Mean Photosynthetically Active Radiation Research Articles

The Effects of Light Intensity and Flow Speed on Biogeochemical Variability Within a Fringing Coral Reef in Onna‐Son, Okinawa, Japan

AbstractGlobal warming and ocean acidification are driving declines in seawater dissolved oxygen (DO) concentrations and pH. Predicting how these changes will affect shallow, near‐shore environments such as coral reefs is challenging due to their high natural biogeochemical variability present over both spatial (m to km) and temporal (diel to seasonal) scales. To make predictions, we must understand the drivers of this variability. The impact of metabolic processes on coral reef biogeochemical variability has been the subject of significant research effort, however, physical factors, including flow speed and light intensity, have received less attention. Here, we measured seawater flow, photosynthetically active radiation (PAR), pH, and DO at three reef habitats (reef flat, lagoon, and outflow channel) in a fringing coral reef system in Okinawa, Japan for 3 weeks in October 2019. During the study, pH ranged from 7.86 to 8.37 units while DO varied from 127 to 369 μmol/kg. Circulation was primarily wave‐driven with mean flow speeds ranging from 14 to 26 cm/s. Flow direction became increasingly consistent at higher flow speeds and traced benthic striations visible in satellite imagery. Multiple linear regression models of daytime changes in pH and DO versus daily mean flow speed and PAR described 25%–74% of the observed variability across all sites while at night, flow speed alone accounted for 7%–75% of the observed variability. These results demonstrate PAR, water flow speed, and the path water takes play important roles in controlling biogeochemical variability within coral reefs and must be considered when assessing their vulnerability to both local and global environmental change.

Impact of radiation variations on temporal upscaling of instantaneous Solar-Induced Chlorophyll Fluorescence

Solar-Induced Chlorophyll Fluorescence (SIF) has been increasingly used as a novel proxy for vegetation productivity. Several space-borne instruments can retrieve SIF at varying overpass time, which complicates the interpretation as SIF is driven by absorbed Photosynthetically Active Radiation (PAR) at the acquisition time. To facilitate comparisons across sensors, satellite-based SIF is upscaled to daily averages with a length-of-day correction factor (DC). In conventional DC calculations, the light intensity over a day is approximated geometrically by the cosine of the Solar Zenith Angle (SZA), neglecting changes in atmospheric extinction and topographic effects. Here, we use reanalysis PAR data for DC calculations to evaluate the impact of atmospheric extinction and diffuse radiation individually. We find that the simple SZA approach is a reliable approximation for flat surfaces, where the overall atmospheric impact on DC is less than 10% as large individual effects on direct and diffuse PAR partially compensate each other. At longer time-scales, a sampling (clear sky) bias might exist due to cloud-filtering of satellite data. We find that in the Amazon the true monthly mean PAR can be 25% lower than the one for cloud-filtered days, potentially inducing seasonal SIF biases on the same order. An additional factor impacting PAR during a day is topography. For complex terrain, direct light in the DC expression requires a correction for surface slopes. For example in the San Gabriel Mountains, California, USA, the modified DC is changed by as much as 500% for strongly tilted surfaces. This modification is especially important for satellite instruments with fine spatial resolutions, where surface slopes are not averaged out and can have a substantial impact on reflectance and SIF. Overall, our refined DC-corrections and averaging strategy can help satellite SIF interpretation as well as intercomparisons over a wide range of spatio-temporal scales and overpass times.

Comparison of the Carbon and Water Fluxes of Some Aggressive Invasive Species in Baltic Grassland and Shrub Habitats

