Photosynthetically Active Photon Flux Density Research Articles

Matching light and nitrogen distributions in the maize canopy to achieve high yield and high radiation use efficiency

Light and nitrogen distribution within a plant's canopy reflect growth adaptation of crop to the environment and are conducive to improving the carbon assimilation ability. So is it possible to rationalize regulation in crop production to maximize yield by improving light and nitrogen distribution without adding additional inputs? In this study, the effects of different nitrogen application rates and planting densities on canopy light and nitrogen distribution of two high-yielding maize cultivars (i.e., XY335 and DH618) and the regulation of canopy physiological characteristics on radiation use efficiency (RUE) and yield were studied based on high yield field experiments in Qitai, China, during 2019 and 2020. The results showed that the distribution of photosynthetically active photon flux density (PPFD) in the maize canopy has been decreasing from top to bottom, while the vertical distribution of specific leaf nitrogen (SLN) initially increased and then decreased from top to bottom in the canopy. When SLN began to decrease, the PPDF of XY335 and DH618 were 0.5 and 0.3, corresponding to 40.6 and 49.3% of the total leaf area index (LAI). The KN/KL in the middle and lower canopy of the XY335 (0.32) was 0.08 higher than that of DH618 (0.24). Yield and RUE of the XY335 (17.2 t ha-1, 1.8 g MJ-1) were 7.0% (1.1 t ha-1) and 13.7% (0.2 g MJ-1) higher than that of DH618 (16.1 t ha-1, 1.6 g MJ-1). Therefore, better light conditions (the proportion of LAI in the upper and middle canopy is small) improved the light distribution when SLN started to decrease, thus helping to mobilize nitrogen distribution and maintain a high nitrogen extinction coefficient (KN) and KN/light extinction coefficient (KL). And KN/KL was a key parameter for yield improvement when maize nutrient requirements were met at 360 kg ha-1. At this time, an appropriately optimized high planting density could promote nitrogen utilization and produce higher yield and higher efficiency. This study will be important for maize high yield and high efficiency cultivation and breeding in the future.

Fatores abióticos e densidade de fótons fotossinteticamente ativos afetam os mecanismos fisiológicos de jabuticaba

ABSTRACT In fruit species, the amount of solar energy absorbed can influence fruit quality; hence, ensuring optimal light distribution management in the canopy of plants is essential. Therefore, the objectives of this study were (i) to analyze the variations in gas exchange through the day and (ii) identify the photosynthetically active photon flux density (PPFD) that promotes higher chlorophyll fluorescence and electron transport rate in jaboticaba seedlings. The experimental design was completely randomized, with treatments consisting of 18 photosynthetic photon flux densities and three evaluations throughout the day. Six replicates were used, with two plants per plot. Gas exchange and chlorophyll a fluorescence in P. peruviana were altered due to fluctuating photosynthetic photon flux density (0; 25; 50; 75; 100; 125; 150; 175; 200; 400; 600; 800; 1,000; 1,200; 1,400; 1,600; 1,800; and 2,000 μmol m-2 s-1) and environmental conditions throughout the day (8:00 a.m., 12:00 and 4:00 p.m.). The higher PPFD (1,384.6 μmol m-2 s-1) and air temperature (39.74 °C) at noon (12:00 p.m.) favored gas exchange in this species. An increase in PPFD of up to 1,000 μmol m-2 s-1 positively influenced the gas exchange and chlorophyll a fluorescence of P. peruviana.

Optimum Water and Nitrogen Management Increases Grain Yield and Resource Use Efficiency by Optimizing Canopy Structure in Wheat

