Abstract

The carbon and oxygen isotopic composition of water and assimilates in plants reveals valuable information on plant responses to climatic conditions. Yet, the carbon and oxygen uptake, incorporation and allocation processes determining isotopic compositions are not fully understood. We carried out a dual-isotope labeling experiment at high humidity with ¹⁸O-enriched water (H₂¹⁸O) and ¹³C-enriched CO₂ (¹³CO₂) with attached Scots pine (Pinus sylvestris L.) branches and detached twigs of hemiparasitic mistletoes (Viscum album ssp. austriacum) in a naturally dry coniferous forest, where also a long-term irrigation takes place. After 4 h of label exposure, we sampled previous- and recent-year leaves, twig phloem and twig xylem over 192 h for the analysis of isotope ratios in water and assimilates. For both species, the uptake into leaf water and the incorporation of the ¹⁸O-label into leaf assimilates was not influenced by soil moisture, while the ¹³C-label incorporation into assimilates was significantly higher under irrigation compared with control dry conditions. Species-specific differences in leaf morphology or needle age did not affect ¹⁸O-label uptake into leaf water, but the incorporation of both tracers into assimilates was two times lower in mistletoe than in pine. The ¹⁸O-label allocation in water from pine needles to twig tissues was two times higher for phloem than for xylem under both soil moisture conditions. In contrast, the allocation of both tracers in pine assimilates were similar and not affected by soil moisture, twig tissue or needle age. Soil moisture effects on ¹³C-label but not on ¹⁸O-label incorporation into assimilates can be explained by the stomatal responses at high humidity, non-stomatal pathways for water and isotope exchange reactions. Our results suggest that non-photosynthetic ¹⁸O-incorporation processes may have masked prevalent photosynthetic processes. Thus, isotopic variation in leaf water could also be imprinted on assimilates when photosynthetic assimilation rates are low.

中文翻译：

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土壤水分，针叶年龄和叶片形态对C和O吸收，吸收和分配的影响：针叶林叶片中13CO2和H218O的双重标记方法。

摘要

植物中水和同化物的碳和氧同位素组成揭示了有关植物对气候条件反应的宝贵信息。然而，尚未完全了解确定同位素组成的碳和氧的吸收，结合和分配过程。我们在高湿度下用¹⁸ O富集水（H ₂ ¹⁸ O）和¹³ C富集CO ₂（¹³ CO ₂）进行了双同位素标记实验，并附有苏格兰松树（Pinus sylvestris L.）树枝和分离的树枝。 hemiparasitic槲寄生的（槲寄生SSP。austriacum）在自然干燥的针叶林中，在那里还会进行长期灌溉。标签暴露4 h后，我们在192 h内对前一年和近年的叶子，嫩枝韧皮部和嫩枝木质部进行了采样，以分析水中和同化物中的同位素比。对于这两个物种，土壤水分均不会影响叶片水分对叶片水分的吸收和¹⁸ O-标记物向叶片同化物中的掺入，而与对照干燥条件相比，灌溉条件下¹³ C-标记物向同化物中的掺入明显更高。叶片形态或针叶年龄的物种特异性差异不影响叶片水吸收¹⁸ O-标记物，但槲寄生中两种示踪剂向同化物中的结合率比松树低两倍。在¹⁸在两种土壤水分条件下，韧皮部中从松针到树枝组织的水中O-标签分配比木质部高出两倍。相反，两种示踪剂在松树同化物中的分配是相似的，并且不受土壤水分，树枝组织或针头年龄的影响。土壤水分对¹³ C-标记而不对¹⁸ O-标记掺入同化物中的影响可以通过高湿度下的气孔响应，水和同位素交换反应的非气孔途径来解释。我们的结果表明，非光合¹⁸ O掺入过程可能掩盖了普遍的光合过程。因此，当光合同化率低时，叶水中的同位素变化也可被印在同化物上。