Optical teledetection of the vertical attenuation coefficient for downward quantum irradiance of photosynthetically available radiation in turbid inland waters

Depth profiles of downward quantum irradiance of photosynthetically available radiation in situ and spectral subsurface irradiance reflectance, obtained from water-leaving radiance, were determined in different inland water types. These included the large, shallow and eutrophic IJssel lagoon in the Netherlands, eighteen Dutch lakes differing in depth and trophic state, and the large, shallow and eutrophic Lake Tai in China. The attenuation coefficient for downward irradiance ranged from 0.7 to 5.4 m^-1. An empirical relationship with the backscattering coefficient in the near-infrared waveband, derived from the reflectance, and three spectral reflectance band ratios explained 91% of the variation. The potential assessment of the spatial distribution of phytoplankton production through shipboard optical teledetection of the concentration of chlorophyll-a and the attenuation coefficient for downward irradiance is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of phytoplankton in determining the underwater light climate in Lake Constance

At all seasons, the underwater light field of meso-eutrophic large (480 km²) deep (mean: 100 m) Lake Constance was studied in conjunction with the assessments of vertical distributions of phytoplankton chlorophyll concentrations. Vertical profiles of scalar, downwelling and upwelling fluxes of photosynthetically available radiation, as well as fluxes of spectral irradiance between 400 and 700 nm wavelength were measured.The overall transparency of the water for PAR is highly dependent on chlorophyll concentration. However, the spectral composition of underwater light is narrowing with water depth regardless of phytoplankton biomass.Green light is transmitted best, even at extremely low chlorophyll concentrations. This is explained by the selective absorption of blue light by dissolved organic substances and red light by the water molecules. Nevertheless, significant correlations were found between vertical attenuation coefficients of downwelling spectral irradiance and chlorophyll concentrations at all wavelengths. The slopes of the regression lines were used as estimates of chlorophyll-specific spectral vertical light attenuation coefficients (K _c()).The proportions of total upwelling relative to total downwelling irradiance (reflectance) increased with water depth, even when phytoplankton were homogeneously distributed over the water column. Under such conditions, reflectance of monochromatic light remained constant. Lower reflectance of PAR in shallow water is explained by smaller bandwidths of upwelling relative to downwelling light near the water surface. In deeper water, by contrast, the spectra of both upwelling and downwelling irradiance are narrowed to the most penetrating components in the green spectral range. Reflectance of PAR was significantly correlated with chlorophyll concentration and varied from 1% and 1-% at low and high phytoplankton biomass, respectively. Over the spectrum, reflectance exhibited a maximum in the green range. Moreover, in deeper layers, a red maximum was observed which is attributed to natural fluorescence by phytoplankton chlorophyll.     

Characterization of the light field in laboratory scale enclosures of eutrophic lake water (Lake Loosdrecht,The Netherlands)

Light conditions in laboratory scale enclosures (LSE) of shallow, eutrophic Lake Loosdrecht (The Netherlands), including a method for simulating a natural incident light course, are described. Total PAR (400–700 nm) and spectral irradiance distribution were measured at sestonic chlorophyll a and dry weight concentrations 100 mg m^–3 and 16 g m^–3, respectively. Phytoplankton was dominated by Oscillatoria spp. The euphotic depth (Z _eu) was 0.7–1.0 m. Shortly after filling the LSE with lake water, diffuse attenuation coefficients ranged from 14 m^–1 for blue to 5 m^–1 for red light. Around Z _eu, attenuation in the blue region was markedly lower and irradiance reflectance (R) continued to increase; these anomalies were caused by lateral incident light from the LSE's waterbath. Spectrophotometry indicated that absorption was mainly by particles, but dissolved humic substances (gilvin) were also important. The particles were likely to be dominated by detritus absorbing more blue relative to red light. Subsurface R in lake water in the LSE had a maximum around 705 nm and low values in the blue band, but was lower than that previously reported for measurements in situ. Wash-out of detritus, presumably both dissolved and particulate fractions, by flow-through with synthetic medium, greatly affected the spectral reflectance measured outside the LSE. The maximum value of R decreased from 0.022 to 0.009, and the peak shifted to 550 nm.     

