Streaming Success: Harnessing Social Media for Dynamic Radiology Education

Social media are increasingly used as tools in radiologists education. This article describes features that aid with the selection of SM platforms, and how to emulate educator roles in the digital world. In addition, we summarize best practices regarding curating and delivering stellar content, building a SM brand, and rules of professionalism when using SM in radiology education.

Training the New Radiologists: Approaches for Education

The field of Radiology is continually changing, requiring corresponding evolution in both medical student and resident training to adequately prepare the next generation of radiologists. With advancements in adult education theory and a deeper understanding of perception in imaging interpretation, expert educators are reshaping the training landscape by introducing innovative teaching methods to align with increased workload demands and emerging technologies. These include the use of peer and interdisciplinary teaching, gamification, case repositories, flipped-classroom models, social media, and drawing and comics. This publication aims to investigate these novel approaches and offer persuasive evidence supporting their incorporation into the updated Radiology curriculum.

Conquering Educational Mountains: Maintaining a Radiology Clinical Education Track

Establishing a clinical education track as part of a radiology residency is essential in shaping future radiology educators. Many obstacles will be encountered while starting, maintaining, and improving these educational pathways. Hurdles may include recruiting suitable residents for the track, recruiting and supporting faculty advisors, sustaining long-term resident engagement, counteracting educational exclusivity, and providing adequate time and financial support. Although every program and institution may face individualized "mountains" to overcome, they are not insurmountable. The goal of this review is to address different conflicts we have encountered while maintaining the clinical education tract at our institution and to provide tips for overcoming them.

Imaging Intracranial Aneurysms in the Endovascular Era: Surveillance and Posttreatment Follow-up

While most intracranial aneurysms (IAs) remain asymptomatic over a patient's lifetime, those that rupture can cause devastating outcomes. The increased usage and quality of neuroimaging has increased detection of unruptured IAs and driven an increase in surveillance and treatment of these lesions. Standard practice is to treat incidentally discovered unruptured IAs that confer high rupture risk as well as ruptured IAs to prevent rehemorrhage. IAs are increasingly treated with coil embolization instead of microsurgical clipping; more recently, flow diversion and intrasaccular flow disruption have further expanded the versatility and utility of endovascular IA treatment. Imaging is increasingly used for posttreatment IA follow-up in the endovascular era. While cerebral angiography remains the standard for IA characterization and treatment planning, advances in CT and CT angiography and MR angiography have improved the diagnostic accuracy of noninvasive imaging for initial diagnosis and surveillance. IA features including size, dome-to-neck ratio, location, and orientation allow rupture risk stratification and determination of optimal treatment strategy and timing. The radiologist should be familiar with the imaging appearance of common IA treatment devices and the expected imaging findings following treatment. In distinction to clipping and coil embolization, flow diversion and intrasaccular flow disruption induce progressive aneurysm obliteration over months to years. Careful assessment of the device; the treated IA; adjacent brain, bone, meninges; and involved extracranial and intracranial vasculature is crucial at posttreatment follow-up imaging to confirm aneurysm obliteration and identify short-term and long-term posttreatment complications. An invited commentary by Chatterjee is available online. Online supplemental material and the slide presentation from the RSNA Annual Meeting are available for this article. ^©RSNA, 2022.

Evolving Radiology Trainee Neuroimaging Workloads: A National Medicare Claims-based Analysis

RATIONALE AND OBJECTIVES

While radiology training programs aim to prepare trainees for clinical practice, the relationship between trainee, and national radiology workforce demands is unclear. This study assesses changing radiology trainee neuroimaging workloads nationwide for neuroimaging studies.

MATERIALS AND METHODS

Using aggregate Medicare claims files from 2002 to 2018, we identified all computed tomography (CT) and magnetic resonance (MR) examinations of the brain, head and neck, and spine (hereafter "neuroimaging") in Medicare fee-for-service beneficiaries nationwide. Using separate Medicare files, we calculated population utilization rates, and work relative value unit (wRVU) weights of all diagnostic neuroradiology services. Using claims modifiers, we identified services rendered by radiology trainees. Using separate national trainee enrollment files, we calculated mean annual per trainee wRVUs.

RESULTS

Between 2002 and 2018, total Medicare neuroimaging claims increased for both radiologists overall (86.1%) and trainees (162.5%), including increases in both CT (102.9% vs 196.8%), and MR (59.9% vs 106.6%). The national percentage of all radiologist neuroimaging wRVUs rendered by trainees increased 46.1% (3.8% of all wRVUs nationally in 2002 to 5.6% in 2018). National trainee increases were present across all neuroimaging services but greatest for head and neck CT (+86.5%). Mean annual per radiology trainee neuroimaging Medicare wRVUs increased +174.9% (42.1 per trainee in 2002 to 115.70 in 2018). Mean per trainee wRVU increases were greatest for spine CT (+394.2%) but present across all neuroimaging services.

CONCLUSION

As neuroimaging utilization in Medicare beneficiaries has grown, radiology trainee neuroimaging workloads have increased disproportionately.

Factors Influential in the Selection of Radiology Residents in the Post-Step 1 World: A Discrete Choice Experiment

OBJECTIVES

Reporting of United States Medical Licensing Examination Step 1 results will transition from a numerical score to a pass or fail result. We sought an objective analysis to determine changes in the relative importance of resident application attributes when numerical Step 1 results are replaced.

METHODS

A discrete choice experiment was designed to model radiology resident selection and determine the relative weights of various application factors when paired with a numerical or pass or fail Step 1 result. Faculty involved in resident selection at 14 US radiology programs chose between hypothetical pairs of applicant profiles between August and November 2020. A conditional logistic regression model assessed the relative weights of the attributes, and odds ratios (ORs) were calculated.

