Characteristics of Persons Who Inject Drugs with Recent HIV Infection in the United States: National HIV Behavioral Surveillance, 2012.

We evaluated characteristics associated with recent HIV infection among persons who inject drugs (PWID) from 19 U.S. cities who participated in 2012 National HIV Behavioral Surveillance. Recent infection was defined as having a reactive HIV test, a Bio-Rad Avidity index cutoff ≤ 30%, no reported HIV diagnosis ≥ 12 months before interview, and no evidence of viral suppression. Of 8667 PWID, 50 (0.6%) were recently HIV infected. Having a greater number of sex partners (≥ 2 partners vs. 0) [prevalence ratio (PR) 4.7, 95% confidence interval (CI) 1.3-17.8], injecting heroin and other drugs (PR 3.0, 95% CI 1.3-6.6) or exclusively non-heroin drugs (PR 5.9, 95% CI 1.7-20.7) compared to injecting only heroin, and having male-male sex in the past year (PR 7.1, 95% CI 3.0-16.6) were associated with recent infection. Promoting not only safe injection practices but also safe sex practices will be key to preventing new HIV infections.

Early HIV Infections Among Men Who Have Sex with Men in Five Cities in the United States.

We tested blood samples from men who have sex with men (MSM) to detect early HIV infection. Early HIV included both acute (infected past 30 days) and recent (estimated recency past 240 days). Acute infections were defined as screen immunoassay (IA) negative/NAAT-positive or IA-positive/Multispot-negative/NAAT-positive. Recent infections were defined as avidity index cutoff <30 % on an avidity-based IA and, (1) not reporting antiretroviral therapy use or, (2) HIV RNA >150 copies/mL. Of 937 samples, 26 % (244) were HIV-infected and of these 5 % (12) were early. Of early infections, 2 were acute and 10 recent; most (8/12) were among black MSM. Early infection was associated with last partner of black race [adjusted relative risk (ARR) = 4.6, confidence intervals (CI) 1.2-17.3], receptive anal sex at last sex (ARR = 4.3, CI 1.2-15.0), and daily Internet use to meet partners/friends (ARR = 3.3, CI 1.1-9.7). Expanding prevention and treatment for black MSM will be necessary for reducing incidence in the United States.

Nitrogen management is essential to prevent tropical oil palm plantations from causing ground-level ozone pollution.

More than half the world's rainforest has been lost to agriculture since the Industrial Revolution. Among the most widespread tropical crops is oil palm (Elaeis guineensis): global production now exceeds 35 million tonnes per year. In Malaysia, for example, 13% of land area is now oil palm plantation, compared with 1% in 1974. There are enormous pressures to increase palm oil production for food, domestic products, and, especially, biofuels. Greater use of palm oil for biofuel production is predicated on the assumption that palm oil is an "environmentally friendly" fuel feedstock. Here we show, using measurements and models, that oil palm plantations in Malaysia directly emit more oxides of nitrogen and volatile organic compounds than rainforest. These compounds lead to the production of ground-level ozone (O(3)), an air pollutant that damages human health, plants, and materials, reduces crop productivity, and has effects on the Earth's climate. Our measurements show that, at present, O(3) concentrations do not differ significantly over rainforest and adjacent oil palm plantation landscapes. However, our model calculations predict that if concentrations of oxides of nitrogen in Borneo are allowed to reach those currently seen over rural North America and Europe, ground-level O(3) concentrations will reach 100 parts per billion (10(9)) volume (ppbv) and exceed levels known to be harmful to human health. Our study provides an early warning of the urgent need to develop policies that manage nitrogen emissions if the detrimental effects of palm oil production on air quality and climate are to be avoided.

Higher allocation to low cost chemical defenses in invasive species of Hawaii.