Seasonal attenuation of quantum irradiance (400–700 NM) in three Nebraska reserviors

The attenuation of down-welling quantum irradiance (400–700 nm) was monitored seasonally in three eutrophic Nebraska reservoirs from July, 1975, to June, 1976. Measurements were made at four stations in McConaughy, three in Pawnee, and three in Yankee Hill using a commercially available, quantum irradiance sensor. The mean vertical attenuation coefficient () for McConaughy varied within a range of thirty-four-fold (0.16 5.45 m^-1), and this range is apparently the greatest reported and this range is apparently the greatest reported for a freshwater system in which data were collected with a quantum sensor. Annual average values for McConaughy, Yankee Hill, and Pawnee were 0.69 m^–1, 1.08 m^–1, and 1.25 m^–1, respectively. The euphotic zone and the 1% level of subsurface irradiance is discussed with respect to the penetration of photosynthetically active radiation (PAR) into natural waters.This study was supported by a grant from the Office of Water Research and Technology, U.S. Dept. of the Interior, project number A-043-NEB, agreement number 14-34-0001-6028, to James R. Rosowski, School of Life Sciences, University of Nebraska, Lincoln.     

盖度与冠层水深对沉水植物水盾草光谱特性的影响

遥感技术可应用于大尺度实时监测沉水植物的分布与生长状况。然而沉水植物的光谱特征受其冠层在水下深度的影响,从而影响湖泊和河流中沉水植物的遥感影像解译与信息提取。应用地物光谱仪,通过野外原位测定和室外控制试验,实测了沉水植物水盾草(Cabomba caroliniana)群落冠层在水下不同深度的反射光谱,分析了冠层水深对水盾草反射光谱的影响,并建立了基于光谱反射率和冠层水深的水盾草群落盖度反演模型。研究结果表明(1)不同盖度的水盾草群落光谱反射率的基本特征主要体现在绿光和近红外波段;(2)水盾草群落的光谱反射率与冠层水深基本呈负相关,相同盖度水盾草群落的光谱反射率随冠层水深的增加而减小,在近红外波段尤其明显;(3)水盾草群落冠层水深越小,其盖度与光谱反射率的相关性越强,且水盾草群落盖度越大,其光谱反射率与冠层水深的相关性越显著;(4)水盾草光谱反射率与盖度相关的最佳波段在692—898 nm,与冠层水深相关最佳的波段在710 nm和806 nm附近;(5)在710 nm和806 nm处建立的结合冠层水深的修正模型,无论是回归方程决定系数(R2),还是水盾草群落盖度的反演精度都明显高于仅用光谱反射率反演盖度的简单模型,因此可有效减除冠层水深对反演精度的影响。本研究的结果可为遥感监测沉水植物的分布和动态变化,以及沉水植物生物物理参量反演提供科学依据。     

Vertically detailed in situ measurements of gelbstoff: demonstrating the impact of a runoff event

The successful performance of a rapid profiling light absorption instrument in making vertically detailed measurements of gelbstoff (dissolved yellow material) in a mesotrophic water supply reservoir is demonstrated. In situ measurements of gelbstoff spectra (g _λ; m^{? 1}) for the photosynthetically active radiation wavelength (λ) interval (400–700 nm), and values of g ₄₄₀, matched closely values obtained through traditional laboratory spectrophotometric analyses. The advantages of the high vertical resolution capability of the in situ absorption instrument (～10 measurements per meter of depth) was demonstrated through the depiction of the vertical details (subsurface peak about 2.25 m thick) of the enrichment in gelbstoff that occurred following a major runoff event, associated with the entry of inflow(s) as a plunging interflow. Values of g ₄₄₀ increased at the depth of the subsurface peak by nearly 100% as a result of this event. The detailed vertical resolution of gelbstoff is valuable to managers of water supply lakes and reservoirs to support selection of intake depths to avoid delivery of water with high levels of color, and to researchers investigating the origins, fate and transport of this material.     

