Leaf water 18O and 2H maps show directional enrichment discrepancy in Colocasia esculenta

Spatial patterns of leaf water isotopes are challenging to predict because of the intricate link between vein and lamina water. Many models have attempted to predict these patterns, but to date, most have focused on monocots with parallel veins. These provide a simple system to study, but do not represent the majority of plant species. Here, a new protocol is developed using a Picarro induction module coupled to a cavity ringdown spectrometer to obtain maps of the leaf water isotopes (¹⁸O and ²H). The technique is applied to Colocasia esculenta leaves. The results are compared with isotope ratio mass spectrometry. In C. esculenta, a large enrichment in the radial direction is observed, but not in the longitudinal direction. The string‐of‐lakes model fails to predict the observed patterns, while the Farquhar–Gan model is more successful, especially when enrichment is accounted for along the radial direction. Our results show that reticulate‐veined leaves experience a larger enrichment along the axis of the secondary veins than along the midrib. We hypothesize that this is due to the lower major/minor vein ratio that leads to longer pathways between major veins and sites of evaporation.     

Effects of environmental parameters, leaf physiological properties and leaf water relations on leaf water delta18O enrichment in different Eucalyptus species

Stable oxygen isotope ratios (delta18O) have become a valuable tool in the plant and ecosystem sciences. The interpretation of delta18O values in plant material is, however, still complicated owing to the complex interactions among factors that influence leaf water enrichment. This study investigated the interplay among environmental parameters, leaf physiological properties and leaf water relations as drivers of the isotopic enrichment of leaf water across 17 Eucalyptus species growing in a common garden. We observed large differences in maximum daily leaf water delta18O across the 17 species. By fitting different leaf water models to these empirical data, we determined that differences in leaf water delta18O across species are largely explained by variation in the Péclet effect across species. Our analyses also revealed that species-specific differences in transpiration do not explain the observed differences in delta18O while the unconstrained fitting parameter 'effective path length' (L) was highly correlated with delta18O. None of the leaf morphological or leaf water related parameters we quantified in this study correlated with the L values we determined even though L was typically interpreted as a leaf morphological/anatomical property. A sensitivity analysis supported the importance of L for explaining the variability in leaf water delta18O across different species. Our investigation highlighted the importance of future studies to quantify the leaf properties that influence L. Obtaining such information will significantly improve our understanding of what ultimately determines the delta18O values of leaf water across different plant species.     

Leaf vein fraction influences the Péclet effect and 18O enrichment in leaf water

The process of evaporation results in the fractionation of water isotopes such that the lighter ¹⁶O isotope preferentially escapes the gas phase leaving the heavier ¹⁸O isotope to accumulate at the sites of evaporation. This applies to transpiration from a leaf with the degree of fractionation dependent on a number of environmental and physiological factors that are well understood. Nevertheless, the ¹⁸O enrichment of bulk leaf water is often less than that predicted for the sites of evaporation. The advection of less enriched water in the transpiration stream has been suggested to limit the back diffusion of enriched evaporative site water (Péclet effect); however, evidence for this effect has been varied. In sampling across a range of species with different vein densities and saturated water contents, we demonstrate the importance of accounting for the relative ‘pool’ sizes of the vascular and mesophyll water for the interpretation of a Péclet effect. Further, we provide strong evidence for a Péclet signal within the xylem that if unaccounted for can lead to confounding of the estimated enrichment within the mesophyll water. This has important implications for understanding variation in the effective path length of the mesophyll and hence potentially the δ¹⁸O of organic matter.     

黑河中游春玉米叶片水δD和δ18O的富集过程和影响因素

植物水的稳定同位素分馏过程是水在土壤-植物-大气连续体中循环的重要环节。以往研究由于叶片水¹⁸O同位素比值(δ¹⁸O _l,b)和氘(D)同位素比值(δD_l,b)(合称δ_l,b)实测数量少只能作为模型验证数据, 导致δ_l,b富集机制研究多集中于模型研究, 缺乏基于野外试验条件的δ_l,b富集的控制机制研究。叶片水δD_l,b和δ¹⁸O _l,b的富集程度(ΔD_l,b和Δ¹⁸O _l,b, 合称Δ_l,b)通常表示为δ_l,b与茎秆水D同位素比值(δD_x)和¹⁸O同位素比值(δ¹⁸O_x) (合称δ_x)之差, 即Δ_l,b = δ_l,b - δ_x。该研究以黑河中游沙漠绿洲春玉米(Zea mays)生态系统为研究对象, 重点采集和分析了季节和日尺度δ_l,b和δ_x数据, 配套开展了大气水汽δ¹⁸O和δD (合称δ_v)等辅助变量的原位连续观测, 探讨了季节和日尺度上的δ_l,b富集特征及其影响因素。结果表明: 叶片水δ_l,b和Δ_l,b的季节变化趋势不明显, 而受蒸腾作用影响表现出白天富集夜间贫化的单峰日变化特征。对于D来说, 无论季节尺度上还是日尺度上, 大气水汽δ_v和相对湿度是δD_l,b和ΔD_l,b的主要环境控制因素; 而对于¹⁸O来说, 无论季节尺度上还是日尺度上, 相对湿度是δ¹⁸O _l,b和Δ¹⁸O _l,b的主要环境控制因素。由于D和¹⁸O在热力学平衡分馏上有约8倍差异, 直接分析叶片水ΔD_l,b和Δ¹⁸O_l,b与影响因素的差异性, 有助于理解叶片水δD和δ¹⁸O富集过程以及对模型发展有一定的指导意义。