Soybean contains a range of compounds with putative health benefits including isoflavones and tocopherols. A study was conducted to determine the effects on these compounds of high temperature stress imposed at specific development stages [i.e., none, pre-emergence, vegetative, early reproductive (R1-4), late-reproductive (R5-8), or all stages]. Two cultivars (AC Proteina and OAC Champion) were grown in growth chambers set at contrasting temperatures [i.e., stress conditions of 33/25 °C (day/night temperature) and control conditions of 23/15 °C] in order to generate these treatments. Isoflavone and tocopherol concentrations in mature seeds were determined using high-performance liquid chromatography. In both cultivars isoflavone response was greatest when stress occurred during the R5-8 stages and during all development stages, these treatments reducing total isoflavone concentration by an average of 85% compared to the control. Stress imposed at other stages also affected isoflavone concentration although the response was smaller. For example, stress during the vegetative stages reduced total isoflavones by 33% in OAC Champion. Stress imposed pre-emergence had an opposite effect increasing daidzein concentration by 24% in AC Proteina. Tocopherol concentrations were affected the most when stress was imposed during all stages of development, followed by stress restricted to stages R5-8; response to stress during other stages was limited. The specific response of tocopherols differed, α-tocopherol being increased by high temperature by as much as 752%, the reverse being observed for δ-tocopherol and γ-tocopherol. The present study demonstrates that while isoflavone and tocopherol concentrations in soybeans are affected the most by stress occurring during seed formation, concentrations can also be affected by stress occurring at other stages including stages as early as pre-emergence.

Isoflavones have been reported to have putative health-beneficial properties, which has led to increased interest and demand for soybeans and soy-based products. This study was conducted to determine the effects of high-temperature stress on isoflavone concentration and expression of four key genes involved in isoflavone synthesis (i.e., CHS7, CHS8, IFS1, and IFS2) in both soybean pods and seeds during their late reproductive stage (i.e., R5-R8). Isoflavone concentrations were quantified using high-performance liquid chromatography (HPLC), and gene expression was studied using quantitative real-time (qRT)-PCR. High-temperature stress [33/25 °C (day/night temperatures)] imposed at the late reproductive stage (R5-R8) reduced total isoflavone concentration by 46-86 and 20-73% in seeds and pods, respectively, the reduction depending on the stage of maturity. At stage R5, the reduction in total isoflavone concentration was greater in seeds than in pods, whereas at subsequent stages, the reverse was observed. High-temperature stress had a large impact on the expression of CHS7, CHS8, IFS1, and IFS2 in both seeds and pods. In seeds, temperature stress reduced the expression of one gene at the R5 stage (CHS8), two genes at the R6 stage (CHS7 and IFS1), and all four genes at the R7 stage, the reduction ranging between 35 and 97%. In pods, high-temperature stress affected the expression of two genes at the R6 stage (CHS7 and IFS2) and all four genes at the R7 stage. Unlike in seeds, at the R6 stage, high temperature increased the expression of CHS7 and IFS2 by 72 and 736%, respectively, whereas at R7 stage the expression of all genes was reduced by an average of 97%. The present study reveals that high-temperature stress initiated at the R5 stage and maintained until maturation (i.e., R8 stage) has a rapid and sustained negative effect on isoflavone concentration in both seeds and pods. High temperature also affects gene expression; however, there was no clear correlation between isoflavone concentration and gene expression.

Soybean [Glycine max (L.) Merr.] seeds contain soyasaponin B, which has putative health benefits. Studies were conducted in multiple environments in Quebec, Canada to determine the effects of genotypes, environments, and seeding dates on soyasaponin B concentration in mature seeds. A growth chamber study was also conducted to determine the impact of high air temperature imposed at specific growth development stages on soyasaponin B in soybeans. Concentrations of individual and total soyasaponin B were determined using high-performance liquid chromatography. Genotype and environment main effects were the main determinants of soyasaponin B concentration in soybean, genotype × environment interactions accounting for less than 5% of the variation for all soyasaponin. Ranking of 20 early maturing soybean genotypes was thus relatively consistent across four environments, total concentration varying between 2.31 and 6.59 μmol g(-1). Seeding date consistently impacted soyasaponin B concentrations, early seeding date resulting in the highest concentrations. There was an 11% difference in total soyasaponin B concentration of soybeans seeded in mid-May compared to that in late-June. The response to high air temperature was complex and cultivar specific. High temperature stress restricted to the seed filling stages increased total soyasaponin B concentration in one cultivar by 28% when compared to that in control nonstressed plants; however, in another cultivar high temperature applied during all growth stages reduced total concentration by 27%. Results from the present study thus demonstrate that environmental factors and crop management both impact soyasaponin B concentration in soybeans.