MICROSENSOR STUDIES OF OXYGEN AND LIGHT DISTRIBUTION IN THE GREEN MACROALGA CODIUM FRAGILE1

Scalar irradiance, oxygen concentration, and oxygenic photosynthesis were measured at 0.1 mm spatial resolution within the tissue of the siphonous green macroalga Codium fragile subsp. tomentosoides (van Goor) Silva by fiber-optic scalar irradiance microsensors and oxygen microelectrodes. The scalar irradiance of visible light was strongly attenuated in the outer 0.2 mm of the tissue but was nearly constant for the subsequent 1.0 mm of photo-synthetic tissue. Far-red scalar irradiance at 750 nm increased below the tissue surface to a maximum of 200% of incident irradiance at 1.2 mm depth due to multiple scattering in the medullary tissue. The constant intensity of visible light below 0.2 mm was thus a result of the combined effects of absorption and backscattering from the medulla. The oxygen exchange between the alga and the surrounding water was diffusion-limited with a steep O₂gradient inside and around the alga. In darkness, the tissue below 0.6 mm became anoxic, and endophytic extracellular space provided an environment where anoxygenic microbial processes may occur. When illuminated at 160 nmol photons·^?2·^?1, O₂ concentrations exceeded ambient levels throughout the thallus, with a maximum of 250% of air saturation just below the surface. The amplitude of oxygen variation was buffered by gas bubbles formed in the medullary tissue.     

Optical parameters of leaves of 30 plant species

Optical parameters (absorption coefficient k, infinite reflectance R∞, scattering coefficient 8) are tabulated for seven wavelengths and analyzed for statistical differences for 30 plant species. The wavelengths are: 550 nm (green reflectance peak), 650 nm (chlorophyll absorption band), 850 nm (infrared reflectance plateau), 1450 nm (water absorption band), 1650 nm (reflectance peak following water absorption band at 1450 nm), 1950 nm (water absorption band), and 2200 nm (reflectance peak following water absorption band at 1950 nm).     

Spectral downwelling irradiance in an Antarctic lake

Summary Spectral downwelling irradiance (400–700 nm) was determined in the ice-covered Lake Hoare located in the dry valleys near McMurdo Sound, Antarctica. Full waveband PAR beneath the ice was <44E·m^-2·s^-1 or <3% of surface downwelling irradiance. Maximum light transmission just beneath the 2.6–4 m ice cover, which contained sediments and air bubbles, occurred between 400–500 nm. In the water column below, attenuation of light by phytoplankton in the 400–500 nm region and between 656–671 nm suggested absorption of light by algal pigments.     

Trophic state and water turn-over time in six choked coastal lagoons in Brazil

Comparison of total phosphorus and chlorophyll-a concentration, nutrient loading, and water turn-over time in six shallow choked lagoons along the coast of the state of Rio de Janeiro, Brazil, established that water turn-over time is related to the trophic state of the lagoons with additional anthropogenic nutrient loading affecting this relationship. Turnover time was calculated as a flushing half-life from rainfall, evaporation, runoff, and tidal exchange data, and trophic state was calculated from the quantity and quality of dissolved inorganic nutrients, total phosphorus, and chlorophyll-a standing stock. Flushing half-life of the lagoons ranged between 1 and 27 days, annual phosphorus areal loading from 3 to 18 mg m^–2d^–1, and chlorophyll-a standing stock from 6 to 160 mg M^–2     

Solar UV radiation modulates daily production and DNA damage of marine bacterioplankton from a productive upwelling zone (36°S), Chile

