Plant Gas Exchange of Two Bunchgrasses in Relation to Herbivory Tolerance

The occurrence of compensatory photosynthesis was examined in the field during the spring for all foliage elements on two Agropyron bunchgrass species that differ in their evolutionary history of grazing pressure. Compensatory photosynthesis did occur in many individual foliage elements during at least part of their ontogeny. For both species, compensatory photosynthesis was related primarily to delayed leaf senescence and increased soluble protein concentrations, but not to an improvement in the water status of clipped plants. Photosynthetic water use efficiency and photosynthetic rates per unit soluble protein of foliage on partially defoliated plants were not increased following the clipping treatments. Light and temperature dependencies of gas exchange measurements were usually very similar between the two Agropyron species. However, gas exchange rates per unit foliage area of leaves exserted late in the spring on~ spicatum plants were significantly different from those on A. desertorum plants when these leaves were senescing. To determine the ecological significance of these differences between species for light and temperature dependencies, the average carbon gain and water loss rate per tiller were estimated. The differences between species for carbon gain and water loss rates per tiller in this environment were substantially less than the individual leaf gas exchange differences between species. Photosynthetic activity and survival of leaves were also determined during the fall, winter, and early spring for the two Agropyron species in the field. A large proportion of the leaves of both species survived the winter. Photosynthetic rates of both species declined as air temperature dropped during the fall, were slightly positive during the winter between periods of snow cover, and increased during the early spring. Even though there is potential for photosynthesis during a winter with intermittent snow cover, total plant saccharide pools were barely maintained over such a winter. Although A. desertorum and A. spicatum were exposed to different levels of grazing pressure during their evolutionary history, the phenology, water status, and gas exchange rates of foliage and tillers were very similar both for undefoliated as well as partially defoliated plants. Therefore, we conclude that compensatory photosynthesis does not appear to be an important ecological component of herbivory tolerance for these species

Chlorophyll Fluorescence Probe of Ultraviolet-B Photoinhibition of Primary Photoreactions in Intact Leaves

Damage to primary photosynthetic reactions caused by environmental stress can be assessed by measurement of chlorophyll fluorescence induction in intact leaves. This approach was applied in studies of ultraviolet-B photoinhibition of photosynthesis in Pisum sativum L. and Rumex patientia L. leaves. At ultraviolet-B dose rates insufficient to cause inhibition of net photosynthesis, changes in the magnitude of fluorescence transients did occur, which suggested direct effects on chloroplast pigments in Pisum and inhibition of photosynthetic electron transport between the photosystems in both species. Leaves of these two species subjected to a much higher dose rate had a significant reduction of net photosynthesis and changes in the magnitude of fluorescence transients that indicated partial loss of water-splitting capability and direct effects on chloroplast pigments. Ultraviolet radiation-induced changes of photosynthetic thylakoid membranes may be ultimately responsible for these disruptions of the primary photosynthetic reactions

Functional responses of plants to elevated atmospheric CO 2 – do photosynthetic and productivity data from FACE experiments support early predictions?

Summary  1 I.   Introduction  2 II.   Early assessments of [CO 2 ] responses in natural ecosystems   2 III.   Global network of FACE sites   4 IV.   Assimilation and leaf N-content   5 V.   Primary productivity  13 VI.   Response of plant functional types  20 VII.  Conclusions   23 Acknowledgements  24 References   24 Summary Results from 16 free-air CO 2 enrichment (FACE) sites representing four different global vegetation types indicate that only some early predictions of the effects of increasing CO 2 concentration (elevated [CO 2 ]) on plant and ecosystem processes are well supported. Predictions for leaf CO 2 assimilation (A net ) generally fit our understanding of limitations to photosynthesis, and the FACE experiments indicate concurrent enhancement of photosynthesis and of partial downregulation. In addition, most herbaceous species had reduced leaf nitrogen (N)-content under elevated [CO 2 ] and thus only a modest enhancement of A net , whereas most woody species had little change in leaf N with elevated [CO 2 ] but a larger enhancement of A net . Early predictions for primary production are more mixed. Predictions that enhancement of productivity would be greater in drier ecosystems or in drier years has only limited support. Furthermore, differences in productivity enhancements among six plant functional types were not significant. By contrast, increases in productivity enhancements with increased N availability are well supported by the FACE results. Thus, neither a resource-based conceptual model nor a plant functional type conceptual model is exclusively supported by FACE results, but rather both species identity and resource availability are important factors influencing the response of ecosystems to elevated [CO 2 ]. © New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01033.xPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66140/1/j.1469-8137.2004.01033.x.pd