The capacity to produce carbon-based secondary compounds (CBSC), such as phenolics (including tannins) and terpenes as defensive compounds against herbivores or against neighboring competing plants can be involved in the competition between alien and native plant species. Since the Hawaiian Islands are especially vulnerable to invasions by alien species, we compared total phenolic (TP), total tannin (Tta), and total terpene (TT) leaf contents of alien and native plants on Oahu Island (Hawaii). We analyzed 35 native and 38 alien woody plant species randomly chosen among representative current Hawaiian flora. None of these CBSC exhibited phylogenetic fingerprinting. Alien species had similar leaf TP and leaf Tta contents, and 135% higher leaf TT contents compared with native species. Alien plants had 80% higher leaf TT:N leaf content ratio than native plants. The results suggest that apart from greater growth rate and greater nutrient use, alien success in Oahu also may be linked to greater contents of low cost chemical defenses, such as terpenes, as expected in faster-growing species in resource rich regions. The higher TT contents in aliens may counterbalance their lower investment in leaf structural defenses and their higher leaf nutritional quality. The higher TT provides higher effectiveness in deterring the generalist herbivores of the introduced range, where specialist herbivores are absent. In addition, higher TT contents may favor aliens conferring higher protection against abiotic and biotic stressors. The higher terpene accumulation was independent of the alien species origin, which indicates that being alien either selects for higher terpene contents post-invasion, or that species with high terpene contents are pre-adapted to invasiveness. Although less likely, an originally lower terpene accumulation in Hawaiian than in continental plants that avoids the increased attraction of specialist enemies associated to terpenes may not be discarded.

Alternative algorithms for human immunodeficiency virus infection diagnosis using tests that are licensed in the United States.

Serodiagnosis of human immunodeficiency virus (HIV) infection in the United States has traditionally relied on a sequential two-test algorithm: an initial screen with an enzyme immunoassay (EIA) and reflex testing of EIA-reactive specimens with a more specific supplemental test such as Western blotting or immunofluorescence. The supplemental tests are tedious, subjective, and expensive. In addition, there have been major improvements in the performance and accuracy of the EIA tests as well as the introduction of rapid serologic tests (RT) and HIV nucleic acid amplification tests (NAAT). Related to these improvements is the possibility that alternative algorithms using combinations of currently approved HIV tests may function as well as if not better than the current algorithm, with more flexibility, improved accuracy, and lower cost. To this end, we evaluated the performance of 12 currently licensed tests and 1 in-house HIV test (6 EIA, 4 RT, and 3 NAAT) on panels of plasma samples from HIV-infected (n = 621 HIV type 1 [HIV-1] and 34 HIV-2) and uninfected (n = 513) people and of sequential specimens from people early in seroconversion (183 specimens from 15 patients). Test combinations were analyzed in two dual-test (sensitivity-optimized and specificity-optimized) algorithms and in a three-test (tie-breaking) algorithm, and performance was compared to the conventional algorithm. The results indicate that alternative algorithm strategies with currently licensed tests compare favorably with the conventional algorithm in detecting and confirming established HIV infection. Furthermore, there was a lower frequency of discordant or indeterminate results that require follow-up testing, and there was improved detection of early infection.

Adaptation to promiscuous usage of CC and CXC-chemokine coreceptors in vivo correlates with HIV-1 disease progression.

OBJECTIVE: To study coreceptor usage of sequential primary HIV-1 isolates in a longitudinal follow-up cohort of HIV-1-infected men to understand its contribution to pathogenesis of HIV disease. DESIGN: Viral coreceptor usage of sequential primary isolates from HIV-1-infected individuals was examined at various timepoints and data was compared with CD4 cell counts, rates of disease progression and beta-chemokine production. METHODS: Fifty-eight sequential primary isolates were obtained from four rapid progressors, six late progressors, and three long-term nonprogressors (LTNP) and their coreceptor usage was examined by infection of peripheral blood mononuclear cells (PBMC) from donors with wild-type or non-functional CC-chemokine receptor (CCR)-5, and by infection of GHOST4 cells expressing CD4 and various chemokine receptors [CCR-1-CCR-5, CXC-chemokine receptor (CXCR)-4, BOB/GPR15, BONZO/STRL33]. Production of RANTES and macrophage inflammatory protein (MIP)-1beta was examined using unstimulated or phytohemagglutinin (PHA)-stimulated PBMC isolated from these individuals at multiple timepoints during infection. RESULTS: A switch from single CCR-5 coreceptor usage to multiple coreceptor usage occurred in all four rapid progressors and three out of six late progressors. In addition to the commonly used coreceptors CXCR-4, CCR-5, and CCR-3, some of the viruses isolated from patients in the terminal stage of infection also used CCR-1, CCR-2b, CCR-4, and BOB as coreceptors. The emergence of viral variants capable of utilizing multiple coreceptors generally preceded CD4 cell decline to < 200 x 10(6)/l and correlated with the onset of AIDS. In contrast, three LTNP maintained exclusive usage of CCR-5 over a period of 7-12 years post-infection. Endogenous production of RANTES and MIP-1beta by PBMC from LTNP was not significantly different from rapid and late progressors. However, PHA-driven production of both chemokines was significantly higher in LTNP, suggesting that in vivo activating stimuli might curtail HIV replication by inducing these chemokines. CONCLUSIONS: Viral variants capable of utilizing a broad range of coreceptors correlated with HIV-1 disease progression. In contrast, LTNP maintain exclusive usage of CCR-5 and produce higher levels of beta-chemokines. Thus, both viral and host determinants leading to the emergence of viral variants capable of using an expanded range of coreceptors may be likely determinants of disease progression.