In upwelling ecosystems, such as the Humboldt Current system (HCS) off Concepción, the effects of solar radiation on bacterioplankton incorporation rates have been related to previous light acclimation and responses to irradiance. In this paper, we study the daily effect of Photosynthetic Active Radiation (PAR, 400-700 nm) and ultraviolet radiation UVR (280-400 nm) on bacterial secondary production (BSP). We also considered the DNA damage-repair response to solar radiation stress by the induction of cyclobutane pyrimidine dimers (CPDs). Experiments were conducted with natural bacterioplankton assemblages (0.2-0.7 μm) collected off Concepción (36°S), during the austral Spring, October-November, 2004. Surface (0.5 m) and subsurface (80 m) bacterioplankton samples were exposed to different solar radiation treatments for 5-20 h. BSP was estimated by ¹⁴C-leucine and ³H-thymidine incorporation at several time intervals, whereas CPDs accumulation was assessed using immunoassay techniques. During high irradiance periods, BSP was mainly affected by PAR in both surface and subsurface assemblages and, to a lesser, but significant (Tukey < 0.05) extent, by UV-A (320-400 nm) and UV-B (280-320 nm) radiation. Maximum inhibition of BSP in surface waters was 78%; growth rates (μ) and bacterial growth efficiency (BGE) were also low (78% and 66% respectively). Subsurface water assemblages, on the contrary, showed a ∼ 25 fold enhancement of BSP, μ, and BGE. Both types of assemblages had a rapid CPDs accumulation (maximum 60 CPDs Mb^− 1) during high irradiance periods. Recovery of BSP inhibition and DNA damage in surface bacteria was total after sunset and after the night incubation period, resembling pre-exposure levels. Despite subsurface BSP enhancement during day-night exposure, residual DNA damage was detected at the end of the experiment (20 CPDs Mb^− 1) suggesting a chronic DNA damage. Our results represent the worst case scenario (i.e., assemblages receiving surface irradiances as may occur in this upwelling zone) and indicate that surface and subsurface bacterial assemblages in the HCS are both highly sensitive to solar irradiance. However, they showed different responses, with surface bacteria having more effective photorepair mechanisms, and sustaining higher BSP than subsurface assemblages.     

Continuous underwater light measurement near Helgoland (North Sea) and its significance for characteristic light limits in the sublittoral region

Underwater irradiance was measured at intervals of 20 min for one year at 2 water depths (2.5 and 3.5 m below M.L.W.S.) and in 3 spectral regions in the sublittoral region of the rocky island of Helgoland. Data are presented for spectral and total irradiance at water depths ranging from 2 to 15 m (below M.L.W.S.). 90% of the total annual light reaching sublittoral habitats is received during the period from April to September, when Jerlov water type 7 (occasionally water type 5) dominates. During the other half of the year, the water is very turbid, and transparency is so low that long dark periods occur even at moderate water depths. The total annual light received at the lower kelp limit (Laminaria hyperborea), at 8 m water depth, is 15 MJ m^–2 year^–1 or 70 E m^–2 year^–1, which corresponds to 0.7% of surface irradiance (visible). At the lower algal limit (15 m water depth) these values are 1 MJ m^–2 year^–1 or 6 E m^–2 year^–1, corresponding to 0.05% of surface irradiance. These data are similar to measurements at the same limits in several different geographical areas, and may determine the depth at which these limits occur.     

Photosynthetic activity and respiration in an equatorial African soda lake

SUMMARY. Photosynthetic activity and respiration in Lake Sonachi (Kenya), a meromictic soda lake lying in a volcanic crater, were measured through diel cycles during a 15-month period. A pattern of thermal stratification in the morning and mixing in the afternoon and night occurred in the mixolimnion. Diel variations in dissolved oxygen at 50 cm were 2.2–7.5 mgO₂ 1^-11% of the incident photosynthetically available irradiance (PAR) reached a depth of 1.3–2.4 m and, as a consequence, the steepest thermal gradients and highest oxygen concentrations occurred in the top 1–2 m. Vertical profiles of dissolved oxygen were used in three ways to estimate photosynthetic and respiration rates. Changes in dissolved oxygen at the depth of maximal photosynthesis (c. 50 cm) during mid-morning were corrected for vertical diffusion to determine net free water oxygen increases of 70-1800 mg O₂ m^-3 h^-1 Variations in areal oxygen content at successive intervals throughout the day and night were corrected for air-water oxygen exchange to calculate net free water oxygen change per h. Maximal rates of increase (550–4850 mg O₂ m^-2 h^-1) usually occurred in late morning or early afternoon; maximal rates of decrease (440–2600 mg O₂ m^-2 h^-1) were common at sunset. The correction for air-water exchange was usually small because of the low wind speeds and the nearness to saturation of the surface water. Summation of daytime and night-time rates of oxygen change provide estimates of net (-3.4–12 gO₂ m ^-2) and gross (-0.7-18.7 g O₂ m^-2) daily photosynthesis and respiration (0.8-7.2 gO₂ m^-2). Photosynthetic rates of bottled samples ranged from 150 to 870 mgO₂m ^-2h ^-1 and 1.4 to 6.8 g O₂, m^-2 day ^-1The efficiency of utilization of PAR incident on the lake surface varied from 1.0 to 7.2 mmol O₂E^-1 periods with higher irradiance typically had lower efficiencies. Free water estimates of photosynthesis usually exceeded the rates measured in bottles. For example, net, free water changes per hour were 1.2–10 times higher than gross areal rates per hour in bottles. Photosynthetic activity in Lake Sonachi in 1973 and 1974 was modest when compared to other tropical African soda lakes.     