RESULTS

There were 212 participants. When a numerical Step 1 score was provided, the most influential attributes were medical school (OR: 2.35, 95% confidence interval [CI]: 2.07-2.67), Black or Hispanic race or ethnicity (OR: 2.04, 95% CI: 1.79-2.38), and Step 1 score (OR: 1.8, 95% CI: 1.69-1.95). When Step 1 was reported as pass, the applicant's medical school grew in influence (OR: 2.78, 95% CI: 2.42-3.18), and there was a significant increase in influence of Step 2 scores (OR: 1.31, 95% CI: 1.23-1.40 versus OR 1.57, 95% CI: 1.46-1.69). There was little change in the relative influence of race or ethnicity, gender, class rank, or clerkship honors.

DISCUSSION

When Step 1 reporting transitions to pass or fail, medical school prestige gains outsized influence and Step 2 scores partly fill the gap left by Step 1 examination as a single metric of decisive importance in application decisions.

Developing a Resident-led First-year Radiology Resident Lecture Series

The first year of radiology residency presents many unique challenges, from transitioning into a completely new, specialized field to preparing for call. Implementation of a longitudinal lecture series dedicated towards the clinical demands of being a first-year radiology resident may improve their knowledge and comfort level, as well as benefit the entire program. In this article, we outline our experience with the development of a resident-led dedicated first-year radiology resident lecture series providing targeted, high-yield instruction on rotation logistics, basic physics and artifacts, examination protocolling, and common and "don't miss" pathology.

Cytotoxic Lesion of the Corpus Callosum in an Adolescent with Multisystem Inflammatory Syndrome and SARS-CoV-2 Infection

Multisystem inflammatory syndrome in children is a recently described complication in the late phase of Severe Acute Respiratory Syndrome coronavirus 2 (SARS-CoV-2) infection involving systemic hyperinflammation and multiorgan dysfunction. The extent of its clinical picture is actively evolving and has yet to be fully elucidated. While neurologic manifestations of SARS-CoV-2 are well-described in the adult population, reports of neurologic complications in pediatric patients with SARS-CoV-2 infection are limited. We present a pediatric patient with SARS-CoV-2 infection with development of multisystem inflammatory syndrome and acute encephalopathy causing delirium who was found to have a cytotoxic lesion of the corpus callosum on neuroimaging. Cytotoxic lesions of the corpus callosum are a well-known, typically reversible entity that can occur in a wide range of conditions, including infection, seizure, toxins, nutritional deficiencies, and Kawasaki disease. We hypothesized that the cytotoxic lesion of the corpus callosum, in the index case, was secondary to the systemic inflammation from SARS-CoV-2 infection, resulting in multisystem inflammatory syndrome in children.

Ischemic Infarction in Young Adults: A Review for Radiologists

Ischemic strokes in young adults are devastatingly debilitating and increasingly frequent. Stroke remains the leading cause of serious disability in the United States. The consequences of this familiar disease in this atypical age group are especially detrimental and long lasting. Ischemic stroke in young adults is now emerging as a public health issue, one in which radiologists can play a key role. The incidence of ischemic infarction in young adults has risen over the past couple of decades. Increased public awareness, increased use of MRI and angiography, and more accurate diagnosis may in part explain the increased detection of stroke in young adults. The increased prevalence of stroke risk factors in young adults (especially sedentary lifestyle and hypertension) may also contribute. However, compared with older adults, young adults have fewer ischemic infarcts related to the standard cardiovascular risk factors and large- or small-vessel disease. Instead, their infarcts most commonly result from cardioembolic disease and other demonstrated causes (ie, dissection). Thus, radiologists must expand their differential diagnoses to appropriately diagnose ischemic strokes and identify their causes in the young adult population. From the more frequent cardioembolism and dissection to the less common vasculitis, drug-related, CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy), moyamoya, and hypercoagulable state-related infarcts, this article covers a wide breadth of causes and imaging findings of ischemic stroke in young adults. ^©RSNA, 2019.

Skull Base-related Lesions at Routine Head CT from the Emergency Department: Pearls, Pitfalls, and Lessons Learned

Routine non-contrast material-enhanced head CT is one of the most frequently ordered studies in the emergency department. Skull base-related pathologic entities, often depicted on the first or last images of a routine head CT study, can be easily overlooked in the emergency setting if not incorporated in the interpreting radiologist's search pattern, as the findings can be incompletely imaged. Delayed diagnosis, misdiagnosis, or lack of recognition of skull base pathologic entities can negatively impact patient care. This article reviews and illustrates the essential skull base anatomy and common blind spots that are important to radiologists who interpret nonenhanced head CT images in the acute setting. The imaging characteristics of important "do not miss" lesions are emphasized and categorized by their cause and location within the skull base, and the potential differential diagnoses are discussed. An interpretation checklist to improve diagnostic accuracy is provided. ^©RSNA, 2019.

High recovery of nitrogenase activity and of Fe-labeled nitrogenase in heterocysts isolated from Anabaena variabilis

Heterocysts were isolated from the N(2)-fixing cyanobacterium Anabaena variabilis after vegetative cells were disrupted by treatment with lysozyme and cavitation in a sonic cleaning bath. The acetylene-reducing (nitrogenase) activity of the isolated heterocysts, ca. 5.0 mumol (mg of chlorophyll a)(-1) min(-1) in the presence of H(2) and light, accounted for an average of 60% of the nitrogenase activity of whole filaments, and was relatively insensitive to inactivation by oxygen. Soluble extracts derived from intact filaments grown with (55)Fe, and from their heterocysts and vegetative cells, were subjected to electrophoresis. The nitrogenase and nitrogenase reductase bands (MoFe protein and Fe protein, or component 1 and component 2, respectively) were identified in these nondenaturing gels, and their radioactivities were quantitated. The isolated heterocysts accounted for an average of 91% of the nitrogenase and 69% of the nitrogenase reductase of the original filaments.

