A pool-weighted perspective on the two-water-worlds hypothesis.

The 'two-water-worlds' hypothesis is based on stable isotope differences in stream, soil and xylem waters in dual isotope space. It postulates no connectivity between bound and mobile soil waters, and preferential plant water uptake of bound soil water sources. We tested the pool-weighted impact of isotopically distinct water pools for hydrological cycling, the influence of species-specific water use and the degree of ecohydrological separation. We combined stable isotope analysis (δ18 O and δ2 H) of ecosystem water pools of precipitation, groundwater, soil and xylem water of two distinct species (Quercus suber, Cistus ladanifer) with observations of soil water contents and sap flow. Shallow soil water was evaporatively enriched during dry-down periods, but enrichment faded strongly with depth and upon precipitation events. Despite clearly distinct water sources and water-use strategies, both species displayed a highly opportunistic pattern of root water uptake. Here we offer an alternative explanation, showing that the isotopic differences between soil and plant water vs groundwater can be fully explained by spatio-temporal dynamics. Pool weighting the contribution of evaporatively enriched soil water reveals only minor annual impacts of these sources to ecosystem water cycling (c. 11% of bulk soil water and c. 14% of transpired water).

Real-time carbon allocation into biogenic volatile organic compounds (BVOCs) and respiratory carbon dioxide (CO2) traced by PTR-TOF-MS, 13CO2 laser spectroscopy and 13C-pyruvate labelling.

Our understanding of biogenic volatile organic compound (BVOC) emissions improved substantially during the last years. Nevertheless, there are still large uncertainties of processes controlling plant carbon investment into BVOCs, of some biosynthetic pathways and their linkage to CO2 decarboxylation at central metabolic branching points. To shed more light on carbon partitioning during BVOC biosynthesis, we used an innovative approach combining δ13CO2 laser spectroscopy, high-sensitivity proton-transfer-reaction time-of-flight mass spectrometry and a multiple branch enclosure system in combination with position-specific 13C-metabolite labelling. Feeding experiments with position-specific 13C-labelled pyruvate, a central metabolite of BVOC synthesis, enabled online detection of carbon partitioning into 13C-BVOCs and respiratory 13CO2. Measurements of trace gas emissions of the Mediterranean shrub Halimium halimifolium revealed a broad range of emitted BVOCs. In general, [2-13C]-PYR was rapidly incorporated into emitted acetic acid, methyl acetate, toluene, cresol, trimethylbenzene, ethylphenol, monoterpenes and sesquiterpenes, indicating de novo BVOC biosynthesis of these compounds. In contrast, [1-13C]-pyruvate labelling substantially increased 13CO2 emissions in the light indicating C1-decarboxylation. Similar labelling patterns of methyl acetate and acetic acid suggested tightly connected biosynthetic pathways and, furthermore, there were hints of possible biosynthesis of benzenoids via the MEP-pathway. Overall, substantial CO2 emission from metabolic branching points during de novo BVOC biosynthesis indicated that decarboxylation of [1-13C]-pyruvate, as a non-mitochondrial source of CO2, seems to contribute considerably to daytime CO2 release from leaves. Our approach, combining synchronised BVOC and CO2 measurements in combination with position-specific labelling opens the door for real-time analysis tracing metabolic pathways and carbon turnover under different environmental conditions, which may enhance our understanding of regulatory mechanisms in plant carbon metabolism and BVOC biosynthesis.

Metabolome and Lipidome Profiles of Populus × canescens Twig Tissues During Annual Growth Show Phospholipid-Linked Storage and Mobilization of C, N, and S.

The temperate climax tree species Fagus sylvatica and the floodplain tree species Populus × canescens possess contrasting phosphorus (P) nutrition strategies. While F. sylvatica has been documented to display P storage and mobilization (Netzer et al., 2017), this was not observed for Populus × canescens (Netzer et al., 2018b). Nevertheless, changes in the abundance of organic bound P in gray poplar trees indicated adaptation of the P nutrition to different needs during annual growth. The present study aimed at characterizing seasonal changes in metabolite and lipid abundances in gray poplar and uncovering differences in metabolite requirement due to specific needs depending on the season. Seasonal variations in the abundance of (i) sugar-Ps and phospholipids, (ii) amino acids, (iii) sulfur compounds, and (iv) carbon metabolites were expected. It was hypothesized that seasonal changes in metabolite levels relate to N, S, and C storage and mobilization. Changes in organic metabolites binding Pi (Porg) are supposed to support these processes. Variation in triacylglycerols, in sugar-phosphates, in metabolites of the TCA cycle and in the amino acid abundance of poplar twig buds, leaves, bark, and wood were found to be linked to changes in metabolite abundances as well as to C, N, and S storage and mobilization processes. The observed changes support the view of a lack of any P storage in poplar. Yet, during dormancy, contents of phospholipids in twig bark and wood were highest probably due to frost-hardening and to its function in extra-plastidic membranes such as amyloplasts, oleosomes, and protein bodies. Consistent with this assumption, in spring sugar-Ps increased when phospholipids declined and poplar plants entering the vegetative growth period and, hence, metabolic activity increases. These results indicate that poplar trees adopt a policy of P nutrition without P storage and mobilization that is different from their N- and S-nutrition strategies.

Seasonal Alterations in Organic Phosphorus Metabolism Drive the Phosphorus Economy of Annual Growth in F. sylvatica Trees on P-Impoverished Soil.

Phosphorus (P) is one of the most important macronutrients limiting plant growth and development, particularly in forest ecosystems such as temperate beech (Fagus sylvatica) forests in Central Europe. Efficient tree internal P cycling during annual growth is an important strategy of beech trees to adapt to low soil-P. Organic P (Porg) is thought to play a decisive role in P cycling, but the significance of individual compounds and processes has not been elucidated. To identify processes and metabolites involved in P cycling of beech trees, polar-metabolome and lipidome profiling was performed during annual growth with twig tissues from a sufficient (Conventwald, Con) and a low-soil-P (Tuttlingen, Tut) forest. Autumnal phospholipid degradation in leaves and P export from senescent leaves, accumulation of phospholipids and glucosamine-6-phosphate (GlcN6P) in the bark, storage of N-acetyl-D-glucosamine-6-phosphate (GlcNAc6P) in the wood, and establishing of a phospholipid "start-up capital" in buds constitute main processes involved in P cycling that were enhanced in beech trees on low-P soil of the Tut forest. In spring, mobilization of P from storage pools in the bark contributed to an effective P cycling. Due to the higher phospholipid "start-up capital" in buds of Tut beeches, the P metabolite profile in developing leaves in spring was similar in beech trees of both forests. During summer, leaves of Tut beeches meet their phosphate (Pi) needs by replacing phospholipids by galacto- and sulfolipids. Thus, several processes contribute to adequate Pi supply on P impoverished soil thereby mediating similar growth of beech at low and sufficient soil-P availability.