The underwater light field in the Bellingshausen and Amundsen Seas (Antarctica)

The underwater light field in the Bellingshausen andAdmundsen Seas was characterised using data collectedduring the R/V Polarstern cruise ANT XI/3, from12.1.94 to 27.3.94. The euphotic zone varied from 24to 100 m depth. Spectral diffuse vertical attenuationcoefficients (K _d ())were determined for 12narrow wavebands as well as for photosyntheticallyavailable radiation (PAR, 400–700 nm): K _d (490)ranged from 0.03 to 0.26 m¹; K _d (550) from0.04 to 0.17 m¹; K _d (683) from 0.04 to0.17 m¹; and K _d (PAR) varied from 0.02 to0.25 m¹. K _d () for wavelengths centred at412 nm, 443 nm, 465 nm, 490 nm, 510 nm, 520 nm and550 nm were significantly correlated with chlorophyllconcentration (ranging from 0.1 to 6 mg m³). Thevertical attenuation coefficients for 340 nm and380 nm ranged from 0.10 to 0.69 m¹ and from 0.05to 0.34 m¹, respectively, and were also highlycorrelated with chlorophyll concentrations. These K _d values indicate that the 1% penetration depthmay reach maxima of 46 m and 92 m for 340 nm and380 nm, respectively. The spectral radiancereflectances (Rr()) for 443 nm, 510 nm and 550 nmwere less than 0.01 sr¹. Rr() for 665 nm and683 nm increased with depth up to 0.2 sr¹ because ofchlorophyll fluorescence. Using a model that predicts downwardirradiances by taking into account the attenuation bywater and absorption by chlorophyll, we show thatchlorophyll fluorescence has a significant influenceon the red downward irradiance (E _d (633, 665, 683))in deeper layers. The ability of the phytoplanktonpopulation to influence the light environment byautofluorescence and absorption processes depends onthe light conditions and on the photoacclimation ofthe cells, represented by the in vivo crosssection absorption coefficient of chlorophyll (a*). Theobtained mean chlorophyll-specific light attenuationcoefficients of phytoplankton in situ (k _d ) are higherthan the in vivo absorption coefficient of chlorophyll,more than to be excepted from the scattering. a*(), m² mg chl¹, decreased due topackaging effect with increasing chlorophyllconcentrations.     

Synoptic water clarity assessment in the Florida Keys using diffuse attenuation coefficient estimated from Landsat imagery

The diffuse attenuation coefficient, K (m^{− 1}), is a measure of the effective attenuation of light in the water column. It characterizes water clarity and is used as a proxy for water quality. Mapping of shallow water benthic habitats using optical means, including daytime visible satellite imagery, requires knowledge of K to correct for water column effects such as light absorption and scattering. Traditionally, K is derived from imagery using a priori knowledge of bottom types at different depths and specific locations, and assuming that light attenuates exponentially with depth. This technique is applied to three Landsat 7 satellite images (February, May, and July 2000) from the Florida Keys Reef Tract between Key Largo and Key West. Interpolated depth data, initially from NOAA vector chart data, with uncertainties of ±0.5 m, were draped over 30 m spatial resolution Landsat satellite imagery. K was derived for 27 sites where bright sand could be observed at depths between 3 and 20 m. The blue and green bands (Landsat bands 1 and 2 at 450–520 nm and 520–600 nm, respectively) provided K values consistent with time and location. Average K values for bands 1 and 2, respectively, were 0.029 and 0.043 m^{− 1} (Lower Keys), 0.050 and 0.072 m^{− 1} (Middle Keys), and 0.063 and 0.082 m^{− 1} (Upper Keys). The red band (band 3, 630–690 nm) provided more ambiguous and erroneous results with several negative K values, attributed primarily to three factors use of a simple atmospheric correction, the high absorption and rapid attenuation of red light in water, and low radiometric sensitivity of the Landsat ETM + sensor. Despite the fact that these observations were a snapshot in time, trends were observed for the Upper, Middle, and Lower Keys, possibly due to the influence of more turbid Florida Bay waters.     

