Delayed stem senescence (DSS) in soybean plants is defined as retarded leaf and stem maturity, even if pods mature normally. This phenomenon is a serious problem for the combine harvesting of soybean in Japan. Our previous research showed that DSS occurred more frequently in June sowing than in July sowing, and the plants sown in June were exposed to higher temperature and higher solar radiation intensity during reproductive growth stage as compared to those sown in July. We additionally observed higher levels of cytokinins, plant hormones with senescence inhibition activity, in xylem exudate after pod-setting stage (R4) under the DSS-induced condition. Thus, this study investigated the effects of the respective environmental factors, temperature and light intensity, after R4 on cytokinin levels in the xylem exudate and the occurrence of DSS. Pot experiments were conducted in the experimental field of Nihon University (Fujisawa-city, Kanagawa, Japan). In temperature treatments, soybean plants grown in the field environment were moved to an air-conditioned room at R4 stage, whereas control of light intensity was achieved by shading using cheese clothes under the field condition. The temperature control experiments revealed that high temperature after R4 promoted the DSS symptom, while low temperature after R4 inhibited the occurrence of DSS. On the other hand, decreased light intensity after R4 did not affect the severity of DSS. The cytokinin levels in the xylem exudate at R5 correlated with the severity of DSS in the temperature treatments, but the change in light intensity did not influence the cytokinin levels. These results indicated that high temperature after pod-setting stage have a profound effect on the occurrence of DSS, and this effect is mediated by the increased cytokinin levels in the xylem exudate.

中文翻译：

---

生长后期温度和光照条件对大豆木质部分泌物中延迟茎衰老和细胞分裂素水平的影响

大豆植物中的延迟茎衰老 (DSS) 被定义为叶和茎成熟延迟，即使豆荚正常成熟。这种现象是日本大豆联合收获的一个严重问题。我们前期的研究表明，6月播种的DSS发生频率高于7月播种，且与7月播种相比，6月播种的植物在生殖生长阶段暴露于更高的温度和更高的太阳辐射强度。我们还观察到在 DSS 诱导条件下结荚期 (R4) 后的木质部分泌物中细胞分裂素、具有衰老抑制活性的植物激素水平较高。因此，本研究调查了 R4 后各个环境因素、温度和光照强度对木质部渗出液中细胞分裂素水平和 DSS 发生的影响。盆栽实验在日本大学（日本神奈川县藤泽市）的实验场进行。在温度处理中，在R4阶段将在大田环境中生长的大豆植物移至空调房间，而在大田条件下通过使用干酪布遮光来控制光照强度。控温实验表明，R4后的高温促进了DSS症状的发生，而R4后的低温抑制了DSS的发生。另一方面，R4 后光照强度降低并不影响 DSS 的严重程度。R5 处木质部渗出液中的细胞分裂素水平与温度处理中 DSS 的严重程度相关，但光强度的变化并不影响细胞分裂素水平。