Contrasting hydraulic strategies in <Emphasis Type="Italic">Salix psammophila</Emphasis> and <Emphasis Type="Italic">Caragana korshinskii</Emphasis> in the southern Mu Us Desert,China

Salix psammophila and Caragana korshinskii are two common shrubs in the southern Mu Us Desert, China. Their hydraulic strategies for adapting to this harsh, dry desert environment are not yet clear. This study examined the hydraulic transport efficiency, vulnerability to cavitation, and daily embolism refilling in the leaves and stems of these two shrubs during the dry season. In order to gain insight into water use strategies of whole plants, other related traits were also considered, including daily changes in stomatal conductance, leaf mass per area, leaf pressure–volume parameters, wood density and the Huber value. The leaves and stems of S. psammophila had greater hydraulic efficiency, but were more vulnerable to drought-induced hydraulic dysfunction than C. korshinskii. The difference between leaf and stem water potential at 50 % loss of conductivity was 0.12 MPa for S. psammophila and 0.81 MPa for C. korshinskii. Midday stomatal conductance decreased by 74 % compared to that at 8:30 in S. psammophila, whereas no change occurred in C. korshinskii. Daily embolism and refilling occurred in the stems of S. psammophila and leaves of C. korshinskii. These results suggest that a stricter stomatal regulation, daily embolism repair in stems, and a higher stem water capacitance could be partially compensating for the greater susceptibility to xylem embolism in S. psammophila, whereas higher leaf elastic modulus, greater embolism resistance in stems, larger difference between leaf and stem hydraulic safety, and drought-induced leaf shedding in C. korshinskii were largely responsible for its more extensive distribution in arid and desert steppes.     

Treatment of raw and ozonated oil sands process-affected water under decoupled denitrifying anoxic and nitrifying aerobic conditions: a comparative study

Batch experiments were performed to evaluate biodegradation of raw and ozonated oil sands process-affected water (OSPW) under denitrifying anoxic and nitrifying aerobic conditions for 33 days. The results showed both the anoxic and aerobic conditions are effective in degrading OSPW classical and oxidized naphthenic acids (NAs) with the aerobic conditions demonstrating higher removal efficiency. The reactors under nitrifying aerobic condition reduced the total classical NAs of raw OSPW by 69.1 %, with better efficiency for species of higher hydrophobicity. Compared with conventional aerobic reactor, nitrifying aerobic condition substantially shortened the NA degradation half-life to 16 days. The mild-dose ozonation remarkably accelerated the subsequent aerobic biodegradation of classical NAs within the first 14 days, especially for those with long carbon chains. Moreover, the ozone pretreatment enhanced the biological removal of OSPW classical NAs by leaving a considerably lower final residual concentration of 10.4 mg/L under anoxic conditions, and 5.7 mg/L under aerobic conditions. The combination of ozonation and nitrifying aerobic biodegradation removed total classical NAs by 76.5 % and total oxy-NAs (O3–O6) by 23.6 %. 454 Pyrosequencing revealed that microbial species capable of degrading recalcitrant hydrocarbons were dominant in all reactors. The most abundant genus in the raw and ozonated anoxic reactors was Thauera (~56 % in the raw OSPW anoxic reactor, and ~65 % in the ozonated OSPW anoxic reactor); whereas Rhodanobacter (~40 %) and Pseudomonas (~40 %) dominated the raw and ozonated aerobic reactors, respectively. Therefore, the combination of mild-dose ozone pretreatment and subsequent biological process could be a competent choice for OSPW treatment.     