Is Ambient Light during the High Arctic Polar Night Sufficient to Act as a Visual Cue for Zooplankton?

The light regime is an ecologically important factor in pelagic habitats, influencing a range of biological processes. However, the availability and importance of light to these processes in high Arctic zooplankton communities during periods of ''complete'' darkness (polar night) are poorly studied. Here we characterized the ambient light regime throughout the diel cycle during the high Arctic polar night, and ask whether visual systems of Arctic zooplankton can detect the low levels of irradiance available at this time. To this end, light measurements with a purpose-built irradiance sensor and coupled all-sky digital photographs were used to characterize diel skylight irradiance patterns over 24 hours at 79°N in January 2014 and 2015. Subsequent skylight spectral irradiance and in-water optical property measurements were used to model the underwater light field as a function of depth, which was then weighted by the electrophysiologically determined visual spectral sensitivity of a dominant high Arctic zooplankter, Thysanoessa inermis. Irradiance in air ranged between 1–1.5 x 10^-5 μmol photons m^-2 s^-1 (400–700 nm) in clear weather conditions at noon and with the moon below the horizon, hence values reflect only solar illumination. Radiative transfer modelling generated underwater light fields with peak transmission at blue-green wavelengths, with a 465 nm transmission maximum in shallow water shifting to 485 nm with depth. To the eye of a zooplankter, light from the surface to 75 m exhibits a maximum at 485 nm, with longer wavelengths (>600 nm) being of little visual significance. Our data are the first quantitative characterisation, including absolute intensities, spectral composition and photoperiod of biologically relevant solar ambient light in the high Arctic during the polar night, and indicate that some species of Arctic zooplankton are able to detect and utilize ambient light down to 20–30m depth during the Arctic polar night.     

Effects of light history on primary productivity in a phytoplankton community dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii

1. The effects of instantaneous irradiance and short‐term light history on primary production were determined for samples from a subtropical water reservoir dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii. ¹⁴C‐bicarbonate uptake incubations were conducted on water samples from the reservoir, for irradiance (photosynthetically active radiation) ranging from 0 to 1654 μmol quanta m⁻² s⁻¹. Prior to the ¹⁴C incubations, cells were pre‐treated at irradiance levels ranging from 0 to 1006 μmol quanta m⁻² s⁻¹. 2. The average irradiance experienced by cells during the 2–2.5 h pre‐treatment incubations affected the productivity–irradiance (P–I) parameters: exposure to high light in pre‐treatment conditions caused a substantial decrease in maximum rate of primary production P_max and the photoinhibition parameter β when compared to cells pre‐treated in the dark. 3. While the data collected in this study were not sufficient to develop a full dynamic model of C. raciborskii productivity, P_max and β were modelled as a function of pre‐treatment irradiance, and these models were applied to predict the rate of primary production as a function of both instantaneous and historical irradiance. The results indicated that while cells with a history of exposure to high irradiance will be the most productive in high irradiance, production rates will be highest overall for dark‐acclimated cells in moderate irradiance. 4. Our results may explain why optically‐deep mixing favours C. raciborskii. If the mixing depth z_m exceeds the euphotic depth z_eu, cells will be dark‐acclimated, which will increase their rate of production when they are circulated through the euphotic zone. These results also predict that production rates will be higher during morning hours than for the same irradiance in the afternoon, which is consistent with other phytoplankton studies. 5. Since the rate of production of C. raciborskii‐dominated systems cannot be described by a single P–I curve, accurate estimates of production rates will require measurements over the daily light cycle.     