Photosynthetic properties of an Arabidopsis thaliana mutant possessing a defective PsbS gene

We describe the properties of npq4-9, a new mutant of Arabidopsis thaliana (L.) Heynh. with reduced nonphotochemical quenching (NPQ) capacity that possesses a single amino acid substitution in the PsbS gene encoding PSII-S, a ubiquitous pigment-binding protein associated with photosystem II (PSII) of higher plants. Growth, photosynthetic pigment contents, and levels of the major PSII antenna proteins were not affected by npq4-9. Although the extent of de-epoxidatin of violaxanthin to antheraxanthin plus zeaxanthin for leaves displaying the mutant phenotype equaled or exceeded that observed for the wild type, the relative effectiveness of de-epoxidized xanthophylls in promoting NPQ was consistently lower for the mutant. Energy partitioning in PSII was analyzed in terms of the competition for singlet chlorophyll a among the processes of fluorescence, thermal dissipation, and photochemistry. The key processes of NPQ and photochemistry in open PSII centers are represented by the relative in vivo rate constants kN and kP0, respectively. The magnitude of kP0 in normal leaves declined only slightly with increasing kN, consistent with localization of NPQ primarily in the antenna complex. Conversely, a highly significant linear decline in kP0 with increasing kN was observed for the mutant, consistent with a role for the PSII reaction center in the NPQ mechanism. Although the PSII absorption cross-section for white light was not significantly different relative to that of the wild type, PSII quantum yield was significantly lower in the mutant. The resulting lower capacity for linear electron transport in the mutant primarily affected reduction of terminal acceptors other than CO2. Parallel measurements of fluorescence and in vivo absorbance at 820 nm indicated a consistently higher steady-state level of reduction of PSII acceptors and accumulation of P700+ for the mutant. This suggests that inter-photosystem electron transport in the mutant is restricted either by a higher transthylakoid delta pH or by diminished accessibility to reduced plastoquinone.

Chlorophyll fluorescence at 680 and 730 nm and leaf photosynthesis

Chlorophyll fluorescence constitutes a simple, rapid, and non-invasive means to assess light utilization in Photosystem II (PS II). This study examines aspects relating to the accuracy and applicability of fluorescence for measurement of PS II photochemical quantum yield in intact leaves. A known source of error is fluorescence emission at 730 nm that arises from Photosystem I (PS I). We measured this PS I offset using a dual channel detection system that allows measurement of fluorescence yield in the red (660 nm < F < 710 nm) or far red (F > 710 nm) region of the fluorescence emission spectrum. The magnitude of the PS I offset was equivalent to 30% and 48% of the dark level fluorescence F(0) in the far red region for Helianthus annuus and Sorghum bicolor, respectively. The PS I offset was therefore subtracted from fluorescence yields measured in the far red spectral window prior to calculation of PS II quantum yield. Resulting values of PS II quantum yield were consistently higher than corresponding values based on emission in the red region. The basis for this discrepancy lies in the finite optical thickness of the leaf that leads to selective reabsorption by chlorophyll of red fluorescence emission originating in deeper cell layers. Consequently, red fluorescence measurements preferentially sense emission from chloroplasts in the uppermost layer of the leaf where levels of photoprotective nonphotochemical quenching are higher due to increased photon density. It is suggested that far red fluorescence, corrected for the PS I offset, provides the most reliable quantitative basis for calculation of PS II quantum yield because of reduced sensitivity of these measurements to gradients in leaf transmittance and quenching capacity.

A nonphotochemical-quenching-deficient mutant of Arabidopsis thaliana possessing normal pigment composition and xanthophyll-cycle activity

Higher-plant chloroplasts alter the distribution of absorbed radiant energy between photosynthesis and heat formation in response to changing illumination level or environmental stress. Fluorescence imaging was used to screen 62 yellow-green T-DNA insertion mutant lines of Arabidopsis thaliana (L.) Heynh. for reduced photoprotective nonphotochemical quenching (NPQ) capacity. Pulse-modulation fluorometry was employed to characterize one line (denoted Lsr1(-)) that exhibited an approximately 50% reduction in NPQ compared to the wild type (WT). The loss in NPQ capacity was associated with the DeltapH-dependent phase of quenching (qE). Under the growth conditions employed, pigment composition and levels of the six photosystem-II light-harvesting chlorophyll a/b proteins were identical in mutant and WT. Changes in the in-vivo levels of the xanthophyll pigments violaxanthin, antheraxanthin, and zeaxanthin in excess light were the same for mutant and WT. However, use of the violaxanthin de-epoxidase inhibitor dithiothreitol indicated that a zeaxanthin-dependent component of NPQ was specifically reduced in the mutant. The mutant exhibited diminished suppression of minimum fluorescence yield (F(o)) in intense light suggesting an altered threshold in the mechanism of response to light stress in the mutant. The NPQ-deficient phenotype was meiotically transmissible as a semidominant trait and mapped near marker T27K12 on chromosome 1. The results suggest that the mutant is defective in sensing the transthylakoid DeltapH that reports exposure to excessive illumination.