Excessive nitrogen (N) application rates and serious over-exploitation of groundwater under farmer practice threatens the sustainable use of resources in the North China Plain (NCP). Crop canopy structure affects light distribution between leaves, which is important to determine crop growth. A field experiment conducted from October 2016 to June 2019 in the NCP was designed to examine whether optimum water and nitrogen management could optimize canopy characteristics to improve yield and resource use efficiency. Field treatments included: (1) an example of local farming practices, which include the addition of 330 kg N ha−1 and irrigated twice (FP), (2) a reduced N rate of 270 kg N ha−1 and irrigated twice (T1), (3) a reduced rate of N rate of 210 kg N ha−1 and irrigated once (T2), and (4) no N applied (0 kg N ha−1) and irrigated once (T3). Results showed that the highest yield was in T1 treatment during 3 years’ winter wheat growing seasons. Water use efficiency (WUE), N use efficiency (NUtE), and N partial factor productivity (PFPN) were significantly higher in T2 treatment than in FP, and the three-year average yield was 9.4% higher than that at FP. Optimum crop management practice (T1 and T2) improved canopy structure characteristics, with less relative photosynthetically active photon flux density (PPFD) penetrated of canopy and higher population uniformity as well as leaf area index, to coordinate the distribution of light within the canopy and maximize canopy light interception, resulting in higher yield and resource use efficiency. Leaf area index (LAI) and specific leaf area (SLA) were closely correlated with grain yield and WUE, while PPFD penetrated of canopy was negatively and significantly correlated with grain yield and WUE. The results indicate that canopy structure characteristics, especially PPFD and population uniformity are good indicators of yield and resource use efficiency.

Nitrate, nitrite, protein, amino acid contents, and photosynthetic and growth characteristics of tatsoi cultivated under various photon flux densities and spectral light compositions

This study evaluated the effects of light intensity and the spectrum produced by light-emitting diodes (LEDs) on the growth and nitrate assimilation of tatsoi (Brassica rapa var. rosularis), grown in a controlled environment. A system of high-powered solid-state lighting modules with blue 445, red 640 and 660 and far-red 731 nm LEDs were used for illumination. Two experiments were performed seeking to explore the light parameter effects on tatsoi growth, nitrate, nitrite, total protein, and free amino acid contents, as well as soluble sugar contents and chlorophyll indexes: (1) investigation of the effects of a photosynthetically active photon flux density (PPFD) of 100, 200, 300, 400 and 500 μmol m–2 s–1 and (2) evaluation of the effects of different combinations of red (R660 and R660 + R640), red supplemented with blue (R660 + B445 and R660 + R640+B445) and far red (R660 + R640+B445+FR731) LEDs. A deficient light intensity resulted in nitrate and nitrite accumulation in tatsoi leaves; under the exposure of 100 μmol m–2 s–1 PPFD, tatsoi contained 3.4 times higher NO3 and 34 times higher NO2 contents, compared to that under 200 μmol m–2 s–1. The photosynthetic photon flux density of ∼300 μmol m2 s−1 allowed the obtaining of an optimal tatsoi production with high biomass and minimal nitrate and nitrite contents in the leaves. Long term exposure of sole red light was stressful for tatsoi growth; however the addition of blue light to the red 660- and 640- nm LEDs resulted in the highest total protein and essential amino acid contents, leaf area and biomass, and significantly reduced nitrate but increased nitrite content.

Rate Constants of PSII Photoinhibition and its Repair, and PSII Fluorescence Parameters in Field Plants in Relation to their Growth Light Environments.

The extent of photoinhibition of PSII is determined by a balance between the rate of photodamage to PSII and that of repair of the damaged PSII. It has already been indicated that the rate constants of photodamage (kpi) and repair (krec) of the leaves differ depending on their growth light environment. However, there are no studies using plants in the field. We examined these rate constants and fluorescence parameters of several field-grown plants to determine inter-relationships between these values and the growth environment. The kpi values were strongly related to the excess energy, EY, of the puddle model and non-regulated energy dissipation, Y(NO), of the lake model, both multiplied by the photosynthetically active photon flux density (PPFD) level during the photoinhibitory treatment. In contrast, the krec values corrected against in situ air temperature were very strongly related to the daily PPFD level. The plants from the fields showed higher NPQ than the chamber-grown plants, probably because these field plants acclimated to stronger lightflecks than the averaged growth PPFD. Comparing chamber-grown plants and the field plants, we showed that kpi is determined by the incident light level and the photosynthetic capacities such as in situ rate of PSII electron transport and non-photochemical quenching (NPQ) [e.g. Y(NO)×PPFD] and that krec is mostly determined by the growth light and temperature levels.