Biological systems are shaped by environmental pressures. These processes are implemented through the organisms exploiting their adaptation abilities and, thus, improving their spreading. Photosynthesis, transpiration, and water use efficiency are major physiological parameters that vary among organisms and respond to abiotic conditions. Invasive species exhibited special physiological performance in the invaded habitat. Photosynthesis and transpiration intensity of Fallopia japonica, Heracleum sosnowskyi, and Rumex confertus of northern and trans-Asian origin were performed in temperate extensive seminatural grassland or natural forest ecotones. The observed photosynthetically active radiation (PAR) ranged from 36.0 to 1083.7 μmol m−2 s−1 throughout the growing season depending on the meteorological conditions and habitat type. F. japonica and H. sosnowskyi settled in naturally formed shadowy shrub habitats characterized by the lowest mean PAR rates of 58.3 and 124.7 μmol m−2 s−1, respectively. R. confertus located in open seminatural grassland habitats where the mean PAR was 529.35 μmol m−2 s−1. Correlating with the available sunlight radiation (r = 0.9), the highest average photo assimilation rate was observed for R. confertus (p = 0.000). The lowest average intensity of photosynthesis rates was exhibited of F. japonica and H. sosnowskyi in shadowy shrub habitats. Transpiration and water use effectivity at the leaf level depended on many environmental factors. Positive quantitative responses of photosynthesis and transpiration to soil and meteorological conditions confirmed positive tolerance strategies of the invasive species succeeded by environmental adaptation to new habitats during their growing period sustained across a range of environments.

Retrieving high-resolution surface photosynthetically active radiation from the MODIS and GOES-16 ABI data

Incident photosynthetically active radiation (PAR) is a key parameter in plant physiological, biological and physical process-based terrestrial ecosystem models. Deriving highly accurate and spatially continuous PAR from remote sensing data can fill in the gaps of insufficient ground-based measurements. In this study, we provide an efficient approach for estimating PAR by employing look-up table (LUT) method using observations from both MODIS and GOES-16. The GOES-16 is the first satellite of the new-generation US Geostationary Operational Environmental Satellite R-series (GOES-R), which can capture dynamic changes in the atmosphere every 5 min. First, the diurnal variation of surface bidirectional reflectance is characterized assuming that the surface characteristics and solar zenith angles in tropical and middle latitudes at a given time of day do not change dramatically within a month. Next, the aerosol optical depth (AOD) and cloud optical depth (COD) are retrieved from the GOES-16 visible observations under both clear-sky and cloudy conditions. Finally, PAR is estimated via the retrieved AOD and COD using the LUT method and a MODIS-based surface albedo product. The estimated PAR was compared against ground-based measurements collected from seven SURFace RADiation budget network (SURFRAD) sites and 23 National Ecological Observatory Network (NEON) sites at instantaneous, hourly and daily scales. The instantaneous PAR validation results achieved R2 values of 0.97 and 0.82, and root mean square errors (RMSEs) of 18.1 and 51.1 W m‐2, for clear and cloudy skies, respectively. The RMSEs of the hourly averaged PAR were reduced to 16.8 W m−2 and 40.4 W m−2, respectively, for clear-sky and cloudy conditions. The daily mean PAR evaluation for all sky conditions shows an overall mean bias error (MBE) of 0.97 W m−2 and an RMSE of 10.5 W m−2. Compared with other global PAR datasets, the estimated PAR values in this study have lower RMSEs and higher spatial-temporal resolution. Uncertainties in the retrieved PAR caused by topography and solar-cloud-geometry effects (SCGE) can be reduced by spatial and temporal averaging. Based on an evaluation of retrievals at different spatial-temporal scales, we recommend that a 20 × 20 km2 domain-averaged PAR be utilized for instantaneous PAR validation, whereas hourly and daily averaged PAR values are nearly independent of spatial scale.

Micorrhizal biotechnology as an alternative to potentialize the strawberry quality

The need to develop and use biotechnology tools to improve management and nutritional techniques in strawberry cultivation are increasing. Based on this, the objective of this study was to test if the mycorrhizal inoculation changes the agronomic and qualitative performance of strawberry fruits. The strawberry daughters plants used were of the cultivar Camarosa. The study was carried out in an agricultural greenhouse, Horticulture Sector of the Faculty of Agronomy and Veterinary Medicine (FAMV) of UPF, in the city of Passo Fundo, Rio Grande do Sul (RS), Brazil. The experiment was developed and maintained from June to December 2015. The treatments consisted of five inoculum: T1 = no inoculum (control); T2 = Acaulospora morrowiae; T3 = Rhizophagus clarus; T4 = mycorrhizal community; T5 = Claroideoglomus etunicatum. The experimental design was a randomized block with four replicates. Each plot consisted of a bag containing 6 plants spaced 0.15 mx 0.15 m. In the pits of the plants, it was applied, with the aid of syringe and water, about 70 infective propagules of the tested inocula. During the conduction of the experiment the temperature and photosynthetically active radiation (PAR) were monitored in the culture environment. It was evaluated the agronomic and qualitative performance of the fruits and the percentage of mycorrhizal colonization of the plants roots. The data were submitted to analysis of variance and the differences between means were compared by the Tukey test at 5% of probability error. The cultivar Camarosa produced more fruits in October. In September, the month before the one with the best agronomic performance, the mean temperature was 17.7 ºC. The mean PAR recorded in the growing environment was low in relation to crop requirements, throughout the growing period. Fruits produced by plants inoculated with A. morrowiae presented higher values of TSS/TTA ratio in September and October. The percentage of mycorrhizal colonization varied from 26.2% for A. morrowiae to 46.2% for R. clarus. The inoculation of arbuscular mycorrhizal fungi does not alter the production of strawberry fruits. However, when the plants are inoculated with A. morrowiae there is an improvement in the fruit flavor.