Effects of O(2) and CO(2) Concentrations on Quantum Yields of Photosystems I and II in Tobacco Leaf Tissue

The interactive effects of irradiance and O(2) and CO(2) levels on the quantum yields of photosystems I and II have been studied under steady-state conditions at 25 degrees C in leaf tissue of tobacco (Nicotiana tabacum). Assessment of radiant energy utilization in photosystem II was based on changes in chlorophyll fluorescence yield excited by a weak measuring beam of modulated red light. Independent estimates of photosystem I quantum yield were based on the light-dark in vivo absorbance change at 830 nanometers, the absorption band of P700(+). Normal (i.e. 20.5%, v/v) levels of O(2) generally enhanced photosystem II quantum yield relative to that measured under 1.6% O(2) as the irradiance approached saturation. Photorespiration is suspected to mediate such positive effects of O(2) through increases in the availability of CO(2) and recycling of orthophosphate. Conversely, at low intercellular CO(2) concentrations, 41.2% O(2) was associated with lower photosystem II quantum yield compared with that observed at 20.5% O(2). Inhibitory effects of 41.2% O(2) may occur in response to negative feedback on photosystem II arising from a build-up in the thylakoid proton gradient during electron transport to O(2). Covariation between quantum yields of photosystems I and II was not affected by concentrations of either O(2) or CO(2). The dependence of quantum yield of electron transport to CO(2) measured by gas exchange upon photosystem II quantum yield as determined by fluorescence was unaffected by CO(2) concentration.

Analysis of changes in minimal and maximal fluorescence yields with irradiance and o(2) level in tobacco leaf tissue

The responses of minimal and maximal fluorescence yields of chlorophyll a to irradiance of actinic white light were determined by pulse modulated fluorimetry in leaf discs from tobacco, Nicotiana tabacum, at 1.6, 20.5, and 42.0% (v/v) O(2). Steady-state maximal fluorescence yield (F(m)', measured during a saturating light pulse) declined with increasing irradiance at all O(2) levels. In contrast, the steady-state minimal fluorescence yield (F(o)', measured during a brief dark interval) increased with irradiance relative to that recorded for the fully dark-adapted leaf (F(o)) or that observed after 5 minutes of darkness (F(o) (*)). The relative magnitude of this increase was somewhat greater and extended to higher irradiances at the elevated O(2) levels compared with 1.6% O(2). Suppression of F(o)' was only observed consistently at saturating irradiance. The results are interpreted in terms of the occurrence of photosystem II units possessing exceedingly slow turnover times (i.e. "inactive" units). Inactive units play an important role, along with thermal deactivation of excited chlorophyll, in determining the response of in vivo fluorescence yield to changes in irradiance. Also, a significant interactive effect of O(2) concentration and the presence or absence of far red light on oxidation of photosystem II acceptors in the dark was noted.

Method for the concentration of infectious pancreatic necrosis virus from hatchery water

A method was developed for concentrating infectious pancreatic necrosis virus from hatchery water using positively charged 1-MDS filters. The method consists of passing large volumes (Ca. 1001) of hatchery water through 1-MDS microporous filter followed by the elution of the adsorbed virus using a high pH buffer. The virus adsorbed efficiently to 1-MDS filters when the pH of the water was 5.5 and was eluted optimally with 3% beef extract solution (pH 10). This procedure permitted the processing of 100 1 of hatchery water which resulted in a 300-fold reduction in the volume of water and greater than 90% recovery of the seeded virus.

Effects of Irradiance on the in Vivo CO(2):O(2) Specificity Factor in Tobacco Using Simultaneous Gas Exchange and Fluorescence Techniques

The effects of gas phase O(2) concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO(2) fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO(2) and 25 to 30 degrees C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O(2) levels relative to 1% O(2) when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O(2) levels only at saturating irradiance. Simultaneous measurements of CO(2) and H(2)O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO(2) fixation and O(2)-dependent, dissipative processes such as photorespiration as a function of leaf internal CO(2) concentration. Changes in the in vivo CO(2):O(2) ;specificity factor' (K(sp)) with increasing irradiance are examined. The magnitude K(sp) was found to decline from a value of 85 at moderate irradiance to 68 at very low light, and to 72 at saturating photon flux rates. The results are discussed in terms of the applicability of the ribulose bisphosphate carboxylase/oxygenase enzyme model to photosynthesis in vivo.

Effects of water vapor pressure deficit on photochemical and fluorescence yields in tobacco leaf tissue

The relationship between photochemical quantum yield (phi(s)) and fluorescence yield have been investigated in leaf tissue from Nicotiana tabacum using CO(2) exchange and a modulated fluorescence measuring system. The quantum yield of CO(2) fixation at 1.6% (v/v) O(2) and limiting irradiance was reduced 20% by increasing the mean H(2)O vapor pressure deficit (VPD) from 9.2 to 18.6 mbars. As [CO(2)] and irradiance were varied, the intrinsic quantum yield of open photosystem II units (phi(s)/q(Q) where q(Q) is the photochemical fluorescence quenching coefficient) declined linearly with the degree of nonphotochemical fluorescence quenching. The slope and y-intercept values for this function were significantly reduced when the mean VPD was 18.4 millibars relative to 8.9 millibars. Susceptibility of the leaf tissue to photoinhibition was unaffected by VPD. Elevated O(2) concentrations (20.5% v/v) reduced the intrinsic quantum yield of net CO(2) uptake due to the occurrence of O(2)-reducing processes. However, the relative effect of high VPD compared to low VPD on intrinsic quantum yield was not dependent on the O(2) level. This suggests that the Mehler reaction does not mediate the response of quantum yield to elevated VPD. The results are discussed with regard to the possible role of transpiration stress in regulating dissipation of excitation by electron transport pathways other than noncyclic electron flow supporting reduction of CO(2) and/or O(2).

