Compound specific hydrogen isotopic analyses have the potential to reveal the biosynthetic pathways of biomarkers and to reconstruct the effects of water stress in a plant, or in an ecosystem. Although C4 graminoids are of great geological interest and are some of the world's leading crops, there are few experimental studies of their biomarker responses to hydrological conditions. Here, we study a C4 graminoid, broomcorn millet, and compare the effects of controlled changes in environmental conditions on the distributions of n-alkane homologues and on the pentacyclic triterpene, miliacin, which is a biomarker for broomcorn millet; both were measured in the seeds of the graminoid. Broomcorn millet plants were propagated in hydroponic solutions with four different δ²H values for each of two growth chambers, differing in relative humidity (58 and 74%). Analyses of δ²H values of the lipid compounds (miliacin and n-alkanes) in seeds and water extracted from transpiring and non-transpiring organs allowed us to quantify the apparent (lipids vs. source water to plant) and biosynthetic (lipids vs. leaf water) fractionations during miliacin and n-alkane syntheses. Miliacin and n-alkane δ²H values were linearly related to leaf water δ²H values, permitting credible biosynthetic fractionations to be calculated for n-alkanes (average −149.5‰ ± 11) and miliacin (−118‰ ± 5). These biosynthetic fractionations were within the range of published values for compounds with their respective biosynthetic pathways, although a ²H-enrichment of miliacin compared to n-alkanes remains unexplained.

Whereas a 16% decrease in relative humidity had no significant impact on the biosynthetic fractionation of miliacin, n-C₂₅ and n-C₂₇ alkanes, it led to a ~25‰ decrease in biosynthetic fractionation for n-C₃₁ and n-C₃₃ alkanes. This could be the consequence of a contribution of more depleted pools of hydrogen atoms in cytoplasmic water (compared to chloroplastic hydrogen pools) during the n-alkyl lipid elongation process. This finding suggests that the respective influences of source water δ²H values and relative humidity on the δ²H values of organic compounds may be discretely inferred by examining the δ²H values of compounds synthesized from distinct sources of hydrogen in cells. This provides clues to the biosynthetic fractionations in a C4 plant for compounds derived from distinct pathways, but also highlights specific issues related to seed lipids which require further research.

化合物特定的氢同位素分析具有揭示生物标记物的生物合成途径并重建植物或生态系统中水分胁迫效应的潜力。尽管C4类动物体具有巨大的地质意义，并且是世界上一些主要农作物，但很少有实验研究它们的生物标记物对水文条件的响应。在这里，我们研究了一种C4类拟min物，room帚小米，并比较了环境条件的受控变化对正构烷烃同系物和五环三萜烯ilia霉素（is帚小米的生物标志物）分布的影响。两者均在类粒动物的种子中测定。黍植物水培溶液进行传播，但有四种不同δ ²两个生长室中每个室的H值，相对湿度不同（分别为58％和74％）。δ的分析^2个的脂质化合物h的值（miliacin和Ñ烷烃）的种子和水蒸散和非蒸腾器官提取使我们能够量化表观（脂质对源水至植物）和生物合成（脂质与紫water碱和正构烷烃合成过程中的叶分馏。Miliacin和Ñ烷烃δ ² ħ值线性相关叶水δ ^2个ħ值，从而允许可信的生物合成分馏针对要计算的Ñ-烷烃（平均-149.5‰±11）和紫ilia碱（-118‰±5）。这些生物合成分馏在具有其各自生物合成途径的化合物的公开值范围内，尽管尚无法解释与正构烷烃相比，紫ilia素的² H富集。

相对湿度降低16％对紫胶素，n -C ₂₅和n -C ₂₇烷烃的生物合成分馏没有显着影响，但导致n -C ₃₁和n -C的生物合成分馏降低〜25 ‰。_33种烷烃。这可能是由于在正烷基脂质延长过程中细胞质水中的氢原子池减少了（与氯塑料氢池相比）的结果。这一发现表明，源水的相应影响δ ^2个的δ值ħ和相对湿度²有机化合物h的值可以通过检查δ离散地推断²从细胞中氢的不同的信号源合成的化合物h的值。这为C4植物中源自不同途径的化合物的生物合成分馏提供了线索，但也突出了与种子脂质相关的特定问题，需要进一步研究。