Boosting proso millet yield by altering canopy light distribution in proso millet/mung bean intercropping systems

To elucidate the mechanism by which intercropping proso millet (Panicum miliaceum L.) with mung bean (Vigna radiata L.) increases proso millet yield and to determine how this higher yield results from maximization of resources use efficiency, we designed and conducted four strip intercropping row arrangements, including two rows of proso millet alternating with two rows of mung bean (2P2M), four rows of proso millet alternating with two rows of mung bean (4P2M), four rows of proso millet alternating with four rows of mung bean (4P4M), two rows of proso millet alternating with four rows of mung bean (2P4M), sole proso millet (SP, control) and sole mung bean (SM, control) in Yulin, Shaanxi, China. Photosynthetically active radiation (PAR) in the canopy, radiation use efficiency (RUE), leaf photosynthetic characteristics, dry matter accumulation and allocation, and yield of proso millet were investigated. The results showed that the intercropping systems had higher PAR than the monoculture. Mean PAR intensities were increased by respectively 2.2%–23.4%, 19.8%–59.7%, and 61.2%–133.3% in the proso millet upper, middle and lower canopies compared with SP. The increase in PAR directly increased RUE, a result attributed mainly to the increase in photosynthetic capacity, including net photosynthetic rate and chlorophyll content. These responses resulted in increased dry matter allocation to plant organs. Yield of intercropped proso millet was 6.8%–37.3% higher than that under monoculture and the land equivalent ratios for the different intercropping patterns were all greater than unity (> 1). In general, yield followed a positive linear function of PAR in the intercropping system. The results indicated that intercropping can boost proso millet yield, evidently by altering light distribution within its canopy and consequently increasing RUE, thereby increasing leaf photosynthetic capacity, dry matter accumulation, and allocation to the grain. The optimum combination for improving the growth and yield of proso millet on the Loess Plateau of China was 2P4M.

Virtual Geographic Simulation of Light Distribution within Three-Dimensional Plant Canopy Models

Virtual geographic environments (VGEs) have been regarded as an important new means of simulating, analyzing, and understanding complex geological processes. Plants and light are major components of the geographic environment. Light is a critical factor that affects ecological systems. In this study, we focused on simulating light transmission and distribution within a three-dimensional plant canopy model. A progressive refinement radiosity algorithm was applied to simulate the transmission and distribution of solar light within a detailed, three-dimensional (3D) loquat (Eriobotrya japonica Lindl.) canopy model. The canopy was described in three dimensions, and each organ surface was represented by a set of triangular facets. The form factors in radiosity were calculated using a hemi-cube algorithm. We developed a module for simulating the instantaneous light distribution within a virtual canopy, which was integrated into ParaTree. We simulated the distribution of photosynthetically active radiation (PAR) within a loquat canopy, and calculated the total PAR intercepted at the whole canopy scale, as well as the mean PAR interception per unit leaf area. The ParaTree-integrated radiosity model simulates the uncollided propagation of direct solar and diffuse sky light and the light-scattering effect of foliage. The PAR captured by the whole canopy based on the radiosity is approximately 9.4% greater than that obtained using ray tracing and TURTLE methods. The latter methods do not account for the scattering among leaves in the canopy in the study, and therefore, the difference might be due to the contribution of light scattering in the foliage. The simulation result is close to Myneni’s findings, in which the light scattering within a canopy is less than 10% of the incident PAR. Our method can be employed for visualizing and analyzing the spatial distribution of light within a canopy, and for estimating the PAR interception at the organ and canopy levels. It is useful for designing plant canopy architecture (e.g., fruit trees and plants in urban greening) and planting planning.