Partitioning of Noncyclic Photosynthetic Electron Transport to O(2)-Dependent Dissipative Processes as Probed by Fluorescence and CO(2) Exchange

The partitioning of noncyclic photosynthetic electron transport between net fixation of CO(2) and collective O(2)-dependent, dissipative processes such as photorespiration has been examined in intact leaf tissue from Nicotiana tabacum. The method involves simultaneous application of CO(2) exchange and pulse modulated fluorescence measurements. As either irradiance or CO(2) concentration is varied at 1% O(2) (i.e. absence of significant O(2)-dependent electron flow), the quantum efficiency of PSII electron transport (phi(se)) with CO(2) as the terminal acceptor is a linear function of the ratio of photochemical:nonphotochemical fluorescence quenching coefficients (i.e. q(Q):q(NP)). When the ambient O(2) concentration is raised to 20.5% or 42% the q(Q):q(NP) is assumed to predict the quantum efficiency of total noncyclic electron transport (phi'(se)). A factor which represents the proportion of electron flow diverted to the aforementioned dissipative processes is calculated as (phi'(se) - phi(se))/phi'(se) where phi(se) is now the observed quantum efficiency of electron transport in support of net fixation of CO(2). Examination of changes in electron allocation with CO(2) and O(2) concentration and irradiance at 25 degrees C provides a test of the applicability of the Rubisco model to photosynthesis in vivo.

Temperature-sensitive gel for virus concentration from urine

Cross-linked, partially hydrolyzed polyacrylamide gels (temperature-sensitive gels) with the property to swell at 4 degrees C and collapse at higher temperatures (greater than 19 degrees C) were used to concentrate bacteriophage T-2 from urine. Samples of urine, 50 ml, seeded with bacteriophage T-2 were reduced to approximately 5 ml, with an average virus recovery of 53%. Subsequent experiments with feline cell-associated herpes virus resulted in a 6-fold decrease in volume with 54% virus recovery. The gel could be used repeatedly without any loss in efficiency.

Carbon Dioxide-Induced Oscillations in Fluorescence and Photosynthesis: Role of Thylakoid Membrane Energization in Regulation of Photosystem II Activity

The response of CO(2) fixation to a sudden increase in ambient CO(2) concentration has been investigated in intact leaf tissue from spinach (Spinacia oleracea) using a dual channel infrared gas analyzer. Simultaneous with these measurements, changes in fluorescence emission associated with a weak, modulated measuring beam were recorded. Application of brief (2-3 seconds) dark intervals enabled estimation of the dark fluorescence level (F(o)) under both steady state and transient conditions. The degree of suppression of F(o) level fluorescence in the light was strongly correlated with nonphotochemical quenching under all conditions. During CO(2)-induced oscillations in photosynthesis under 2% O(2) the changes in nonphotochemical quenching anticipate changes in the rate of uptake of CO(2). At such low levels of O(2) and constant illumination, changes in the relative quantum efficiency of open photosystem II units were estimated as the ratio of the rate of CO(2) uptake and the photochemical quenching coefficient. Under the same conditions the relative quantum efficiency of photosystem II was found to vary inversely with the degree of nonphotochemical quenching. The relationship between changes in the rate of CO(2) uptake: photochemical quenching coefficient and nonphotochemical quenching was altered somewhat when the same experiment was conducted under 20% O(2). The results suggest that electron transport coupled to reduction of O(2) occurs to varying degrees with time during oscillations, especially when ambient O(2) concentrations are high.

Relationship between Steady-State Fluorescence Yield and Photosynthetic Efficiency in Spinach Leaf Tissue

The relationship between steady-state photosynthetic efficiency, as moles CO(2) per mole of incident visible photons under 2% O(2), and chlorophyll fluorescence quenching has been investigated in intact leaf tissue of Spinacia oleracia. Fluorescence yield was measured using a pulse amplitude modulation technique that permitted rapid and sensitive resolution and quantitation of photochemical and nonphotochemical quenching coefficients. A highly linear relationship was observed between photosynthetic efficiency and the ratio of photochemical:nonphotochemical quenching coefficients for values of the latter less than 1.6. This relationship applied whether irradiance or CO(2) concentration was varied. The observed relationships between photochemical yield and fluorescence yield were compatible with the photosystem II model proposed by Butler and Kitajima (1975 Biochim Biophys Acta 376: 116-125). The results are discussed with respect to the proposed role of nonphotochemical quenching in regulating radiant energy utilization and also the applicability of fluorescence measurements as a means of estimation of the rate of photosynthetic electron transport.

Propagation and quantitation of animal herpesviruses in eight cell culture systems

A comparative study was carried out to determine the relative sensitivities of eight different cell culture systems to six different herpesviruses of animals. The cells used were: OFL (ovine fetal lung), ML (mink lung), FK (ferret kidney), PTK-2 (potoroo kidney), TEK (turkey embryo kidney), ED (equine dermal), BT (bovine turbinate), and PK15 (porcine kidney). The viruses tested were: PRV (pseudorabies) of swine, CPHV (caprine herpesvirus), IBRV (infectious bovine rhinotracheitis virus), DN-599 strain of bovine herpesvirus type 4, EHV-1 (equine herpesvirus), and CHV (canine herpesvirus). On the basis of virus titers obtained and the time of appearance of CPE (cytopathic effects), ML cells were found to be the most useful because of their sensitivity to all six viruses tested. BT and OFL cells were also found to be highly sensitive to all viruses with the exception of CHV.