Temporal variations in air‐sea CO2 exchange near large kelp beds near San Diego, California

AbstractThis study presents nearly continuous air‐sea CO2 flux for 7 years using the eddy covariance method for nearshore water near San Diego, California, as well as identifying environmental processes that appear to control temporal variations in air‐sea CO2 flux at different time scales using time series decomposition. Monthly variations in CO2 uptake are shown to be positively influenced by photosynthetically active photon flux density (PPFD) and negatively related to wind speeds. In contrast to the monthly scale, wind speeds often influenced CO2 uptake positively on an hourly scale. Interannual variations in CO2 flux were not correlated with any independent variables, but did reflect surface area of the adjacent kelp bed in the following year. Different environmental influences on CO2 flux at different temporal scales suggest the importance of long‐term flux monitoring for accurately identifying important environmental processes for the coastal carbon cycle. Overall, the study area was a strong CO2 sink into the sea (CO2 flux of ca. −260 g C m−2 yr−1). If all coastal areas inhabited by macrophytes had a similar CO2 uptake rate, the net CO2 uptake from these areas alone would roughly equal the net CO2 sink estimated for the entire global coastal ocean to date. A similar‐strength CO2 flux, ranging between −0.09 and −0.01 g C m−2 h−1, was also observed over another kelp bed from a pilot study of boat‐based eddy covariance measurements.

Influences of vegetation status on seedling survival of a river-endemic plant Aster kantoensis in the floodplain

To assess the vegetation status and substrate condition on seedling survival and flowery of an endangered plant species Aster kantoensis, I carried out an in situ seed-sowing experiment in the gravelly/sandy floodplain of the Kinu River, central Japan. The coverage of plant species, especially that of an alien species Eragrostis curvula, increased, and the subsequent reduction in the relative photosynthetically active photon flux density (PPFD) (%) after the large-scale flood in 1998 caused a decline in the percentage of seedling survival and flowery of A. kantoensis. Performances of A. kantoensis were compared under four types of seminatural conditions, i.e., combinations of existence or absence of E. curvula combined with gravelly or sandy substrates. The highest percentage of survival and flowery were observed on the gravelly substrate under the E. curvula,-free conditions, but values were low in areas invaded by E. curvula. Aggressive invasion of E. curvula was the principal cause of loss of safe sites for colonization of river-endemic plants.

The influence of light intensity and leaf age on the photosynthetic capacity of leaves within a tomato canopy

SummaryIn dense crop stands, the decrease in leaf photosynthetic capacity (Amax) is paralleled by a decrease in photosynthetically active photon flux density (PPFD) and an increase in leaf age. In greenhouse horticulture, assimilation lighting is traditionally applied from above the canopy. Recently a new lighting technique has been developed in which assimilation lighting is applied within the canopy: intracanopy lighting. This development raises the question whether the decrease in the Amax of lower, thus older and shaded, leaves in a crop is solely due to the lower PPFD, or also partly due to ageing of these leaves. We investigated whether leaf ageing influenced changes in the Amax of tomato leaves during their usual life-span during cultivation in commercial crop systems (i.e., up to 70 d).To uncouple leaf age from the PPFD, tomato plants were grown horizontally, so that the PPFD was similar for all leaves. To investigate the effect of PPFD during leaf development (PPFDld), Amax-leaf age profiles were determined for the leaves of plants grown under conditions with distinctly different natural patterns of PPFD (i.e., Winter, early Spring, and late Spring). In addition, in half of the plants used per experiment, all fully-developed leaves were shaded to 25% of the normal PPFD in the greenhouse using a neutral density filter. Photosynthetic capacity and chlorophyll contents were higher in late Spring than in Winter, but were hardly affected by leaf age. In early Spring, the Amax and chlorophyll contents were higher in younger leaves than in older leaves. To a large extent, this was due to the differences in PPFDld, and hardly due to leaf ageing. Shading fully-developed, mature leaves dramatically decreased their Amax and chlorophyll contents within a few days. We conclude that, during the normal 70 d life-span of tomato leaves in commercial cultivation, the decrease in PPFD within the canopy, and not leaf-ageing, is the most important factor causing changes in Amax with canopy depth.