Identification of mixed HIV-1/HIV-2 infections in Brazil by polymerase chain reaction.

Analysis of sera from hospitalized Brazilian patients by whole-virus lysate-based enzyme immunoassay and Western blot indicated that 0.4% were reactive to HIV-2 alone while 4% were reactive to both HIV-1 and HIV-2. When these sera were tested for HIV antibody by type-specific peptide enzyme immunoassays, dual seropositivity was confirmed in only 0.4% of patients. To define genetically the HIV strains within the population, we analyzed peripheral blood mononuclear cells from selected seropositive patients for the presence of HIV-1 and HIV-2 proviral DNA using the polymerase chain reaction (PCR). Independent primers/probes sets were used for the amplification and detection of viral sequences from the long terminal repeat (LTR), gag, and protease (prt) gene regions. Our findings confirmed the serologic evidence of HIV-2 in Brazil and determined the extent of mixed HIV-1 and HIV-2 infections. Detailed evaluation of the amplified viral protease sequences by endonuclease restriction analysis and DNA sequencing independently confirmed mixed HIV-1 and HIV-2 infections in the two patients seropositive for HIV-1 and HIV-2. The data further indicated that these isolates are distinct from the HIV laboratory standards. We interpret the combination of culture and PCR findings to demonstrate the presence of both HIV-1 and HIV-2 in Brazil.

Presence of human T lymphotropic virus types I and II in Ghana, west Africa.

Until recently, HTLV-I was considered to be an Old World virus and HTLV-II was thought to be endemic in the Americas. However, the presence of HTLV-II among Pygmies and other populations of Africa has raised doubts as to whether HTLV-II is primarily a New World virus. The large serosurveys conducted in the urban and rural areas of southern Ghana have identified a 1-2% prevalence for HTLV-I/II. To define the HTLV type, we have used a Western blot assay (HTLV-2.3 blot) that allows simultaneous confirmation and differentiation between HTLVs. Samples (n = 139) were chosen on the basis of previous reactivity with either an enzyme immune assay or r21e-spiked WB results. The WB 2.3 analysis of these specimens identified 55 (40%) to be HTLV positive, 70 (50%) to be HTLV indeterminant, and 14 (10%) to be HTLV negative for HTLV. HTLV seroindeterminant patterns ranged from both gag and env (14 were r21+, p24+, and/or p19+ [all were RIPA negative]) to gag only (21 were p24+/p19+, 16 were p19+, and 7 were p24+), and env only (8 were r21+ and 4 were rgp46+) reactivities. Of the 55 HTLV-positive specimens, 41 were typed as HTLV-I, 9 were HTLV-II, and 5 could not be typed (HTLV-I/II). Of the nine HTLV-II-positive specimens, three were from patients with Burkitt's lymphoma and six were from healthy individuals (two pregnant women) with no obvious risk factors for HTLV-II.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcriptional activation of the intercellular adhesion molecule 1 (CD54) gene by human T lymphotropic virus types I and II Tax is mediated through a palindromic response element.