Effects of CO(2) and O(2) on Photosynthesis and Growth of Autotrophic Tobacco Callus

Mean inhibition of net photosynthesis in autotrophic tobacco callus by 21 and 40% O(2) was 30 and 47%, respectively, similar to intact leaves. Increasing CO(2) concentrations (500-2000 microliters per liter) produced a steady decline in percent inhibition at both O(2) concentrations, indicating that O(2) inhibition resulted primarily from photorespiration. Net photosynthetic rate was plotted as a function of CO(2) concentration at 1, 21, and 40% O(2) for calculation of kinetic constants. Values for V(max) were similar at all O(2) concentrations (x = 5.27 mumol CO(2) per gram fresh weight per hour), indicating that O(2) inhibition of net photosynthesis was fully reversible by CO(2). To determine whether CO(2) and O(2) produced similar effects on growth, autotrophic callus was incubated for three weeks in atmospheres of normal air, high CO(2), high O(2) and high CO(2)/high O(2). Growth in high CO(2) was nearly double that in normal air. High O(2) decreased growth significantly relative to air, but growth in high CO(2)/high O(2) was similar to that in air. Lack of CO(2) reversal of growth inhibition by O(2) indicates that prolonged exposure to high O(2) results in toxicity arising from a nonphotorespiratory source. Growth of heterotrophic callus (2% sucrose), however, was not inhibited by 40 or 60% O(2), suggesting that O(2) toxicity in autotrophic callus results primarily from disruption of photosynthetic functions.

Quantitation of the O(2)-Dependent, CO(2)-Reversible Component of the Postillumination CO(2) Exchange Transient in Tobacco and Maize Leaves

The postillumination transient of CO(2) exchange and its relation to photorespiration has been examined in leaf discs from tobacco (Nicotiana tabacum) and maize (Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O(2) in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO(2) exchange equivalents (i.e. nanomoles CO(2)) versus time were derived from the analyzer responses of individual transients. In tobacco (C(3)), subtraction of the time course of cumulative CO(2) exchange under photorespiratory conditions (21 or 43% O(2)) from that obtained under nonphotorespiratory conditions (1% O(2)) revealed the presence of an O(2)-dependent and CO(2)-reversible component within the first 60 seconds following darkening. This component was absent in maize (C(4)) and at low external O(2):CO(2) ratios (i.e. <100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O(2). This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO(2) evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration.

Photorespiration stoichiometry in leaves estimated by combined physical and stereochemical methods: allowance for isomerase-catalyzed 3H losses in ribulose bisphosphate regeneration

We showed previously [K.R. Hanson and R.B. Peterson (1986) Arch. Biochem. Biophys. 246, 332-346] that under steady-state photosynthetic conditions the fraction of ribulose bisphosphate oxidized and the fraction of glycolate carbon photorespired (the stoichiometry of photorespiration) may be estimated in leaves by a combination of physical and stereochemical methods. The calculations assumed that when (3R)-D-[3-3H1,3-14C]glyceric acid is supplied to illuminated leaf discs the only loss of 3H from the combined photosynthetic and photorespiratory system is the result of glycolate oxidase action; i.e., the isomerase-catalyzed losses in the regeneration of ribulose bisphosphate are negligible. The present study of tobacco leaf discs under zero-photorespiration conditions (low O2 and high CO2 concentrations), and also of maize leaf discs, shows that some 3H losses occur (between 8 and 13% of the 3H at C-1 of ribulose 5-phosphate). The calculated loss varied moderately with temperature but did not vary when the flux of ribulose bisphosphate formation was altered by changing the irradiance. The calculated loss under zero-photorespiration conditions, therefore, may be used to calculate ribulose bisphosphate and glycolate partitioning under other conditions. Earlier experiments on the influence of O2 and CO2 concentrations of temperature on the partitioning of ribulose bisphosphate and glycolate have been reexamined. The loss corrections decreased all values for the fraction of ribulose bisphosphate oxidized and increased all values for the stoichiometry of photorespiration. Essentially all stoichiometry values were above the theoretical lower limit of 25%. The previous conclusion that the stoichiometry of photorespiration substantially exceeds 25% at higher O2 concentrations and higher temperatures is unchanged. The results with maize leaf discs implied that there is very little oxidation of ribulose 1,5-bisphosphate under normal-air conditions; i.e., photorespiration is indeed suppressed, not merely hidden, by efficient refixation of CO2.

Regulation of photorespiration in leaves: evidence that the fraction of ribulose bisphosphate oxygenated is conserved and stoichiometry fluctuates

Under steady-state conditions the combined system of the reductive photosynthetic cycle and the oxidative photorespiratory loop may be defined by two partitioning terms: the fraction of ribulose bisphosphate oxygenated and the fraction of glycolate carbon photorespired (the stoichiometry of photorespiration). A combination of physical and stereochemical methods [K.R. Hanson, and R. B. Peterson, (1985) Arch. Biochem. Biophys. 237,300-310] has been used to estimate these partitionings for tobacco leaf discs. Inverted discs, as compared to normally oriented discs, were found to have greater net photosynthesis; their ratio of photorespiration to net photosynthesis was less, and less of their glycolate carbon was photorespired. An eightfold reduction of irradiance below that of full sunlight for inverted discs in normal air at 32 degrees C reduced both photosynthesis and photorespiration about threefold but had little effect on the partitioning of ribulose bisphosphate and glycolate. Increasing the temperature from 22 to 40 degrees C for inverted discs in normal air and 1000 microE m-2 s-1 irradiance had little effect on net photosynthesis but increased the ratio of photorespiration to net photosynthesis almost threefold; ribulose bisphosphate partitioning was little changed but the fraction of glycolate carbon photorespired more than doubled. If field-grown plants respond to temperature in a similar fashion, genetic intervention to reduce the increase in photorespiration stoichiometry with temperature could increase total daily carbon assimilation and hence improve crop yields.