Estimating the three-dimensional structure of canopy foliage based on the light measurements in a Betula ermanii stand

In order to more accurately evaluate the functional activity of forest stands by canopy production and evapotranspiration, we improved the methods for field measurements and statistical modeling to estimate foliage configuration (spatial distribution of leaves) while simultaneously reconstructing the three-dimensional photosynthetically active photon flux density (PPFD) distribution (PPFD pattern) in a forest canopy. By using a sensor (photodiode) array, a PPFD pattern was observed in summer 2002 under the canopy in an even-aged, pure stand of Japanese mountain birch Betula ermanii Cham. (17-years old) in Hokkaido, northern Japan. A Markov chain Monte Carlo (MCMC) sampling technique is developed such that a set of foliage configurations generated by the model referred to as the Gibbs foliage canopy (GFC) approximates the field-measured PPFD pattern. The posterior distribution of the foliage configurations is generated by the parallel tempering MCMC of eight independent series of foliage configurations. The GFC model generated the posterior distribution of the LAI estimates (mean 4.56) that appeared to be appropriate in comparison to other LAI estimates of the B. ermanii stand based on the indirect and nondestructive methods by LAI-2000 (LAI = 3.43) and litterfall traps (LAI = 5.56) because they could be under- and overestimated, respectively. Our evaluations of the canopy production and evapotranspiration rates suggest that the relationship between LAI and canopy functions was not very simple because it depended on the nonlinear functional forms of the leaf responses of photosynthesis and transpiration to PPFD. The current study demonstrates an application of MCMC techniques that can generate a set of possible structures of unobserved/unobservable objects based on the high-resolution dataset obtained by some indirect (or remote-sensing) methods.

Water relations and carbon dioxide exchange of epiphytic lichens in the Namib fog desert

Although there is only negligible rainfall, frequent nocturnal fog, dew and high air humidity support a luxurious lichen vegetation in the coastal zone of the central Namib Desert (Namibia). In earlier publications, we have studied ecophysiological performance of a series of epilithic and terrestrial lichens. Here, we have extended this work to three epiphytic species ( Heterodermia namaquana, Ramalina lacera, and Xanthoria turbinata) that inhabit the sparse perennial shrubs growing in this area. Our intention, monitoring lichen CO 2 exchange, their water relations and microclimate conditions, was to determine the functional mechanisms that allow these epiphytes to exist under the special conditions of a fog desert. Measurements were conducted mainly during the spring season. The epiphytic lichens showed response patterns very similar to the epilithic and epigaeic species at the same site. Their metabolism was activated through moistening by dew and/or fog during the night and, in the very early morning, they exhibited the typical brief peak of net photosynthesis (NP) between sunrise and desiccation. The thalli were almost completely dry for the remainder of the day. Average duration of the positive NP during the morning peak was about 3 h. Dew condensation, alone, resulted in activation that provided 58–63% of integrated carbon income (ΣNP) as compared to fog (plus dew). In the late afternoon, there was a tendency for hydration to increase again, due to water vapour uptake at higher air humidity, and this allowed on some days a brief additional period of very low rates of photosynthesis shortly before sunset. Light response of photosynthesis showed “sun-plant” characteristics with saturation around 1000 μmol m −2 s −1 photosynthetically active photon flux density (PPFD). Light compensation point (LCP) of CO 2 exchange after sunrise was highly dependent on actual water content (WC) for X. turbinata: at low hydration it was ca. 10 μmol m −2 s −1 PPFD whilst, at high WC, it was almost 80 μmol m −2 s −1 PPFD. In contrast, LCP of R. lacera was almost independent of WC. This phenomenon was probably due to differences in thallus structure. Maximal attained NP and daily ΣNP both showed a saturation-type response to previous maximal nocturnal WC. Neither parameter was increased substantially when higher maximal thallus WCs were produced by experimental moistening in the night. All three species, despite their different morphologies, performed optimally at the highest nocturnal moistening achieved by natural fog and were not able to make use of higher hydration. The three studied epiphytes were similar in their chlorophyll-related rates of NP. Due to lower chlorophyll content, dry weight and carbon-related NP of X. turbinata was only about one-third of that of the other two species. The average carbon income on days with fog and/or dew hydration during the spring season amounted to 2.4 and 2.1 mgC (gC) −1 day −1 (related to thallus carbon content) for H. namaquana and R. lacera, respectively. This primary production was of similar magnitude to those found for the terrestrial species at the same site.