Effects of irradiance, water temperature and nutrients on the growth of sporelings of the crustose coralline alga Lithophyllum yessoense Foslie (Corallinales, Rhodophyceae)

Lithophyllum yessoense Foslie is a markedly dominant subtidal, crustose coralline alga in south–western Hokkaido, Japan. In this study, the effects of irradiance, water temperature and nutrients (nitrate and phosphate) on the growth of sporelings of the alga were examined. The relative growth rate (RGR) was saturated at 17.6% d^?1 at a high irradiance (240 umol photon m²s^?1). Even at a low irradiance (10.7–49.9 umol photon m^?2s^?1), RGR was 7.1–12.7% d^?1 The survival rate of sporelings was greater than 80% at irradiance above 10.7 μmol photon m^?2s^?1 throughout the culture period. The growth of L. yessoense sporelings was promoted at 15°C and 20°C, but inhibited at 5°C. The half‐saturation constants (Ks) for growth were about 0.5 umol L^?1 and 0.14 umol L^?1 for nitrate and phosphate, respectively. Saturated nitrate and phosphate concentrations for the growth were about 4.0 μmol L^?1 and 0.4 μmol L^?1, respectively, suggesting that L. yessoense is adaptable to a relatively high water temperature, a wide range of irradiance, and low ambient nitrate and phosphate concentrations. The results provide a possible explanation of why L. yessoense is dominant in the environments of south‐western Hokkaido.     

Effects of irradiance on growth and flowering in the shade plant, cineraria

Effects of total irradiance on growth and flowering were studied in cineraria cv. Cindy Blue grown under warm (mean 21°C) glasshouse conditions. Efficiency of light conversion for leaf and shoot dry weight increase were reduced from 0.08 to 0.02 as the mean daily light integral increased from 0.9 to 4.4 MJ m^-2 day^-1 but no significant difference in leaf area were associated with this. Specific leaf area decreased exponentially from 0.07 to 0.02 m²g¹ over the cumulative irradiance range 23 to 127 MJ m^-2 after the start of treatments and thereafter remained stable. A light integral of 19.2 MJ m^-2 were required for initiation of one leaf in plants grown under a daily integral of 4.4 MJ m^-2 day^-1, as compared with only 5.1 MJ m^-2day^-1 required per leaf in plants grown at less than 0.9 MJ m^-2day^-1. Neither chronological duration of juvenile development nor leaf number below the flower was affected by irradiance. However, as the rate of leaf initiation increased with irradiance up to 2.4 MJ m^-2day^-1 so the rate of progress to flower visibility increased linearly with irradiance over the same range. This rate then remained constant from 2.4 to 4.4 MJ m^-2day^-1. Length of the main flowering shoot decreased and the number of flowering shoots increased as irradiance increased from 0.9 to 2.4 MJ m^-2 day^-1 and then remained unchanged by further increases in irradiance.     

Effects of soil structural discontinuity on root and shoot growth and water use of maize

The role of roots penetrating various undisturbed soil horizons beneath loose layer in water use and shoot growth of maize was evaluated in greenhouse experiment. 18 undisturbed soil columns 20 cm in diameter and 20 cm in height were taken from the depths 30–50 cm and 50–70 cm from a Brown Lowland soil, a Pseudogley and a Brown Andosol (3 columns from each depth and soil). Initial resistance to penetration in undisturbed soil horizons varied from 2.5 to 8.9 MPa while that in the loose layer was 0.01 MPa. The undisturbed horizons had a major effect on vertical arrangement of roots. Root length density in loose layer varied from 96 to 126 km m^-3 while in adjacent stronger top layers of undisturbed horizons from 1.6 to 20.0 km m^-3 with higher values in upper horizons of each soil. For specific root length, the corresponding ranges were 79.4–107.7 m g^-1 (on dry basis) and 38.2–63.7 m g^-1, respectively. Ratios of root dry weight per unit volume of soil between loose and adjacent undisturbed layers were much lower than those of root length density indicating that roots in undisturbed horizons were produced with considerably higher partition of assimilates. Root size in undisturbed horizons relative to total roots was from 1.1 to 38.1% while water use from the horizons was from 54.1 to 74.0%. Total water use and shoot growth were positively correlated with root length in undisturbed soil horizons. There was no correlation between shoot growth and water use from the loose layers.