Canopy stomatal uptake of NOX, SO2 and O3 by mature urban plantations based on sap flow measurement

Abstract Canopy stomatal uptake of NOX (NO, NO2), SO2 and O3 by three mature urban plantations (of Schima superba, Eucalyptus citriodora and Acacia auriculaeformis) were studied using the sap flow-based approach under free atmospheric conditions. The annual mean concentration for NO, NO2, SO2 and O3 were 18.2, 58.1, 12.8 and 42.4 μg m−3, respectively. The atmospheric concentration exhibited a spring or winter maximum for NO, NO2 and SO2, whereas the concentration maximum for O3 occurred in the autumn. Despite the daytime mean canopy stomatal conductance (GC) being positively related with the photosynthetically active radiation (PAR) and negatively with the vapour pressure deficit (VPD), the maximal daytime mean GC did not appear when the PAR was at its highest level or the VPD was at its lowest level because a positive correlation was noted between the daytime mean PAR and VPD (P G c = a × lnPAR − b (P G c = G sref − m × lnVPD (P

Effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in Picea engelmannii and Abies lasiocarpa at treeline, Medicine Bow Mountains, Wyoming, USA

Here we describe the dynamic effects of cumulus clouds on microclimate and shoot-level photosynthetic gas exchange in saplings of two treeline conifer species—Picea engelmannii and Abies lasiocarpa. Measurements were made during both clear-sky and partly cloudy conditions (∼10–70% cumulus cloud cover) throughout the 2012 growing season within an alpine-treeline ecotone. Cumulus clouds generated dynamic fluctuations in photosynthetically active radiation (PAR), higher maximum PAR (>2500μmolm−2s−1), 2–4 fold increases in diffuse PAR, reduced daily mean and cumulative PAR, lower needle temperatures, and reduced leaf-to-air vapor pressure differences relative to clear-sky conditions. Onset of cloud-shade corresponded with declines in photosynthesis, needle temperatures, and evapotranspiration, which were proportional to cloud duration and opacity. Despite increased diffuse light and greater sunlight intensity during cloud-gaps, photosynthesis was never higher on partly cloudy days compared to clear days in either species, during cloud-gaps or cloud-shade. However, reduced transpiration paired with photosynthesis comparable with clear-sky levels during cloud-gaps resulted in greater instantaneous water use efficiency relative to clear-sky measurements. There was no apparent photoinhibition of photosynthesis reflected in gas exchange measurements in response to abrupt and dramatic changes in PAR levels caused by cumulus clouds. We conclude that cumulus clouds reduce instantaneous and daily carbon gain, although lower needle temperatures and associated reductions in transpirational water loss may alleviate daily and seasonal water stress, thereby enhancing carbon gain over the growing season.

Observation and estimation of photosynthetic photon flux density in Southern China

Observations of photosynthetically active radiation (PAR) and global solar radiation (G) at Sanya, Southern China during 2005–2011 were used for investigating their relationships under various sky conditions. Monthly mean PAR generally increased from 24.73 mol m−2 day−1 in December to 40.97 mol m−2 day−1 in July with annual mean value being about 32.88 mol m−2 day−1. Monthly mean daily PAR fraction (F p) changed from 1.82 mol MJ−1 in March (or April) to 2.03 mol MJ−1 in October with annual mean being 1.90 mol MJ−1. Clearness index (K t) was used for characterizing sky conditions and partly cloudy skies (41.45 %) were the dominated sky condition in this area. Meanwhile, an efficient PAR model has been introduced by investigating the dependence of PAR on K t and cosine of solar zenith angle under any sky conditions. The model has been tested at Sanya, Lhasa, and Inner Mongolia, China through the statistical indices: mean bias error (MBE), mean absolute percentage error (MAPE), and root–mean–square error (RMSE), which were generally lower than −3.6, 8.24, and 10.26 %, respectively. PAR values during 1992–2012 were then reconstructed and it was discovered that annual mean PAR decreased at the rate of −1 mol m−2 day−1 per decade.