Recognition of Superior Photosynthetic Efficiency in the Field Using the CO(2)-Depletion Technique

The relationship between CO(2) exchange rate (CER) and growth of crops in the field was investigated in Connecticut Broadleaf tobacco (Nicotiana tabacum) using the CO(2)-depletion technique. A particular objective was to determine if modest (i.e. <10%) varietal differences could be distinguished in mean CER. Statistical analysis of numerous CER values obtained over a wide range of irradiances during the course of the season indicated that differences of as little as 7% in the mean CER between varieties would be significant (n approximately 400). The usefulness of the CO(2)-depletion technique in detecting modest differences in photosynthetic efficiency has thus been demonstrated. These results are discussed in relation to the prospects for introducing and detecting genetic traits which would diminish photorespiration and increase CER and growth.

The stoichiometry of photorespiration during C3-photosynthesis is not fixed: evidence from combined physical and stereochemical methods

The stoichiometry of photorespiration, S, is defined as the fraction of glycolate carbon photorespired. It is postulated that under steady-state conditions there are two determinants of the ratio of photorespiration to net photosynthesis: the partitioning of ribulose bisphosphate between oxidation and carboxylation, and the partitioning of glycolate between reactions leading to complete oxidation to CO2 (S = 100%) and those yielding CO2 plus serine (S = 25%). S may be calculated using two independent probes of the system. The physical probe, using an infrared gas analyzer, measured photorespiration and net photosynthesis, and hence their ratio PR/NPS = pn(phys). The metabolic probe employed tracer (3R)-D-[3-3H1,3-14C]glyceric acid to determine r, the fraction of 3H retained in the triose phosphates leaving the chloroplasts. It is deduced from the postulated model that S = pn(phys) . r/(1 - r). Experiments have been performed with illuminated tobacco leaf discs (inverted) under varying concentrations of O2 and CO2. Increasing O2 at constant CO2 increased pn(phys) and decreased r, whereas increasing CO2 at constant O2 had the opposite effect. S more than doubled at 32 degrees C on going from 16 to 40% O2 (340 microliters CO2/liter) and decreased 40% on going from 200 to 700 microliters CO2/liter (21% O2). For discs in normal air S was somewhat greater than 27%. It is suggested that net photosynthesis, and therefore crop yields, could be increased by selecting for crop plants with reduced photorespiration stoichiometry.

A Numerical Approach to Measurement of CO(2) Exchange Transients by Infrared Gas Analysis

Open flow-through systems coupled to infrared gas analyzers have been frequently employed in the study of CO(2) exchange transients such as the postillumination burst observed in leaves of C(3) plants. A major limitation associated with use of such systems is their non-steady state response to rapid changes in leaf CO(2) exchange rate. A previous publication outlined a numerical approach to model the analyzer response as a function of CO(2) exchange rate and thus permit estimation of the postillumination burst (Peterson 1983 Plant Physiol 73: 978-982). The model is critically analyzed within the framework of the physics of solute dispersion as previously described for linear flow systems. Thus, the numerical simulation is validated on the basis of physical principle. Additional improvements to the previous model are described which enhance the accuracy and efficiency of use of this technique for estimation of photorespiration.

Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: II. Effects of O(2), CO(2), and Temperature

An open system associated with an infrared gas analyzer was employed to study transients in CO(2) exchange generated upon darkening preilluminated leaf discs of tobacco (Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO(2) exchange rate. A computer was used to evaluate parameters of the leaf CO(2) release rate to provide an estimate of the initial rate of postillumination CO(2) evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O(2), the decline in rate of CO(2) evolution upon darkening followed first order kinetics. Initial rates of CO(2) evolution following darkening were relatively independent of the prior ambient CO(2) concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO(2) concentration at 21% O(2). Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO(2) fixation rate at 32 degrees C and high irradiance. The rapid initial CO(2) evolution observed upon darkening at 21% O(2) was absent in 3% O(2). Rates of photorespiration under normal atmospheric concentrations of CO(2) and O(2) as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C(3) leaf tissue.

Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: I. A Nonsteady State Model for Measurement of CO(2) Exchange Transients

Although open systems have been used for the study of transients in leaf CO(2) exchange such as the postillumination burst, these systems frequently do not permit reliable estimates of transient rates due to their nonsteady state nature. A nonsteady state mathematical approach is described which predicts changes in CO(2) concentration in the leaf chamber and infrared gas analyzer measuring cell as a function of leaf CO(2) exchange rate in Nicotiana tabacum vars John Williams Broadleaf and Havana Seed. With the aid of a computer, a numerical formula simulates the mixing and dilution which occurs as CO(2) passes through the finite volume of the measuring cell of the analyzer. The method is presented with special relevance to photorespiration as manifested by the postillumination burst of CO(2). The latter is suggested to decline with the first order kinetics following darkening of a C(3) leaf. This approach provides a basis for reliable estimation of the initial and, hence, maximal rate of CO(2) evolution during the postillumination burst under a variety of environmental conditions.

Relationship between Net CO(2) Assimilation and Dry Weight Accumulation in Field-Grown Tobacco

To assess the variability of net photosynthetic CO(2) exchange per unit leaf area and to construct budgets for stands of field-grown tobacco (Nicotiana tabacum, Connecticut Broadleaf), a number of short-time measurements were made on all available leaf positions on two varieties using a hand-held transparent chamber for conducting gas exchange measurements on leaves. Measurements of net CO(2) exchange were carried out on 18 separate days during a 35-day period, beginning 22 days after the seedlings were transplanted to the field. Gas exchange assays on leaves were conducted under ambient conditions of temperature and light intensity at all times of day. Solar radiation was monitored throughout the period, and losses of respiratory CO(2) from stems, roots, and leaves (in the dark) were estimated. A simple model was proposed to relate daily total CO(2) input to irradiance and total leaf area. The total leaf area was assumed to be a function of day number. Dark respiratory losses accounted for 41% to 47% of total CO(2) assimilation. Analysis of variance indicated that the two varieties were not significantly different in whole plant rate of CO(2) fixation per unit of leaf area. CO(2) input was closely associated with leaf area within each variety. Throughout the experiment, the difference between the two varieties in total leaf area per plant was the largest single factor in determining net CO(2) inputs. The cumulative dry weight increase for each variety was similar to the prediction of net dry matter input obtained by gas exchange measurements, thus confirming the close relationship between total plant net CO(2) assimilation and dry weight yield.

