Effects of planting density on photosynthetic production and yield formation of soybean varieties from different eras
Planting density is an effective way to harness the production potential of elite soybean varieties,yet the response dif-ferences to planting density among soybean varieties over a breeding time span of up to 100 years remain unclear.To clarify the effects of planting density on the photosynthetic production and yield formation of soybean varieties released in different eras,this study used 50 soybean varieties released from the 1930s to the 2020s.The response differences in leaf area index (LAI),leaf area index growth rate (LGR),leaf area duration (LAD),dry matter accumulation,crop growth rate (CGR),lower leaf senescence,and yield were explored under different planting densities (normal density of 200,000 plants hm-2 and high density of 300,000 plants hm-2) for soybean varieties from different eras (1930s-1940s,1950s-1960s,1970s-1980s,1990s-2000s,and 2010s-2020s).The results showed that with the advancement of breeding eras,both the photosynthetic production capacity and yield of soybeans gradually increased.Compared with old varieties (1930s-1940s,1950s-1960s,and 1970s-1980s),new varieties (1990s-2000s and 2010s-2020s) showed better growth status at high density.The increase in LAI of new varieties at high density was more substantial,with increases of 17.79％ and 23.06％ at the R4 stage,and the decrease in LAI from the R4 to R6 stage was slower,resulting in a more significant increase in LAD.At the R6 stage,the dry matter accumulation of new varieties at high density in-creased by 25.28％ and 28.96％,respectively,and their CGR also significantly increased (P<0.05) by 21.66％ and 25.38％,re-spectively.Moreover,the new varieties showed strong senescence resistance at high density,with smaller decreases in the amount of upward displacement of yellow leaf nodes and leaf SPAD values of the lower leaves.In terms of yield,new varieties experi-enced greater increases in the number of seeds and pods per unit area at high density,with a smaller decrease in 100-seed weight,resulting in a significant (P<0.05) yield increases of 4.49％ and 5.04％,respectively.In conclusion,at the beginning of the growth period,new varieties showed a strong ability to increase the 'source' under high density,with high and stable leaf source values,greater light energy interception,and substantial dry matter accumulation,promoting the rapid development of the seed 'sink'.During the later growth period,the slower decrease in LAI,robust photosynthetic capacity of the population and thorough seed filling significantly increased the number of seeds and pods per unit area,thereby compensating for the slight decrease in 100-seed weight and achieving a significant increase in soybean yield.
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