The availability of iron (Fe) is highly limited for plant uptake in calcareous soils. To overcome this challenge, plants have developed different strategies of Fe mobilization. Non-gramineous species employ a reduction-based strategy (strategy I), which is assisted by the release of Fe(III)-chelating and -reducing coumarins. In this study, the coumarin release by three strategy-I plant species of the Brassicaceae family, i.e. Brassica napus L., Raphanus sativus L. and Sinapis alba L., on calcareous soil was examined using the RHIZOtest approach. With this experimental setup, the plants were first grown hydroponically in a cylindrical growth unit for 12 days, which was then subsequently brought in contact with calcareous, Fe deficient soil for another 9 days. Root exudates were collected in both experimental phases. Total carbon (C) release, coumarin exudation rates, trace metal plant uptake as well as trace metal mobilization were determined in response to Fe deficiency after 3, 6 and 9 days of soil exposure. High total C exudation rates indicated a clear stress response upon Fe deficiency during the soil contact period. Six different coumarins were detected, with scopoletin being predominantly released by all three plant species during the hydroponic stage with zero Fe supply. The exudation rate of all detected coumarins was significantly lower during soil exposure and represented only a very small fraction of the total organic compounds released. Although the role of coumarins in Fe mobilization is very complex and still not completely understood, it seems that coumarins are less important in Fe mobilization by the three investigated plant species when grown on soil compared to phytosiderophore-releasing strategy-II plants. Our data further indicate that plants growing on a Fe-deficient soil release less coumarins than in an artificial nutrient solution setup with zero Fe supply, which highlights the importance of determining root exudation rates from soil-grown plants, since data from hydroponic studies are likely not representative for soil-grown plants.

铁（Fe）的可用性对于钙质土壤中的植物吸收非常有限。为了克服这一挑战，植物开发了不同的铁动员策略。非禾本科物种采用基于还原的策略（策略 I），该策略由 Fe(III) 螯合和还原香豆素的释放辅助。在本研究中，芸苔科植物的三种策略Ⅰ植物物种，即欧洲油菜、萝卜和白白菜的香豆素释放量L., 在石灰质土壤上使用 RHIZOtest 方法进行了检查。在这个实验装置中，植物首先在圆柱形生长单元中水培生长 12 天，然后与钙质、缺铁的土壤再接触 9 天。在两个实验阶段收集根系分泌物。土壤暴露 3、6 和 9 天后，响应铁缺乏，测定了总碳 (C) 释放、香豆素渗出率、微量金属植物吸收以及微量金属动员。高总 C 渗出率表明在土壤接触期间对 Fe 缺乏有明显的应激反应。检测到六种不同的香豆素，东莨菪碱主要由所有三种植物在水培阶段释放，铁供应为零。所有检测到的香豆素的渗出率在土壤暴露期间显着降低，仅占释放的总有机化合物的很小一部分。尽管香豆素在 Fe 动员中的作用非常复杂并且仍未完全了解，但与植物铁黄素释放策略 II 植物相比，香豆素在土壤上生长的三种研究植物物种的 Fe 动员中似乎不太重要。我们的数据进一步表明，在缺铁土壤上生长的植物释放的香豆素比在零铁供应的人工营养液设置中释放的香豆素少，这突出了确定土壤生长植物根系渗出率的重要性，因为来自水培研究的数据很可能不代表土壤生长的植物。