Regulation of Glycine Decarboxylase and l-Serine Hydroxymethyltransferase Activities by Glyoxylate in Tobacco Leaf Mitochondrial Preparations

Glyoxylate at a concentration of 10 millimolar caused 50% inhibition of decarboxylation of 20 millimolar [1-(14)C]glycine and accompanying synthesis of serine in a mitochondria-enriched preparation from tobacco (Nicotiana tabacum var. John Williams Broadleaf) leaves. None of the other compounds tested including formate, acetate, oxalate, aspartate, and glutamate appreciably affected activity. Occasional inhibition produced by glycolate may have resulted from residual glycolate oxidase in these preparations. Added glyoxylate was not converted to glycine in these preparations and about 98% of it could be recovered at the end of the reaction. Hence, the observed inhibition by glyoxylate did not result from dilution of radioactivity in the substrate.Glyoxylate also regulated synthesis of HCHO from l-serine catalyzed by l-serine hydroxymethyltransferase in mitochondrial preparations. Control of this enzyme activity by glyoxylate was complex and was characterized by enhancement of activity at low glyoxylate concentrations (less than 10 millimolar) and inhibition by concentrations generally above 10 millimolar. These results define potential sites of biochemical regulation of important steps in the pathway of photorespiratory carbon flow and are considered in the light of other observations of effects of exogenous glyoxylate on photorespiration in leaf tissue.

Enhanced Incorporation of Tritium into Glycolate during Photosynthesis by Tobacco Leaf Tissue in the Presence of Tritiated Water

Tobacco (Nicotiana tabacum var. Havana Seed) leaf discs were allowed to photosynthesize for 3 to 20 minutes in the presence of (14)CO(2) and (3)H(2)O. Several metabolites of the Calvin cycle and photorespiratory pathway were isolated and purified and the (3)H:(14)C values measured. Glycolate had a 5- to 10-fold higher (3)H:(14)C than the Calvin cycle intermediate 3-phosphoglyceric acid, or its end product sucrose. The glycolate oxidase inhibitor alpha-hydroxy-2-pyridinemethanesulfonic acid caused glycolate to accumulate in the tissue and lowered the (3)H:(14)C in glycolate to a value similar to that in 3-phosphoglyceric acid. Phosphoglycolate, a possible precursor of glycolate arising from the Calvin cycle, exhibited a (3)H:(14)C value similar to 3-phosphoglyceric acid under all conditions. The finding of a (3)H enrichment in glycolate suggests that another source of glycolate, possibly the reduction of glyoxylate, exists in leaf tissue. Analyses of incorporation of (3)H into the pro-2R and pro-2S hydrogens of glycolate, in the presence and absence of alpha-hydroxy-2-pyridinemethanesulfonic acid, suggest an alternative source of glycolate. Biochemical mechanisms to account for (3)H enrichment into glycolate are evaluated.

Energy transfer from phycobiliproteins to photosystem I in vegetative cells and heterocysts of Anabaena variabilis

The presence of phycobilins in heterocysts of Anabaena variabilis is established on the basis of absorption and fluorescence spectroscopy. At 77 K heterocysts exhibit fluorescence emission bands at 645 and 661 nm indicative of phycocyanin and allophycocyanin, respectively. Both allophycocyanin levels and fluorescence emission at 695 nm were low in heterocysts relative to whole filaments. In situ fluorescence microscopy confirmed the presence of phycobilins in individual heterocysts, but the pigment levels varied considerably among cells. Heterocysts exhibited Photosystem I activity, as evidenced by photooxidation of P-700, but no Photosystem II activity. The quantum efficiency of phycobilins in sensitizing P-700 photooxidation was 50-70% that of chlorophyll a. Phycoibins were also effective in promoting light-dependent reduction of acetylene to ethylene. The results are discussed in terms of the role of the heterocyst in nitrogen fixation and of the significance of energy transfer from phycobilins to Photosystem I in heterocysts.

Modes of reduction of nitrogen in heterocysts isolated from Anabaena species

N2 fixation (acetylene reduction) has been studied with heterocysts isolated from Anabaena cylindrica and Anabaena 7120. In the presence of ATP and at very low concentrations of sodium dithionite, reducing equivalents for activity of nitrogenase in these cells can be derived from several compounds. In the dark, D-glucose 6-phosphate, 6-phosphogluconate and DL-isocitrate support acetylene reduction via NADPH. In the light, reductant can be generated by Photosystem I.

Localization of an uptake hydrogenase in anabaena

Occurrence and localization of an uptake hydrogenase were examined in three strains of the blue-green alga, Anabaena. In vivo H(2) uptake was detected (0.60-1.44 mumoles/[mg of chlorophyll a per hour]) in all three strains when grown with N(2) as the sole source of nitrogen. H(2) uptake (in vivo and in vitro) was severely suppressed in cultures grown on NH(4) (+) and lacking heterocysts. H(2) uptake in cell-free extracts could be readily measured with a methyl viologen-ferricyanide electron acceptor system. Solubilization kinetics during cavitation of aerobically grown Anabaena 7120 indicates that the uptake hydrogenase is localized solely in the heterocyst. When the same organism is grown on N(2)/CO(2), vegetative cells may account for up to 21% of the total hydrogenase activity in the filaments. The results are discussed in terms of a proposed functional relationship between nitrogenase and hydrogenase.