Functional convergence in regulation of net CO2 flux in heterogeneous tundra landscapes in Alaska and Sweden

Summary Arctic landscapes are characterized by extreme vegetation patchiness, often with sharply defined borders between very different vegetation types. This patchiness makes it difficult to predict landscape‐level C balance and its change in response to environment. Here we develop a model of net CO2 flux by arctic landscapes that is independent of vegetation composition, using instead a measure of leaf area derived from NDVI (normalized‐difference vegetation index). Using the light response of CO2 flux (net ecosystem exchange, NEE) measured in a wide range of vegetation in arctic Alaska and Sweden, we exercise the model using various data subsets for parameter estimation and tests of predictions. Overall, the model consistently explains ~80% of the variance in NEE knowing only the estimated leaf area index (LAI), photosynthetically active photon flux density (PPFD) and air temperature. Model parameters derived from measurements made in one site or vegetation type can be used to predict NEE in other sites or vegetation types with acceptable accuracy and precision. Further improvements in model prediction may come from incorporating an estimate of moss area in addition to LAI, and from using vegetation‐specific estimates of LAI. The success of this model at predicting NEE independent of any information on species composition indicates a high level of convergence in canopy structure and function in the arctic landscape.

Water relations and CO 2 exchange of the terrestrial lichen Teloschistes capensis in the Namib fog desert: Measurements during two seasons in the field and under controlled conditions

Although the coastal zone of the Central Namib Desert (Namibia) has negligible rainfall, frequent fog, dew and high air humidity support a luxurious lichen flora. Large areas of soil crust communities are dominated by the multibranched, fruticose Teloschistes capensis interspersed by a (still indeterminable) Ramalina species. In earlier communications, based on field measurements in autumn, we began the analysis of functional mechanisms that allow these lichens to exist under the special conditions of a fog desert. We have extended this work by monitoring lichen CO 2 exchange and water relations in spring and by experiments under controlled conditions. In both seasons, nocturnal hydration, by fog and/or dew, activated dark respiration of the lichens which was followed, after sunrise, by a short period of positive net photosynthesis (NP) that continued until metabolic inactivation occurred from desiccation. Dry thalli of T. capensis were able to reactivate NP through water vapour uptake alone, beginning at an air relative humidity of 82%, i.e. at a water potential of −26.3 MPa; the moisture compensation point during desiccation was at 13% thallus water content (WC, dry weight related). Optimal WC for photosynthesis was around 100%, and both species showed a large and extended suprasaturation depression of CO 2 assimilation. Light response showed “sun-plant” characteristics with saturation >1000 μmol m −2 s −1 photosynthetically active photon flux density (PPFD). However, due to rapid desiccation, the combination of light saturation with optimal WC very rarely occurred under field conditions. Light compensation point after sunrise was highly dependent on actual WC: at low hydration, it amounted to only ca. 10 μmol m −2 s −1 PPFD so that even the smallest levels of hydration could be used for carbon gain before desiccation took place again. This phenomenon was probably due to a hydration gradient in the thallus branches during transient moistening so that the outer photobiont layer was favoured in contrast to the internal mycobiont which remained dry longer and did not contribute respiratory CO 2 loss. Fully hydrated thalli had light compensation points around 50 μmol m −2 s −1 PPFD. Extended desiccation of 1–3 days had no impact on the magnitude and recovery of photosynthesis but, imposed desiccation of 10 days reduced NP in lab and field experiments and caused an extended period of recovery. “Resaturation respiration” was not detected in the field data, although it was present after experimental moistening of dry thalli. In spring, the higher fog frequency and intensity increased maximal nocturnal WC, maximal attained NP as well as integrated daily carbon income (ΣNP) compared to the autumn measurements. NP max and ΣNP depended on maximal nocturnal WC with a saturation-type response. In terms of carbon gain both species seem to be optimally adapted to nocturnal moistening up to 160% WC and were not able to make use of higher degrees of hydration, a feature that might well influence their habitat selection. Maximal daily carbon-related ΣNP for T. capensis was 4.6 mg C (g C) −1 day −1. A rough estimate of the annual (projected) area-related carbon balance (photosynthetic income minus respiratory losses) based on published fog and dew frequencies and personal observations was 15–34 mg C m −2 yr −1.