Environmental controls on coccolithophore calcification

Coccolithophores are major contributors to global marine planktonic calcification, and in nature coccolithophores are invariably calcified through almost all of their life cycle. The response of calcification to environmental factors is essential in understanding the persistence of coccolithophores through at least 220 million years of changing global environments, and their prospects for current environmental change. So far the responses examined have been at the level of acclimation rather than adaptation in evolution. Variation in results of CO2 manipulation experiments can be tentatively attributed to variation among genotypes rather than differences in experimental procedure. Comparisons of methods using the same genotype, and of several genotypes using a single method, suggest significant variation among genotypes. The general response is a decreased particulate inorganic carbon (PIC) to particulate organic carbon (POC) ratio in higher than present CO2 concentrations and vice versa for lower CO2 concentrations. Fewer studies have investigated the effect of other environmental factors. Decreased availability of phosphorus and, to a lesser extent, nitrogen, as well as decreasing photosynthetically active radiation (PAR) down to a certain low value increase PIC: POC, while variable results have been found for changes in ultraviolet radiation (UVR). Many of these results can be accommodated by considering the restriction of calcification to the G1 phase of the cell cycle and the length of this phase under different growth conditions. Fewer studies have investigated the interactions among environmental factors which change with increased CO2 and increasing sea surface temperature; the shoaling of the thermocline will increase the mean PAR and UVR whilst decreasing nitrogen and phosphorus availability. More studies of these interactions, as well as of genetic adaptation in response to changed environmental factors, are needed.

Optical properties of canopies of the tropical seagrass Thalassia testudinum estimated by a three‐dimensional radiative transfer model

Three‐dimensional models of seagrass canopies were constructed by treating leaves as flexible strips that fall under their own weight into naturalistic canopy structures. Canopy structures incorporated shoot density, canopy height, and physical and along‐length optical characteristics of leaves from an empirical data set of six seagrass canopies from a reef lagoon in Puerto Morelos, Mexico. Multiple runs of a radiosity method radiative transfer model elucidated the dependence of various canopy optical properties on incident radiance zenith angle, such as within‐canopy diffuse attenuation, the absorption of photosynthetically active radiation (PAR) by different canopy‐complex components, along‐leaf variation in PAR absorption, and canopy bidirectional reflectance distribution functions (BRDFs). Intersite variation in mean PAR absorption by green leaf sections was primarily associated with leaf area index (LAI). Variation in PAR absorption with incident radiance angle was generally small for incident angles within Snella's window (< 49°). Within canopies, absorption by water itself had a negligible effect. Canopy BRDFs were not Lambertian and exhibited features related to canopy architecture. Model outputs validated well in terms of energy conservation and empirical measurements of within‐canopy PAR attenuation. The model framework has clear applications to improve understanding of seagrass canopy light harvesting and photosynthetic carbon fixation and to aid the development of quantitative biooptical remote sensing methods for seagrass beds.

Shade effect on photosynthesis and photoinhibition in olive during drought and rewatering

Olive tree ( Olea europaea L.) is commonly grown under environmental conditions characterised by water deficit, high temperatures and irradiance levels typical of Mediterranean semi-arid regions. Measurement of gas exchange, chlorophyll content, chlorophyll fluorescence and photoinhibition was carried out on two-year-old olive trees (cv. ‘Coratina’) subjected to a 21-day period of water deficit followed by 23 days of rewatering. At the beginning of the experiment, plants were divided in to two groups and subjected to different light regimes: exposed plants (EP) under a mean photosynthetically active radiation (PAR) at mid-day of 1800 μmol m −2 s −1 and shaded plants (SP) under a mean PAR of 1200 μmol m −2 s −1. The effect of drought and high irradiance levels caused a reduction of gas exchange and photosystem 2 (PSII) efficiency, in terms of quantum yield of PSII ( Φ PSII) both in EP and SP. Shading conditions allowed plants to maintain a high photosynthetic activity at low values of stomatal conductance, whereas in EP the reductions in photosynthetic efficiency and intrinsic water efficiency were due to non-stomatal components of photosynthesis. The decrease in photosynthetic activity and the increase of photoinhibition under drought were more marked in EP than in SP. Full sunlight caused in EP a higher non-photochemical quenching, whereas SP showed a better photochemical efficiency. The information here obtained can be important to understand the mechanisms by which olive plants can minimize photoinhibition when subjected to simultaneous abiotic stresses.