Ecophysiological Responses of Three Riparian Graminoids to Changes in the Soil Water Table

We examined the effect of changes in soil water table on the water relations, leaf gas exchange, and aboveground biomass of three riparian graminoids native to the semiarid western United States: Carex lanuginosa, Juncus balticus, and Carex nebrascensis. All three species co-occur at the wettest microhabitats within riparian corridors, but J. balticus and C. nebrascensis extend into drier areas. Lowering the water table to 1 m had little effect on the leaf gas exchange characteristics of the three graminoids. In the greenhouse, experimental reductions of the water table when plants had three fully mature leaves did not affect gas exchange rates or water potential in any of the three species. Lowering the water table when plants had one fully mature leaf resulted in limited differences between plants grown under high and low water table in J. balticus and C. lanuginosa. Further, these differences were only apparent after long periods of depressed water table (19 wk). In the field, rates of leaf gas exchange did not differ between plants growing near the creek from those occurring distant from the creek. Three factors contribute to the ability of these riparian graminoids to maintain favorable gas exchange and water relations across a range of water table depths. Each species appears to adjust rooting depth to, or just above, the shallow saturated zone. In the held, C. nebrascensis and J. balticus exhibited reductions of aboveground live biomass at locations far from the creek compared to those near the creek. Small adjustments of osmotic potential and the bulk modulus of elasticity help cells of C. nebrascensis and J. balticus maintain turgor as water table drops during the season. The limited distribution of C. lanuginosa near the creek may result, in part, from a higher biomass allocation to leaves and a less efficient water transport from roots to leaves, particularly when depressions of the water table occur during early growth stages

Predicting the outcomes of treatment to eradicate the latent reservoir for HIV-1

Massive research efforts are now underway to develop a cure for HIV infection, allowing patients to discontinue lifelong combination antiretroviral therapy (ART). New latency-reversing agents (LRAs) may be able to purge the persistent reservoir of latent virus in resting memory CD4+ T cells, but the degree of reservoir reduction needed for cure remains unknown. Here we use a stochastic model of infection dynamics to estimate the efficacy of LRA needed to prevent viral rebound after ART interruption. We incorporate clinical data to estimate population-level parameter distributions and outcomes. Our findings suggest that approximately 2,000-fold reductions are required to permit a majority of patients to interrupt ART for one year without rebound and that rebound may occur suddenly after multiple years. Greater than 10,000-fold reductions may be required to prevent rebound altogether. Our results predict large variation in rebound times following LRA therapy, which will complicate clinical management. This model provides benchmarks for moving LRAs from the lab to the clinic and can aid in the design and interpretation of clinical trials. These results also apply to other interventions to reduce the latent reservoir and can explain the observed return of viremia after months of apparent cure in recent bone marrow transplant recipients and an immediately-treated neonate.Comment: 8 pages main text (4 figures). In PNAS Early Edition http://www.pnas.org/content/early/2014/08/05/1406663111. Ancillary files: SI, 24 pages SI (7 figures). File .htm opens a browser-based application to calculate rebound times (see SI). Or, the .cdf file can be run with Mathematica. The most up-to-date version of the code is available at http://www.danielrosenbloom.com/reboundtimes