In vitro infection of T cells with human T lymphotropic virus types I and II (HTLV-I and HTLV-II) resulted in constitutive expression of ICAM-1. Higher levels of ICAM-1 mRNA were expressed in HTLV-transformed cell lines (MT-2, MoT, C8166) when compared with uninfected T cell lines (A301). We demonstrate that this activation is conferred through a site on the ICAM-1 promoter that is activated in trans by the Tax protein of HTLV-I and HTLV-II. Enhanced promoter activity was detected when the ICAM-1 construct (-1162/+1) was transfected into HTLV-I-infected (MT-2), HTLV-II-infected (MoT, AI 1050), or an HTLV-I Tax-only-expressing (C8166) cell line as compared to the uninfected T cell line (A3.01). Cotransfection of the uninfected T cell line A3.01 with the ICAM construct along with Tax-I and Tax-II expression plasmid also resulted in increased promoter activity. Furthermore, experiments with deletion constructs of the ICAM-1 promoter region indicated that a region between -88 and -53 bp relative to the transcription start site is sufficient for Tax-inducible CAT expression. This segment includes an 11-bp palindromic segment (TTTCCGGGAAA) that has homology with the IFN-gamma and IL-6 response element. An 11-bp segment containing this regulatory region proved to be sufficient to confer Tax-I and Tax-II inducibility on a heterologous promoter (TK-CAT). Taken together these findings indicate that constitutive expression of ICAM-1 by HTLV-infected cells is influenced by the viral trans-activator protein Tax. This increased expression of ICAM-1 in response to the Tax protein may play an important role in the lymphoproliferation associated with HTLV infection.

Omeprazole is more effective than cimetidine in the prevention of recurrence of GERD-associated heartburn and the occurrence of underlying oesophagitis.

BACKGROUND: There is documentation of the long-term use of omeprazole 10 mg o.d. in patients with reflux oesophagitis but not in the large number of gastrooesophageal reflux disease (GERD) patients without oesophagitis. There is also a paucity of data on the long-term use of cimetidine in GERD patients. METHODS: One hundred and fifty-six patients (100 male) who previously had symptomatic non-ulcerative oesophagitis (81%) or symptoms without oesophagitis (19%), were recruited. All patients were in symptomatic remission following 4 weeks of omeprazole 20 mg o.d. or cimetidine 400 mg q.d.s. and, if required, a further 4 weeks of omeprazole 20 mg o.d. Patients were randomized to receive, double-blind, either omeprazole 10 mg o.m. (n = 77) or cimetidine 800 mg nocte (n = 79) for 24 weeks. RESULTS: A greater proportion of patients receiving omeprazole, compared with cimetidine, were in symptomatic remission after 12 (69 vs. 27%) and 24 weeks (60 vs. 24%) (each P < 0.0001). The median time to symptomatic relapse was longer for patients receiving omeprazole (169 vs. 15 days) (P = 0.0001). Of patients leaving the study in symptomatic remission, a greater proportion receiving omeprazole, compared with cimetidine, was free of oesophagitis (84 vs. 53%) (P < 0.05). CONCLUSION: Omeprazole 10 mg o.m. is more effective than cimetidine 800 mg nocte in the prevention of recurrence of GERD-associated heartburn and the occurrence of underlying oesophagitis.

Evaluation of fetal monitoring by telemetry.

A radiotelemetry system for the direct monitoring of fetal heart rate and intrauterine pressure during labor has been evaluated at 5 different institutions. A conventional fetal scalp electrode and a special intrauterine pressure sensor are connected to a radio transmitter placed on the patient's thigh. The receiver can be located up to 50 feet away from the transmitter and is either a self-contained monitor or a unit that converts a conventional fetal monitor to a telemetric one. The telemetry recordings are of similar quality to those obtained from conventional monitors. Telemetry allows for greater patient comfort and mobility as well as greater convenience to the clinical staff. Continuous data can be obtained from patients while they are ambulatory or sitting in a chair as well as while they are in bed.

Ehlers-Danlos syndrome simulating child abuse.

We report an unusual insight into the impact of an inherited skin disorder on persons and their families through a child abuse evaluation that was concluded by the diagnosis of Ehlers-Danlos syndrome (EDS) in a 6-year-old girl. To our knowledge, no similar case has been reported in the literature, although other such cases of confusion in diagnosis have been confirmed.

Disruptive behaviors in the classroom: initial standardization data on a new teacher rating scale.

