable: twelve traits separate the groups when the plant is the unit, and only one when the cross is the unit. Pooled-threshold selection repeated the error, assigning 42 out of 78 candidates to one population. Within-population selection retained 20 plants from all seven crosses.