Acid Rain Tolerance in Soybeans: Evaluation of Genetic Variability and Identification of Novel Germplasms Using Multiple Criteria

Acid rain (AR) adversely affects soybean growth and seed yield. However, genetic variation of soybean response to AR stress has not been well evaluated. Here, 441 cultivated soybean genotypes were subjected to simulated rainfall with either pH 4.2 (AR) or pH 5.6 (control) from the three-leaf stage to maturity in 2009 and 2010. After maturity, 14 traits potentially related to AR tolerance were determined. Analyses of variance showed that genetic variation in AR tolerance was statistically highly significant for all surveyed traits. Analysis of AR tolerance coefficients (ARTC) distinguished the 14 traits into four types. The Types I, II, and III variables were affected by AR stress to various degrees and displayed variations in AR sensitivity among genotypes with different sizes. Nevertheless, the Type IV variables were not influenced and showed little genetic variation. Among the three evaluation criteria for measuring AR tolerance of soybean genotypes, the ARTC and the ARTI (AR tolerance index) of seed yield were moderately associated, whereas the ARTC of seed yield and the average membership grade of ARTCs of the seven selected traits for comprehensive evaluating were strongly correlated. Applying the comprehensive evaluation method, 22 highly AR-tolerant and 23 highly AR-susceptible germplasms were identified. Seed yield changes after AR exposure of these screened genotypes were primarily attributed to changes in total pod number per plant, which consequently caused corresponding alterations in fruited pod number per plant and seed number per plant, and were secondarily due to changes in hundred seed weight. These novel germplasms will serve as materials for subsequent investigations aiming to explore the genetic mechanisms underlying AR tolerance and to develop AR-tolerant varieties in soybean.