Estimation of Incident Photosynthetically Active Radiation from GOES Visible Imagery

Abstract Incident photosynthetically active radiation (PAR) is an important parameter for terrestrial ecosystem models. Because of its high temporal resolution, the Geostationary Operational Environmental Satellite (GOES) observations are very suited to catch the diurnal variation of PAR. In this paper, a new method is developed to derive PAR using GOES data. What makes this new method distinct from the existing method is that it does not need external knowledge of atmospheric conditions. The new method retrieves both atmospheric and surface conditions using only at-sensor radiance through interpolation of time series of observations. Validations against ground measurement are carried out at four “FLUXNET” sites. The values of RMSE of estimated and ground-measured instantaneous PAR at the four sites are 130.71, 131.44, 141.16, and 190.22 μmol m−2 s−1, respectively. At the four validation sites, the RMSE as the percentage of estimated mean PAR value are 9.52%, 13.01%, 13.92%, and 24.09%, respectively; the biases are −101.54, 16.56, 11.09, and 53.64 μmol m−2 s−1, respectively. The independence of external atmospheric information enables this method to be applicable to many situations in which external atmospheric information is not available. In addition, topographic impacts on surface PAR are examined at the 1-km resolution at which PAR is retrieved using the GOES visible band data.

The impact of light quality and leaf wetness on photosynthesis in north-west Andean tropical montane cloud forest

Photosynthesis was limited by low-intensity photosynthetically active radiation (PAR) and leaf wetness in a lower montane cloud forest (LMCF) of Cauca, Colombia. Mean PAR intensity remained below the saturation level for leaf-scale net photosynthesis (Pn) throughout the solar day during the wet season and for most of the solar day during the dry season. PAR represented a smaller fraction of total solar radiation (K↓) in LMCF than in lowland rain forest (LRF). In LMCF trees and shrubs, mean PAR-saturated Pn ranged from 4.3–10.6 μmol C m−2 s−1 at 1450 m, and from 3.5–10.2 μmol C m−2 s−1 at 2150 m. Pn was reduced by abaxial wetness in leaves of some trees and shrubs, and eliminated in others. This study indicates that persistent cloudiness and interception of cloud water by leaves limit LMCF productivity.

Comparative analysis of the bryophyte vegetation in acidophilous forest communities in the Bükk Mts (NE Hungary)

The bryophyte vegetation of three acidophilous forest communities of four habitats on radiolarian bedrock were investigated in the Bükk Mts. The bryophyte layers of Deschampsio-Fagetum sylvaticae Soó 1962, Genisto pilosae-Quercetum petraeae Zólyomi et al. 1958 and Genisto tinctoriae-Quercetum petraeae Klika 1932 were compared on the basis of the dominance and frequency of 15 bryophyte species in the communities using ordination and cluster analysis methods. The soil pH and the photosynthetically active radiation (PAR) values in the four sampling areas were also compared. The results refer to a strong connection of the bryophyte species composition considering the dominance values and frequency of the species with the community type in different spatial scale. The soil pH values do not differ significantly in the habitats and they refer to a high amount of hidden acidity which is the effect of the radiolarian bedrock type. The PAR values have been statistically analysed and represent the homogeneity or the heterogeneity of the canopy layer and the effect of the exposition. The results show significant difference in the variances of PAR values between Genisto pilosae-Quercetum and the other two community types and the mean PAR value of Deschampsio-Fagetum sylvaticae differ significantly from the mean values of the two Genisto tinctoriae-Quercetum petraeae stands.

Water vapor transfer from a vegetated surface: A numberical study of bulk transfer coefficients and canopy resistances

The semi-analytical model outlined in previous studies (Massman, 1987a, b) to describe momentum and heat exchange between the atmosphere and vegetated surfaces is extended to include water vapor exchange. The methods employed are based on one-dimensional turbulent diffusivities and use numerical solutions to the steady-state diffusion equation. The model formulates stomatal response as a function of vapor pressure deficit and the within-canopy profile of mean photosynthetically-active radiation (PAR). It is then used to assess the influence that foliage structure, density, and sheltering can have upon the bulk transfer coefficient, kB v -1, and the canopy resistance. A general analytical formulation of the canopy resistance based on the mean within-canopy profile of PAR is proposed and found to agree with the model's solutions for canopy resistance to within a few percent.