This study presents initial standardization data on the Sutter-Eyberg Student Behavior Inventory (SESBI), a teacher-completed measure of disruptive classroom behaviors. SESBIs were completed on 1116 children in kingergarten through fifth grade in a rural eastern Washington school district. Various analyses (Cronbach's alpha, corrected item-total correlations, average interitem correlations, principal components analyses) indicated that the SESBI provides a homogeneous measure of disruptive behaviors. Support was also found for three factors within the scale (e.g., overt aggression, oppositional behavior, and attentional difficulties). While the child's age did not have a significant effect on the SESBI, the child's gender did have a significant effect on scale scores as well as on most of the items, with males being rated more problematic than females. The SESBI was also able to discriminate between children in treatment for behavioral problems or learning disabilities and children not in treatment.

Survey of veterinary conference attendees for evidence of zoonotic infection by feline retroviruses.

OBJECTIVE: To examine exposure risks, possibility of zoonosis, and potential disease associations for feline retroviruses among a group of occupationally exposed individuals. DESIGN: Unlinked voluntary cross-sectional epidemiologic survey. SAMPLE POPULATION: 204 veterinarians, laboratory scientists, and other occupationally exposed individuals who attended a veterinary conference on feline geriatric medicine. PROCEDURE: Blood was collected from participants who also completed a 13-question survey requesting demographic, occupational, exposure, and health information. Blood specimens were fractionated into plasma and mononuclear cell components. Plasma was tested for antibodies against feline immunodeficiency virus (FIV) and feline foamy virus (FeFV), as well as p27 antigen of FeLV. Mononuclear cell lysates were tested for FeLV provirus. RESULTS: Subjects reported extensive duration of work with cats (mean, 17.3 years) and multiple high-risk exposures (eg, cat bites, scratches, and injuries with sharp instruments) per year. However, neither serologic nor molecular evidence of zoonosis with any of the 3 feline retroviruses was detected. CONCLUSIONS AND CLINICAL RELEVANCE: Veterinarians encounter occupational exposures to animal material that place them at high risk for zoonoses. For feline retroviruses, the risk of zoonosis among healthy adult humans appears to be extremely small. However, potential for retroviral zoonosis, especially for viruses such as FeLV and FeFV that can replicate in human cells, cannot be eliminated, and universal precautions to reduce potential exposures should be used when handling sick cats.

Time perspective of female prisoners related to success in a training program.

This study compared scores on future time perspective of 50 female prison inmates who were participating in a vocational training program within the Julia Tutwiler Prison. There was no control group. Zimbardo's measure of future time perspective, administered a year earlier, was associated with success or failure in the program. Associations of future time perspectives with education, age, and ethnicity were investigated. An analysis of covariance between the 21 graduates and 16 dropouts while controlling for education was significant. Despite the incomplete design, some implications are of interest.

Modulation of morphine-induced antinociception in mice by histamine H3-receptor ligands.

The effects of compounds active at histamine H3-receptors on morphine-induced antinociception have been investigated in thermal and chemical tests in mice; tail-immersion (50 degrees C) and hot-plate (49 degrees and 55 degrees C) tests and acetic acid-induced writhing. Neither (R)alpha-methylhistamine, a specific agonist, (S)alpha-methylhistamine, a chemical control, nor thioperamide, an antagonist, had any antinociceptive action alone but thioperamide (3 mg kg-1) attenuated the effects of morphine in the tail-immersion test while (R)alpha-methylhistamine (1 mg kg-1), but not the (S) isomer, potentiated its effects in the hot-plate test at 55 degrees C. These results are consistent with the morphine potentiation seen with H1-antagonists and suggest that central histaminergic mechanisms can modulate opioid actions.

Cloning and expression of a human T-lymphotropic virus type 1 protein with reverse transcriptase activity.

Unlike most other characterized retroviruses, there is little published information on the biochemical properties of human T-lymphotropic virus type 1 (HTLV-1) reverse transcriptase (RT). Specifically, no reports of a cloned functional RT enzyme have been published. Since the RT enzyme is an essential component of the virus, our objective was to clone, express, and purify a functional RT enzyme from HTLV-1. Our approach was to clone and express a protein of approximately 60 to 65 kDa that we hypothesized would correspond to the RT region encoded by the pol reading frame. The predicted region encoding the RT enzyme comprised nucleotides 2617 to 4312 of the HTLV-1 MT-2 isolate. A putative RT gene was obtained by PCR and was ligated into various prokaryotic expression vectors. A novel cloning approach allowed us to generate a stable clone in the prokaryotic expression vector pGEX-4T-1 and produce a recombinant protein of approximately 60 to 65 kDa. The partially purified protein displays RT activity in both amplification RT (AMP-RT) assays and traditional RT assays. This is the first report of a cloned protein from HTLV-1 which displays RT activity and is the first step in the characterization of HTLV-1 RT.

