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Plants have evolved to grow under prominently fluctuating environmental conditions. In experiments under controlled conditions, temperature is often set to artificial, binary regimes with constant values at day and at night. This study investigated how such a diel (24 hr) temperature regime affects leaf growth, carbohydrate metabolism and gene expression, compared to a temperature regime with a field‐like gradual increase and decline throughout 24 hr. Soybean (Glycine max) was grown under two contrasting diel temperature treatments. Leaf growth was measured in high temporal resolution. Periodical measurements were performed of carbohydrate concentrations, carbon isotopes as well as the transcriptome by RNA sequencing. Leaf growth activity peaked at different times under the two treatments, which cannot be explained intuitively. Under field‐like temperature conditions, leaf growth followed temperature and peaked in the afternoon, whereas in the binary temperature regime, growth increased at night and decreased during daytime. Differential gene expression data suggest that a synchronization of cell division activity seems to be evoked in the binary temperature regime. Overall, the results show that the coordination of a wide range of metabolic processes is markedly affected by the diel variation of temperature, which emphasizes the importance of realistic environmental settings in controlled condition experiments., Dynamically changing temperature affects soybean leaf growth and regulatory mechanisms in non‐intuitive ways. The results may help improving knowledge transfer between experiments performed under controlled conditions and experiments performed in the field.

The carbon isotopic composition (δ13C) of foliage is often used as proxy for plant performance. However, the effect of NO3– vs. NH4+ supply on δ13C of leaf metabolites and respired CO2 is largely u...

Recent studies suggest that isotope ratios of the carbon-bound non-exchangeable hydrogen (δ2H) in plant cellulose and lipids can indicate changes in the primary carbon and energy metabolism; however, systematic investigations are scarce.Here, we studied δ2H patterns in two different tobacco (N. sylvestris) model systems, where severe changes in the plant primary metabolism were known: 1) along a nitrogen (N) supply gradient and 2) in a starch-less knockout mutant (pgm). Specifically, we measured δ2H of water, bulk soluble sugars, transitory starch, and cellulose in leaves and roots, using a novel hot water vapor equilibration method and TC/EA-IRMS. Besides, we measured δ2H values of leaf n-alkanes with GC-IRMS.We observed clear δ2H differences in sugars and starch along the N gradient and a 2H-enrichment of both assimilates in pgm compared to a wild type control. The photosynthetic 2H-fractionation between leaf water and sugars/starch reached a maximum of ca. 100‰ in both model systems and was related to changes in concentrations of primary metabolites (e.g. sugars, starch, organic and amino acids), enzymatic activities, gas-exchange, and growth. The signal of the primary carbon metabolism was also visible in δ2H of leaf and root cellulose in both system, but dampened compared to those of sugars and starch. In contrast, the signal was absent in leaf n-alkanes in both systems.Our results provide the first direct evidence that changes in the primary leaf carbon metabolism are imprinted on δ2H of plant carbohydrates in leaf and roots. The metabolic signal might therefore be reconstructed from plant material of important paleo archives (e.g. tree-ring cellulose, lake sediments) and help to better understand plant-climate interactions. The absence of the signal in δ2H of leaf n-alkanes is surprising and suggests a strong difference in metabolic fluxes between carbohydrates and lipids. Yet, this observation may help to further disentangle the processes shaping hydrogen isotopes in plants.

Using five eddy covariance flux sites (two forests and three grasslands), we investigated ecosystem physiological responses to the 2018 drought across elevational gradients in Switzerland. Flux measurements showed that at lower elevation sites (below 1000 m.a.s.l.; grassland and mixed forest) annual ecosystem productivity (GPP) declined by approximately 20% compared to the previous 2 years (2016 and 2017), which led to a reduced annual net ecosystem productivity (NEP). At the high elevation sites, however, GPP increased by approximately 14% and as a result NEP increased in the alpine and montane grasslands, but not in the subalpine coniferous forest. There, increased ecosystem respiration led to a reduced annual NEP, despite increased GPP and lengthening of the growing period. Among all ecosystems, the coniferous forest showed the most pronounced negative stomatal response to atmospheric dryness (i.e. vapour pressure deficit, VPD) that resulted in a decline in surface conductance and an increased water-use efficiency during drought. While increased temperature enhanced the water-use efficiency of both forests, de-coupling of GPP from evapotranspiration at the low-elevation grassland site negatively affected water-use efficiency due to non-stomatal reductions in photosynthesis. Our results show that hot droughts (such as in 2018) lead to different responses across plants types, and thus ecosystems. Particularly grasslands at lower elevations are the most vulnerable ecosystems to negative impacts of future drought in Switzerland. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’. ISSN:0962-8436 ISSN:1471-2970 ISSN:0080-4622

