Effect of thinning and shade removal on green stem disorder in soybean

Abstract

Green stem disorder (GSD) in soybean (Glycine max (L.) Merrill) retains green stems and leaves as the pods mature, thereby reducing the harvest efficiency and impairing seed quality. In order to elucidate the causes of GSD, the factors that promote GSD need to be identified. In our experiments, we adjusted plant density at the developmental growth stage R1 (the beginning of flowering) or at R5 (the beginning of seed filling), from dense (22.2 plants m-2) to sparse (5.56 plants m-2) by thinning. We found that GSD occurrence was increased when plant density was changed, compared to the treatments that were maintained under either dense or sparse conditions. GSD was promoted more strongly when thinning was conducted at R5 than at R1 stage. Shading equipment surrounding plants, except for their uppermost leaves, was implemented to determine the association of shading and GSD. The results of the shade experiment revealed that GSD occurrence generally increased in treatments subjected to shade removal, compared to those that were shaded until R8 stage (full maturity) or never shaded since the time of sowing. GSD was strongly promoted by shade removal at R5 than at R1 stage. The shading results coincide with the results of the plant density experiment, indicating that an increase in light availability enhances source activity relative to sink at R5 stage, thereby promoting GSD occurrence in soybean. Thinning is expected to be used as an easy experimental method to create GSD for research purpose.

Effect of thinning on cultivar differences of green stem disorder in soybean

“…In the previous study (Yamazaki et al, 2018), thinning did not affect the developmental progression of the reproductive stage, which was evaluated based on pod maturation, suggesting that GSD promoted by thinning was characterized by the delayed maturation of the leaves and stems. In the present results, the days from sowing to R8 were also not significantly different between treatments and between cultivars (Table 3), suggesting that the GSD occurrences promoted by thinning or cultivar differences in our experiments were characterized by the delayed maturation of the leaves and stems.…”

“…The GSD scores of the thinning treatments at R1 and R1 + 14 d were not significantly different from those of the dense or sparse treatments or from the thinning treatments at R5 and R5 + 14 d, showing intermediate values (Table 2). There was no Yamazaki et al (2018).…”

“…The dates of growth stages R1, R5, and R8 were recorded for each plant, following the method of Fehr and Caviness (1977). The severity of GSD was assessed for each plant at the R8 stage using a scoring method slightly modified from that of Furuya and Umezaki (1993) (Yamazaki et al, 2018). In the scoring method, the GSD score (1-5) was assigned based on the stem color and number of leaves left on the stem at R8.…”

“…The N concentration in the main stem sampled after the R8 stage in 'Tachinagaha' and 'Sachiyutaka' with thinning was significantly higher than that in the dense treatment (Table 3). The dense treatment of 'Tachinagaha' also tended to have higher N concentrations when compared Yamazaki et al (2018) was used as the GSD scores of 'Sachiyutaka'. c N (mg g −1 ) is dry matter N concentration in main stem.…”

“…In our previous study, it was found that thinning at the R5 growth stage, which can be performed more easily than depodding at the field scale, promoted GSD occurrences in soybean (Yamazaki et al, 2018). Yamazaki et al (2018) discussed that the improved light availability at R5 stage by thinning enhanced source relative to sink, and then promoted the occurrence of GSD. Thus, GSD occurrences promoted by thinning also could be related to source-sink balance as well as depodding.…”