Quantification of leaf-scale light energy allocation and photoprotection processes in a Mediterranean pine forest under extensive seasonal drought

Abstract Photoprotection strategies in a Pinus halepensis forest at the dry timberline that shows sustained photosynthetic activity during 6-7 months summer drought were characterized and quantified under field conditions. Measurements of chlorophyll fluorescence, leaf-level gas exchange and pigment concentrations were made in both control and summer-irrigated plots, providing the opportunity to separate the effects of atmospheric from soil water stress on the photoprotection responses. The proportion of light energy incident on the leaf surface ultimately being used for carbon assimilation was 18% under stress-free conditions (irrigated, winter), declining to 4% under maximal stress (control, summer). Allocation of absorbed light energy to photochemistry decreased from 25 to 15% (control) and from 50% to 30% (irrigated) between winter and summer, highlighting the important role of pigment-mediated energy dissipation processes. Photorespiration or other non-assimilatory electron flow accounted for 15-20% and less than 10% of incident light energy during periods of high and low carbon fixation, respectively, representing a proportional increase in photochemical energy going to photorespiration in summer but a decrease in the absolute amount of photorespiratory CO2 loss. Resilience of the leaf photochemical apparatus was expressed in the complete recovery of photosystem II efficiency (ΦPSII) and relaxation of the xanthophyll de-epoxidation state (DPS) on the diurnal cycle throughout the year, and no seasonal decrease in pre-dawn maximal photosystem II efficiency (Fv/Fm). The response of CO2 assimilation and photoprotection strategies to stomatal conductance and leaf water potential appeared independent of whether stress was due to atmospheric or soil water deficits across seasons and treatments. The range of protection characteristics identified provide insights into the relatively high carbon economy under these dry conditions, conditions which are predicted for extended areas in the Mediterranean and other regions due to global climate change

Critical Behavior of the Random-Field Ising Model

We study the critical properties of the random field Ising model in general dimension d using high-temperature expansions for the susceptibility, χ=∑j[〈σiσj⟩T-〈σi⟩T〈σj⟩T]h and the structure factor, G=∑j[〈σiσj⟩T]h, where 〈⟩T indicates a canonical average at temperature T for an arbitrary configuration of random fields and [ ]h indicates an average over random fields. We treated two distributions of random fields, the bimodal in which each hi=±h0 and a Gaussian distribution in which each hi has variance h02. We obtained series for χ and G in the form ∑n=1,15an(g,d)(J/T)n, where J is the exchange constant and the coefficients an(g,d) are polynomials in g≡h02/J2 and in d. We assume that as T approaches its critical value, Tc, one has χ~(T-Tc)−γ and G~(T-Tc)−γ. For dimensions above d=2 we find a range of values of g for which the critical exponents obtained from our series seem not to depend on g. For large values of g our results show a g dependence which is attributable to either a tricritical point or a first-order transition. All our results for critical exponents suggest that γ¯=2γ, in agreement with the two-exponent scaling picture. In addition we have also constructed series for the amplitude ratio, A=(G/χ2)(T2)/(gJ2). We find that A approaches a constant value as T→Tc (consistent with γ¯=2γ) with A~1. It appears that A is somewhat larger for the bimodal than for the Gaussian model, in agreement with a recent analysis at high d

Evidence for Two Exponent Scaling in the Random Field Ising Model

Novel methods were used to generate and analyze new 15 term high temperature series for both the (connected) susceptibility χ and the structure factor (disconnected susceptibility) χd for the random field Ising model with dimensionless coupling K=J/kT, in general dimension d. For both the bimodal and the Gaussian field distributions, with mean square field J2g, we find that (χd-χ)/K2gχ2=1 as T→Tc(g), for a range of [h2]=J2g and d=3,4,5. This confirms the exponent relation γ¯=2γ (where χd~t−γ¯, χ~t−γ, t=T-Tc) providing that random field exponents are determined by two (and not three) independent exponents. We also present new accurate values for γ

American Gut: an Open Platform for Citizen Science Microbiome Research

McDonald D, Hyde E, Debelius JW, et al. American Gut: an Open Platform for Citizen Science Microbiome Research. mSystems. 2018;3(3):e00031-18