Impact of elevated UV‐B radiation on photosynthetic electron transport, primary productivity and carbon allocation in estuarine epipelic diatoms

Epipelic diatoms are important components of microphytobenthic biofilms. Cultures of four diatom species (Amphora coffeaeformis, Cylindrotheca closterium, Navicula perminuta and Nitzschia epithemioides) and assemblages of mixed diatom species collected from an estuary were exposed to elevated levels of ultraviolet-B (UV-B) radiation. Short exposures to UV-B resulted in decreases in photosystem II (PSII) photochemistry, photosynthetic electron transport, photosynthetic carbon assimilation and changes in the pattern of allocation of assimilated carbon into soluble colloidal, extracellular polysaccharides (EPS) and glucan pools. The magnitude of the effects of the UV-B treatments varied between species and was also dependent upon the photosynthetically active photon flux density (PPFD) to which the cells were also exposed, with effects being greater at lower light levels. Both increases in nonphotochemical quenching of excitation energy in the pigment antennae and photodamage to the D1 reaction centres contributed to decreases in PSII photochemistry. All species demonstrated a rapid ability to recover from perturbations of PSII photochemistry, with some species recovering during the UV-B exposure period. Some of the perturbations induced in carbon metabolism were independent of effects on PSII photochemistry and photosynthetic electron transport. Elevated UV-B can significantly inhibit photosynthetic performance, and modify carbon metabolism in epipelic diatoms. However, the ecological effects of UV-B at the community level are difficult to predict as large variations occur between species.

Response of chlorophyll fluorescence to dynamic light in three alpine species differing in plant architecture

A study was carried out to examine the effect of dynamic photosynthetically active photon flux density (PPFD) on photoinhibition and energy use in three herbaceous species, prostrate Saussurea superba, erect-leaved S. katochaete, and half-erect-leaved Gentiana straminea, from the Qinghai—Tibet Plateau. Chlorophyll fluorescence response was measured under each of three sets of high–low PPFD combinations: 1700–0, 1400–300, and 1200–500 μmol m −2 s −1, illuminating in four dynamic frequencies: 1, 5, 15, and 60 cycles per 2 h. The total light exposure time was 2 h and the integrated PPFD was the same in all treatments. The highest frequency of PPFD fluctuation resulted in the lowest photochemical activity, the highest level of non-photochemical quenching, and the greatest decrease of F v/ F m (maximal photochemical efficiency of PSII). The 5 and 15 cycles per 2 h treatments resulted in higher photochemical activity than the 1 cycle per 2 h treatment. The 1700–0 PPFD combination led to the lowest photochemical activity and more serious photoinhibition in all species. S. superba usually exhibited the highest photochemical activity and CO 2 uptake rate, the lowest reduction of F v/ F m, and the smallest fraction of energy in thermal dissipation. With similar fractions of thermal dissipation, S. katochaete had relatively less photoinhibition than G. straminea owing to effective F 0 quenching. The results suggest that high frequency of fluctuating PPFD generally results in photoinhibition, which is more serious under periods of irradiation with high light intensity.

Patterns in microphytobenthic primary productivity: Species‐specific variation inmigratory rhythms and photosynthetic efficiency in mixed‐species biofilms

The importance of temporal changes in the vertical distribution of microphytobenthic algae on the overall functioning of intertidal biofilms were investigated with low‐temperature scanning electron microscopy and high‐resolution single‐cell fluorescence imaging of photosystem II efficiency (estimated by the fluorescence parameter F’q/F’m) in intact cores maintained in tidal mesocosms. Early morning biofilms consisted of smaller naviculoid and nitzschioid taxa or euglenoid species. By midday, Gyrosigma balticum and Pleurosigma angulatum were dominant. Some taxa (e.g., Plagiotropis vitrea) disappeared from surface layers after midday. Species composition continued to change toward the end of the photoperiod, with G. balticum dominating in diatom‐rich biofilms. In Euglena‐rich biofilms, initial dense surface films of euglenids became progressively dominated by smaller diatoms. F’q/F’m (measured at a photosynthetically active photon flux density (PPFD) of 220 µmol m−2 s−1) of individual cells of all taxa declined significantly after midday, but increased toward dusk. There were significant differences in F’q/F’m between species, particularly after midday. F’q/F’m versus irradiance curves and relative electron transport rate (rETRmax) showed higher efficiencies and rETRmax for euglenids, whereas G. balticum, Nitzschia dubia, and small Nitzschia sp. were shade‐adapted with low values of F’q/F’m, rETRmax, and Esat. G. balticum, P. vitrea, and N. dubia showed rapid vertical migration away from the surface with increasing irradiance. Euglenids, P. angulatum, and N. dubia exhibited their highest rETRmax values at midday. Esat for algal cells was between 500 and 600 µmol m−2 s−1, except for N. dubia and small Nitzschia sp., which had an Esat of 300 µmol m−2 s−1. Differences in behavioral and photophysiological traits between microphytobenthic taxa could be a form of niche separation and need to be incorporated into conceptual models of daily patterns of production in intertidal biofilms.