Modeling the effects of extracellular potassium on bursting properties in pre-Bötzinger complex neurons

There are many types of neurons that intrinsically generate rhythmic bursting activity, even when isolated, and these neurons underlie several specific motor behaviors. Rhythmic neurons that drive the inspiratory phase of respiration are located in the medullary pre-Bötzinger Complex (pre-BötC). However, it is not known if their rhythmic bursting is the result of intrinsic mechanisms or synaptic interactions. In many cases, for bursting to occur, the excitability of these neurons needs to be elevated. This excitation is provided in vitro (e.g. in slices), by increasing extracellular potassium concentration (K _out ) well beyond physiologic levels. Elevated K _out shifts the reversal potentials for all potassium currents including the potassium component of leakage to higher values. However, how an increase in K _out , and the resultant changes in potassium currents, induce bursting activity, have yet to be established. Moreover, it is not known if the endogenous bursting induced in vitro is representative of neural behavior in vivo. Our modeling study examines the interplay between K _out , excitability, and selected currents, as they relate to endogenous rhythmic bursting. Starting with a Hodgkin-Huxley formalization of a pre-BötC neuron, a potassium ion component was incorporated into the leakage current, and model behaviors were investigated at varying concentrations of K _out . Our simulations show that endogenous bursting activity, evoked in vitro by elevation of K _out , is the result of a specific relationship between the leakage and voltage-dependent, delayed rectifier potassium currents, which may not be observed at physiological levels of extracellular potassium.     

Reliability of signal transmission in stochastic nerve axon equations

We introduce a method for computing probabilities for spontaneous activity and propagation failure of the action potential in spatially extended, conductance-based neuronal models subject to noise, based on statistical properties of the membrane potential. We compare different estimators with respect to the quality of detection, computational costs and robustness and propose the integral of the membrane potential along the axon as an appropriate estimator to detect both spontaneous activity and propagation failure. Performing a model reduction we achieve a simplified analytical expression based on the linearization at the resting potential (resp. the traveling action potential). This allows to approximate the probabilities for spontaneous activity and propagation failure in terms of (classical) hitting probabilities of one-dimensional linear stochastic differential equations. The quality of the approximation with respect to the noise amplitude is discussed and illustrated with numerical results for the spatially extended Hodgkin-Huxley equations. Python simulation code is supplied on GitHub under the link https://github.com/deristnochda/Hodgkin-Huxley-SPDE.     

A stochastic model of input effectiveness during irregular gamma rhythms

Gamma-band synchronization has been linked to attention and communication between brain regions, yet the underlying dynamical mechanisms are still unclear. How does the timing and amplitude of inputs to cells that generate an endogenously noisy gamma rhythm affect the network activity and rhythm? How does such ”communication through coherence” (CTC) survive in the face of rhythm and input variability? We present a stochastic modelling approach to this question that yields a very fast computation of the effectiveness of inputs to cells involved in gamma rhythms. Our work is partly motivated by recent optogenetic experiments (Cardin et al. Nature, 459(7247), 663–667 2009) that tested the gamma phase-dependence of network responses by first stabilizing the rhythm with periodic light pulses to the interneurons (I). Our computationally efficient model E-I network of stochastic two-state neurons exhibits finite-size fluctuations. Using the Hilbert transform and Kuramoto index, we study how the stochastic phase of its gamma rhythm is entrained by external pulses. We then compute how this rhythmic inhibition controls the effectiveness of external input onto pyramidal (E) cells, and how variability shapes the window of firing opportunity. For transferring the time variations of an external input to the E cells, we find a tradeoff between the phase selectivity and depth of rate modulation. We also show that the CTC is sensitive to the jitter in the arrival times of spikes to the E cells, and to the degree of I-cell entrainment. We further find that CTC can occur even if the underlying deterministic system does not oscillate; quasicycle-type rhythms induced by the finite-size noise retain the basic CTC properties. Finally a resonance analysis confirms the relative importance of the I cell pacing for rhythm generation. Analysis of whole network behaviour, including computations of synchrony, phase and shifts in excitatory-inhibitory balance, can be further sped up by orders of magnitude using two coupled stochastic differential equations, one for each population. Our work thus yields a fast tool to numerically and analytically investigate CTC in a noisy context. It shows that CTC can be quite vulnerable to rhythm and input variability, which both decrease phase preference.     

