Cadmium (Cd) concentrations in cacao often exceed food limits. A better understanding of the molecular mechanisms of Cd accumulation in cacao trees is necessary to advance Cd mitigation strategies. This study explores Cd distribution and speciation (i.e., chemical form) within the vegetative organs of a high Cd accumulating cacao cultivar growing in soil with background Cd concentrations (0.28 mg Cd kg) by synchrotron radiation-based micro- and nano-X-ray fluorescence imaging and X-ray absorption spectroscopy on duplicate samples of roots, young branches, mature branches, and leaves. In both the roots and branches, the incorporation of Cd in Ca-oxalate crystals with binding to oxygen-ligands was identified as a major mechanism of Cd regulation. The Ca-oxalate crystals were more abundant in the branches than in the roots. We suggest that the incorporation of Cd into the Ca-oxalate crystals may limit the transfer of Cd to other organs like the leaves and beans. The roots also featured Cd-sulfur species in dilated cells of the wood, probably for the retention of Cd in the roots by strong binding with Cd to thiol-containing ligands. A Cd hotspot in large, secreting channels of the branches suggested that mucilage may play a role in the transport of Cd within cacao trees. The leaves had local Cd enrichments in the parenchymatous tissues and displayed a higher Cd concentration in the mesophyll compared to the epidermis. These insights in Cd distribution and speciation underscore that L. develops original Cd detoxification strategies, strongly different from non-tolerant species such as cereals.

中文翻译：

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草酸钙晶体参与高镉积累可可品种的镉储存

可可中的镉 (Cd) 浓度通常超过食品限量。更好地了解可可树中镉积累的分子机制对于推进镉减排策略是必要的。本研究通过基于同步辐射的微纳米 X 射线，探讨了生长在背景镉浓度（0.28 mg Cd kg）土壤中的高镉积累可可品种的营养器官内的镉分布和形态（即化学形态）对根、幼枝、成熟枝和叶的重复样品进行荧光成像和 X 射线吸收光谱分析。在根和枝中，镉掺入草酸钙晶体中并与氧配体结合被认为是镉调节的主要机制。草酸钙晶体在枝条中比在根中更丰富。我们认为，将镉掺入草酸钙晶体中可能会限制镉向其他器官（如叶子和豆类）的转移。根部的木材扩张细胞中也含有镉硫物质，可能是通过与含硫醇配体与镉的强烈结合而将镉保留在根中。树枝的大分泌通道中存在镉热点，表明粘液可能在可可树内镉的运输中发挥作用。叶子的薄壁组织中存在局部镉富集，并且与表皮相比，叶肉中的镉浓度更高。这些对镉分布和形态形成的见解强调了 L. 开发了原创的镉解毒策略，与谷物等非耐受物种有很大不同。