Martín-Gómez Paula, Serrano Luis, Ferrio Juan Pedro
Department of Crop and Forest Sciences, AGROTECNIO Center, Universitat de Lleida, Avda Rovira Roure 191, E-25198 Lleida,Spain.
Department of Crop and Forest Sciences, AGROTECNIO Center, Universitat de Lleida, Avda Rovira Roure 191, E-25198 Lleida, Spain.
Tree Physiol. 2017 Apr 1;37(4):511-522. doi: 10.1093/treephys/tpw115.
In ecohydrology, it is generally assumed that xylem water reflects the water source used by plants. Several studies have reported isotopic enrichment within woody tissues, particularly during dormancy periods or after long periods of inactivity. However, little is known about the short-term dynamics of this process. Here we assessed the magnitude and dynamics of xylem isotopic enrichment in suberized twigs of pines and oaks. We performed a series of laboratory experiments, in which we monitored hourly changes in water content and isotopic composition under two contrasting scenarios of sap flow restriction. First, we simulated the effect of extreme hydraulic failure by excising twigs to restrict sap flow, while sealing the wounds to ensure that water loss took place only through the leaves or bark, as would be the case for evaporation in attached stems. Second, we studied the effect of reduced leaf transpiration by darkening with aluminium foil all the leaves of healthy, well-watered saplings growing in pot conditions. We found evidence of fast evaporative enrichment in metabolically active stems, as a consequence of a temporal decline in sap flow rates, and not necessarily linked to a traceable decline in stem water content. The excision experiments showed significant isotopic changes (~+1‰ in oxygen) appearing in <1 h. Similarly, the pot experiment showed a progressive increase in isotope composition (up to +8‰ in oxygen in a 3-day cycle) when the leaves were covered, and a rapid recovery to initial values when sap flow rates were re-established. We conclude that evaporative enrichment of xylem water in stems is a highly dynamic process that may have significant effects even during short periods of restricted water flow. This has important implications for the study of plant water uptake, as well as for ecosystem- and global-scale hydrological models.
在生态水文学中，一般认为木质部水分反映了植物所利用的水源。几项研究报告了木质组织内的同位素富集现象，尤其是在休眠期或长时间不活动之后。然而，对于这一过程的短期动态变化知之甚少。在此，我们评估了松树和橡树栓化嫩枝中木质部同位素富集的程度和动态变化。我们进行了一系列实验室实验，在两种不同的液流限制情况下，每小时监测水分含量和同位素组成的变化。首先，我们通过切除嫩枝来限制液流，模拟极端水力衰竭的影响，同时密封伤口以确保水分仅通过叶片或树皮流失，这与附着茎干中的蒸发情况相同。其次，我们通过用铝箔覆盖盆栽条件下生长的健康、水分充足的幼树的所有叶片，研究了叶片蒸腾作用降低的影响。我们发现，在代谢活跃的茎干中存在快速蒸发富集的证据，这是液流速率暂时下降的结果，不一定与茎干水分含量的可追溯下降有关。切除实验表明，在不到1小时内出现了显著的同位素变化（氧同位素约增加1‰）。同样，盆栽实验表明，当叶片被覆盖时，同位素组成逐渐增加（在3天的周期内氧同位素增加高达8‰），而当液流速率恢复时，同位素迅速恢复到初始值。我们得出结论，茎干中木质部水分的蒸发富集是一个高度动态的过程，即使在短时间的水流限制期间也可能产生重大影响。这对植物水分吸收的研究以及生态系统和全球尺度的水文模型具有重要意义。