Optimality of nitrogen distribution among leaves in plant canopies

The vertical gradient of the leaf nitrogen content in a plant canopy is one of the determinants of vegetation productivity. The ecological significance of the nitrogen distribution in plant canopies has been discussed in relation to its optimality; nitrogen distribution in actual plant canopies is close to but always less steep than the optimal distribution that maximizes canopy photosynthesis. In this paper, I review the optimality of nitrogen distribution within canopies focusing on recent advancements. Although the optimal nitrogen distribution has been believed to be proportional to the light gradient in the canopy, this rule holds only when diffuse light is considered; the optimal distribution is steeper when the direct light is considered. A recent meta-analysis has shown that the nitrogen gradient is similar between herbaceous and tree canopies when it is expressed as the function of the light gradient. Various hypotheses have been proposed to explain why nitrogen distribution is suboptimal. However, hypotheses explain patterns observed in some specific stands but not in others; there seems to be no general hypothesis that can explain the nitrogen distributions under different conditions. Therefore, how the nitrogen distribution in canopies is determined remains open for future studies; its understanding should contribute to the correct prediction and improvement of plant productivity under changing environments.     

Expression of a putative dioxygenase gene adjacent to an insertion mutation is involved in the short internodes of columnar apples (<Emphasis Type="Italic">Malus</Emphasis> × <Emphasis Type="Italic">domestica</Emphasis>)

Determining the molecular mechanism of fruit tree architecture is important for tree management and fruit production. An apple mutant ‘McIntosh Wijcik’, which was discovered as a bud mutation from ‘McIntosh’, exhibits a columnar growth phenotype that is controlled by a single dominant gene, Co. In this study, the mutation and the Co gene were analyzed. Fine mapping narrowed the Co region to a 101 kb region. Sequence analysis of the Co region and the original wild-type co region identified an insertion mutation of an 8202 bp long terminal repeat (LTR) retroposon in the Co region. Segregation analysis using a DNA marker based on the insertion polymorphism showed that the LTR retroposon was closely associated with the columnar growth phenotype. RNA-seq and RT-PCR analysis identified a promising Co candidate gene (91071-gene) within the Co region that is specifically expressed in ‘McIntosh Wijcik’ but not in ‘McIntosh’. The 91071-gene was located approximately 16 kb downstream of the insertion mutation and is predicted to encode a 2-oxoglutarate-dependent dioxygenase involved in an unknown reaction. Overexpression of the 91071-gene in transgenic tobaccos and apples resulted in phenotypes with short internodes, like columnar apples. These data suggested that the 8202 bp retroposon insertion in ‘McIntosh Wijcik’ is associated with the short internodes of the columnar growth phenotype via upregulated expression of the adjacent 91071-gene. Furthermore, the DNA marker based on the insertion polymorphism could be useful for the marker-assisted selection of columnar apples.     

Effects of soil warming and nitrogen foliar applications on bud burst of black spruce

Key message

In mature black spruce, bud burst process is anticipated by soil warming, while delayed by foliar applications of nitrogen; however, the effects depend on growth conditions at the site.

Abstract

The observation of phenological events can be used as biological indicator of environmental changes, especially from the perspective of climate change. In boreal forests, the onset of the bud burst is a key factor in the length of the growing season. With current climate change, the major factors limiting the growth of boreal trees (i.e., temperature and nitrogen availability) are changing and studies on mature trees are limited. The aim of this study was to investigate the effects of soil warming and increased nitrogen (N) deposition on bud burst of mature black spruce [Picea mariana (Mill.) BSP]. From 2008 onwards, an experimental manipulation of these environmental growth conditions was conducted in two stands (BER and SIM) at different altitudes in the boreal forest of Quebec, Canada. An increase in soil temperature (H treatment) and a canopy application of artificial rain enriched with nitrogen (N treatment) were performed. Observations of bud phenology were made during May–July 2012 and 2013. In BER, H treatment caused an anticipation (estimated as 1–3 days); while N treatment, a delay (estimated as 1–2 days but only in 2012) in bud burst. No treatments effect was significant in SIM. It has been demonstrated that soil temperature and N availability can play an important role in affecting bud burst in black spruce but the effects of these environmental factors on growth are closely linked with site conditions.