Learning Nearest Neighbor Graphs from Noisy Distance Samples

We consider the problem of learning the nearest neighbor graph of a dataset of n items. The metric is unknown, but we can query an oracle to obtain a noisy estimate of the distance between any pair of items. This framework applies to problem domains where one wants to learn people\u27s preferences from responses commonly modeled as noisy distance judgments. In this paper, we propose an active algorithm to find the graph with high probability and analyze its query complexity. In contrast to existing work that forces Euclidean structure, our method is valid for general metrics, assuming only symmetry and the triangle inequality. Furthermore, we demonstrate efficiency of our method empirically and theoretically, needing only O(n log(n)Δ-2) queries in favorable settings, where Δ-2 accounts for the effect of noise. Using crowd-sourced data collected for a subset of the UT Zappos50K dataset, we apply our algorithm to learn which shoes people believe are most similar and show that it beats both an active baseline and ordinal embedding

Finding All ∈-Good Arms in Stochastic Bandits

The pure-exploration problem in stochastic multi-armed bandits aims to find one or more arms with the largest (or near largest) means. Examples include finding an ∈-good arm, best-arm identification, top-k arm identification, and finding all arms with means above a specified threshold. However, the problem of finding all ∈-good arms has been overlooked in past work, although arguably this may be the most natural objective in many applications. For example, a virologist may conduct preliminary laboratory experiments on a large candidate set of treatments and move all ∈-good treatments into more expensive clinical trials. Since the ultimate clinical efficacy is uncertain, it is important to identify all ∈-good candidates. Mathematically, the all-∈-good arm identification problem presents significant new challenges and surprises that do not arise in the pure-exploration objectives studied in the past. We introduce two algorithms to overcome these and demonstrate their great empirical performance on a large-scale crowd-sourced dataset of 2.2Mratings collected by the New Yorker Caption Contest as well as a dataset testing hundreds of possible cancer drugs

Invasion Triangle: An Organizational Framework for Species Invasion

Species invasion is a complex, multifactor process. To encapsulate this complexity into an intuitively appealing, simple, and straightforward manner, we present an organizational framework in the form of an invasion triangle. The invasion triangle is an adaptation of the disease triangle used by plant pathologists to help envision and evaluate interactions among a host, a pathogen, and an environment. Our modification of this framework for invasive species incorporates the major processes that result in invasion as the three sides of the triangle: (1) attributes of the potential invader; (2) biotic characteristics of a potentially invaded site; and (3) environmental conditions of the site. The invasion triangle also includes the impact of external influences on each side of the triangle, such as climate and land use change. This paper introduces the invasion triangle, discusses how accepted invasion hypotheses are integrated in this framework, describes how the invasion triangle can be used to focus research and management, and provides examples of application. The framework provided by the invasion triangle is easy to use by both researchers and managers and also applicable at any level of data intensity, from expert opinion to highly controlled experiments. The organizational framework provided by the invasion triangle is beneficial for understanding and predicting why species are invasive in specific environments, for identifying knowledge gaps, for facilitating communication, and for directing management in regard to invasive species

Relativistic Diskoseismology. I. Analytical Results for 'Gravity Modes'

We generalize previous calculations to a fully relativistic treatment of adiabatic oscillations which are trapped in the inner regions of accretion disks by non-Newtonian gravitational effects of a black hole. We employ the Kerr geometry within the scalar potential formalism of Ipser and Lindblom, neglecting the gravitational field of the disk. This approach treats perturbations of arbitrary stationary, axisymmetric, perfect fluid models. It is applied here to thin accretion disks. Approximate analytic eigenfunctions and eigenfrequencies are obtained for the most robust and observable class of modes, which corresponds roughly to the gravity (internal) oscillations of stars. The dependence of the oscillation frequencies on the mass and angular momentum of the black hole is exhibited. These trapped modes do not exist in Newtonian gravity, and thus provide a signature and probe of the strong-field structure of black holes. Our predictions are relevant to observations which could detect modulation of the X-ray luminosity from stellar mass black holes in our galaxy and the UV and optical luminosity from supermassive black holes in active galactic nuclei.Comment: 31 pages, 6 figures, uses style file aaspp4.sty, prepared with the AAS LATEX macros v4.0, significant revision of earlier submission to include modes with axial index m>