The carbon isotopic composition (δ

Zinc (Zn) deficiency is a widespread problem in wheat-cultivated lands, and understanding important mechanisms determining Zn phytoavailability is of high importance. Accordingly, a two-year field experiment was conducted to investigate the effects of two Zn-deficiency tolerant wheat cultivars on Zn binding forms in a calcareous saline soil. Changes in soil Zn fractions were also found to be related to Zn uptake by wheat. Rhizosphere and bulk soil samples and the above-ground biomass were collected at grain maturity and analyzed for Zn. By using a sequential extraction procedure, soil Zn was divided into the following operationally defined fractions which are conceptually designated: ‘exchangeable Zn’ (EXCH-Zn), ‘Zn bound to carbonates’ (CAR-Zn), ‘organically bound Zn’ (ORG-Zn), ‘Zn bound to iron and manganese oxides’ (FeMnOX-Zn), and ‘residual Zn’ (RES-Zn). EXCH-Zn and ORG-Zn in the rhizosphere soil were found to be positively correlated with Zn uptake into the aboveground biomass of both wheat cultivars, thereby indicating that these fractions could be considered as labile pools. Higher EXCH-Zn and ORG-Zn fractions were determined in the rhizosphere, not in the bulk soil, whereas the non-labile Zn pools was decreased, suggesting strong Zn solubilization due to the input of acidity and organic compounds by root and the associated microbial activity. The differences between rhizosphere and bulk soil in EXCH-Zn and ORG-Zn were associated with the lower pH and the higher concentrations of the total (TOC) and dissolved organic carbon (DOC) in the rhizosphere. The fact, these effects (lower pH and higher DOC in the rhizosphere, rather than in the bulk soil) were stronger for Back Cross Rushan than were for Kavir; this was in line with the higher Zn uptake of Zn-deficiency tolerant Back Cross Rushan.

Over the past decades, average global wheat yields have increased by about 250 %, mainly due to the cultivation of high-yielding wheat cultivars. This selection process not only affected aboveground parts of plants, but in some cases also reduced the root biomass, with potentially large consequences for the amount of organic carbon (OC) transferred to the soil. To study the effect of wheat breeding for high-yielding cultivars on subsoil OC dynamics, two old and two new wheat cultivars from the Swiss wheat breeding program were grown for one growing season in 1.5 m-deep lysimeters and pulse-labelled with 13CO2, to quantify the amount of assimilated carbon that was transferred belowground and potentially stabilized in the soil. The results show that although the old wheat cultivars with higher root biomass transferred more assimilated carbon belowground compared to more recent cultivars, no significant differences in net soil organic carbon (SOC) stabilization were found between the different cultivars. As a consequence, the long-term effect of wheat cultivar selection on SOC stocks will depend on the amount of root biomass that is stabilized in the soil. Our results suggest that the process of wheat selection for high-yielding cultivars resulted in lower amounts of belowground carbon translocation, with potentially important effects on SOC stocks. Further research is necessary to quantify the long-term importance of this effect.

Drought and heat events, such as the 2018 European drought, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO 2 exchange. Each of these quantities may aggravate or mitigate the drought, heat, their side effects on productivity, water scarcity and global warming. We used measurements of 56 eddy covariance sites across Europe to examine the response of fluxes to extreme drought prevailing most of the year 2018 and how the response differed across various ecosystem types (forests, grasslands, croplands and peatlands). Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004–2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought affected. These received on average 9% more solar radiation and released 32% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased net CO 2 uptake by 17.8%, but did not significantly change net evapotranspiration. The response of these fluxes differed characteristically between ecosystems; in particular, the general increase in the evaporative index was strongest in peatlands and weakest in croplands. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

The aim of this 2-year field experiment was to investigate agronomic and economic efficiency of ground tire rubber and rubber ash as zinc (Zn) sources for wheat (Triticum aestivum L. cvs. Kavir and Back Cross) compared with a commercial zinc sulfate (ZnSO4). A similar rate of Zn was used by soil incorporation of 40 kg/ha ZnSO4, 200 kg/ha waste tire rubber ash, and 1000 kg/ha ground rubber. A no Zn added treatment was also considered as control. All Zn fertilizers significantly increased grain yield of wheat over the control, although effectiveness of rubber ash was greater than the other Zn sources. Wheat plants treated with rubber ash accumulated higher Zn in their grains compared with those treated with ground rubber and ZnSO4. Tire rubber ash had the highest agronomic and economic efficiency and contained low levels of cadmium (Cd) and lead (Pb). Therefore, it can be used as an economic substitution for commercial ZnSO4.