Spontaneous lymphocyte proliferation in human T-cell lymphotropic virus type I (HTLV-I) and HTLV-II infection: T-cell subset responses and their relationships to the presence of provirus and viral antigen production.

Spontaneous lymphocyte proliferation (SLP) during in vitro culture of mononuclear cells (MCs) characterizes over half of asymptomatic individuals infected with human T-cell lymphotropic virus type I (HTLV-I) or HTLV-II. Both CD4 and CD8 T-cell subsets within MC cultures are activated during SLP, as judged by high-density CD25 (CD25bright) expression; it is unclear, however, whether both cell subsets can directly undergo SLP. In the present investigation, the SLP capacities of purified CD8 and CD4 cells were examined in subjects infected with HTLV-I (n = 19) or HTLV-II (n = 54) in relation to the SLP status of MCs from each subject. No increase in SLP was observed for CD8 or CD4 cells from SLP-negative (SLP-) HTLV-infected subjects, whereas robust SLP characterized CD8 cells from all SLP-positive (SLP+) individuals, regardless of HTLV type. In contrast, SLP+ CD4 cells characterized only 23% (7 of 31) of HTLV-II+ SLP+ individuals, whereas SLP+ CD4 cells characterized 100% of HTLV-I+ SLP+ individuals. In cocultures of HTLV-II+ SLP+ CD8 cells and autologous SLP- CD4 cells, sizable proportions of both CD8 cells and CD4 cells coexpressed CD25bright, suggesting that SLP- CD4 cells were activated in the presence of SLP+ CD8 cells. PCR analysis for tax sequences detected provirus in most CD4- and CD8-cell preparations from HTLV-seropositive individuals, regardless of type and the SLP status of cell subsets. To determine whether SLP was associated with activation of viral genes, levels of HTLV-I and HTLV-II core antigen (Ag) in supernatants were measured. Viral Ag production and SLP responses were significantly correlated for both CD4 and CD8 cells in both HTLV-I and HTLV-II infections. However, inhibition of CD8- or CD4-cell SLP by cyclosporin A or anti-Tac (anti-CD25) did not reduce Ag production, indicating that Ag production is not coupled to SLP. These findings show that CD4 cells from SLP+ HTLV-I+ and SLP+ HTLV-II+ individuals differ in SLP capacity, that the absence of SLP does not indicate a lack of infection, and that production of viral Ag is associated with, but not dependent on, SLP.

Performance characteristics and applications of a proton transfer reaction-mass spectrometer for measuring volatile organic compounds in ambient air.

Data illustrating the performance characteristics of a proton transfer reaction-mass spectrometer (PTR-MS) under both laboratory and field conditions are presented. Under laboratory conditions, we demonstrate that PTR-MS measures (within 10%) a 2.6 ppbv concentration of gaseous dimethyl sulfide. Using a stepwise dilution of a gaseous isoprene standard, we demonstrate the linearity of the response of PTR-MS across 3 orders of magnitude of mixing ratios, from 100 ppbv to less than 100 pptv. By combining this data set with that of its monosubstituted 13C isotopic analogue, we demonstrate the ability of the instrumentto reliably measure concentrations as low as approximately 50 pptv and to detect concentrations at significantly lower levels. We conclude our laboratory characterization by investigating the components of the instrument noise signal (drift, mean, and range) and develop an expression (noise statistic) that reliably predicts the instrumental noise associated with any signal across a wide range of masses. In the field, we deployed a PTR-MS at a clean-air coastal site and an urban kerbside monitoring station to demonstrate the measurement of atmospheric dimethyl sulfide and benzene concentrations, respectively. At both sites, we were able to monitor diurnal variations in concentrations at unprecedented temporal resolutions (<5 min between successive measurements). We then demonstrate how the noise statistic can be applied to enable real fluctuations in atmospheric VOC concentrations to be reliably distinguished from instrument noise. We conclude by demonstrating how PTR-MS can be used to measure real-time VOC emission rate changes from vegetation in response to external forcing by examining the effect varying photon-flux density has upon emissions of isoprene from a Sitka spruce tree.