Feeding 14CO2 was crucial to uncovering the path of carbon in photosynthesis. Feeding 13CO2 to photosynthesizing leaves emitting isoprene has been used to develop hypotheses about the sources of carbon for the methylerythritol 4-phosphate pathway, which makes the precursors for terpene synthesis in chloroplasts and bacteria. Both photosynthesis and isoprene studies found that products label very quickly (<10 min) up to 80–90% but the last 10–20% of labeling requires hours indicating a source of 12C during photosynthesis and isoprene emission. Furthermore, studies with isoprene showed that the proportion of slow label could vary significantly. This was interpreted as a variable contribution of carbon from sources other than the Calvin–Benson cycle (CBC) feeding the methylerythritol 4-phosphate pathway. Here, we measured the degree of label in isoprene and photosynthetic metabolites 20 min after beginning to feed 13CO2. Isoprene labeling was the same as labeling of photosynthesis intermediates. High temperature reduced the label in isoprene and photosynthesis intermediates by the same amount indicating no role for alternative carbon sources for isoprene. A model assuming glucose, fructose, and/or sucrose reenters the CBC as ribulose 5-phosphate through a cytosolic shunt involving glucose 6-phosphate dehydrogenase was consistent with the observations.

中文翻译：

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加尔文-本森循环中间体中12C的来源以及以13CO2饲喂的植物叶片释放的异戊二烯

饲喂14CO2对于揭示光合作用中的碳路径至关重要。向散发异戊二烯的光合作用叶片中喂入13CO2已被用于开发关于甲基赤藓糖醇4-磷酸途径的碳源的假设，该假设使叶绿体和细菌中的萜烯合成成为前体。光合作用和异戊二烯研究均发现，产品标记速度非常快（<10分钟），最高可达80–90％，但最后10–20％的标记需要数小时，这表明光合作用和异戊二烯释放过程中会产生12C的碳源。此外，异戊二烯的研究表明，慢速标记的比例可能有很大差异。这被解释为来自加尔文-本森循环（CBC）以外的其他来源的碳对甲基赤藓糖醇4-磷酸途径的贡献。这里，在开始喂入13CO2后20分钟，我们测量了异戊二烯和光合代谢产物中标记的程度。异戊二烯标记与光合作用中间体标记相同。高温会使异戊二烯和光合作用中间体中的标记减少相同的量，表明异戊二烯的替代碳源没有作用。假设葡萄糖，果糖和/或蔗糖通过核糖5-磷酸通过涉及葡萄糖6-磷酸脱氢酶的胞质分流重新进入CBC的模型与观察结果一致。高温会使异戊二烯和光合作用中间体中的标记减少相同的量，表明异戊二烯的替代碳源没有作用。假设葡萄糖，果糖和/或蔗糖通过核糖5-磷酸通过涉及葡萄糖6-磷酸脱氢酶的胞质分流重新进入CBC的模型与观察结果一致。高温会使异戊二烯和光合作用中间体中的标记减少相同的量，表明异戊二烯的替代碳源没有作用。假设葡萄糖，果糖和/或蔗糖通过核糖5-磷酸通过涉及葡萄糖6-磷酸脱氢酶的胞质分流重新进入CBC的模型与观察结果一致。