Microsite variation in light availability and photosynthesis in a cool‐temperate deciduous broadleaf forest in central Japan

AbstractAn empirical light simulation model was applied to estimate stand scale photosynthesis in a deciduous broadleaved forest in central Japan. Based on diurnal courses of photosynthetically active photon flux density (PPFD), we characterized the components of incoming light within the forest canopy, and found that the instantaneous relative PPFD (PPFD under the canopy relative to that above the canopy) under diffuse light condition was a reliable estimator of the intensity and duration of PPFD. We calculated the daily photosynthesis (Aday) for each PPFD class using photosynthesis–light response curves. Model simulated Aday were corroborated with the estimates obtained from the nearby CO2 flux tower. The result demonstrated the potential of the light simulation model. The light use efficiency of two dominant species, Betula ermanii as overstory and Sasa senanensis as understory species, were then evaluated. At the forest understory, PPFD under 50 μmol m−2 s−1 contributed to 77% of the sunshine duration on a completely clear day. Therefore, a higher apparent quantum yield for S. senanensis enhanced the utilization of low PPFD for photosynthesis. On the other hand, at the upper forest canopies, B. ermanii with a higher light‐saturated photosynthetic rate used high PPFD efficiently. Consequently, potential of daily net photosynthesis for both B. ermanii and S. senanensis was high under each light condition. Such interspecific difference in the patterns of light utilization was suggested as one of factors allowing coexistence of the two species in the study forest.

Leaf Orientation, Incident Sunlight, and Photosynthesis in the Alpine Species Suassurea superba and Gentiana straminea on the Qinghai-Tibet Plateau

The extremely high level of solar radiation on the Qinghai-Tibet Plateau may induce photoinhibition and thus limit leaf carbon gain. To assess the effect of high light, we examined gas exchange and chlorophyll fluorescence for two species differing in light interception: the prostrate Saussurea superba and the erect-leaved Gentiana straminea. In controlled conditions with favorable water and temperature, neither species showed apparent photoinhibition in gas exchange measurements. In natural environment, however, their photosynthetic rate decreased remarkably at high light. Photosynthesis depression was aggravated under high leaf temperature or soil water stress. Relative stomatal limitation was much higher in S. superba than in G. straminea and it remarkably increased in the later species at midday when soil was dry. F v /F m as an indicator for photoinhibition was generally higher in S. superba than in the other species. F v /F m decreased significantly under high light at midday in both species, even when soil moisture was high. F 0 linearly elevated with the increment of leaf temperature in G. straminea, but remained almost constant in S. superba. Electron transport rate (ETR) increased with photosynthetically active photon flux density (PPFD) in S. superba, but declined when PPFD was high than about 1000 μmol m−2 s−1 in G. straminea. Compared to favorable environment, the estimated daily leaf carbon gain at PPFD above 800 μmol m−2 s−1 was reduced by 32% in S. superba and by 17% in G. straminea when soil was moist, and by 43% and 53%, respectively, when soil was dry. Our results suggest that the high radiation induces photoinhibition and significantly limits photosynthetic carbon gain, and the limitation may further increase at higher temperature and in dry soil.

Effects of water vapour pressure deficit and stomatal conductance on photosynthesis, internal pressurization and convective flow in three emergent wetland plants

Internal pressurization and convective gas flow in emergent wetland plants is a function of the water vapour pressure deficit (WPD) and stomatal conductance (Gs) separating the external atmosphere from the internal aerenchyma. We have compared the effects of WPD and Gs under a range of light intensities on static pressures and convective flows in Phragmites australis, Typha orientalis and Baumea articulata. The capacity of the three species to generate flows per unit leaf area differed, being greatest in P. australisand lowest in B. articulata. In all three species, decreasing light intensity from full sunlight (2200 μmol m−2 s−1 photosynthetically active photon flux density (PPFD)) to < 200 and < 10 μmol m−2 s−1PPFD caused immediate decreases in photosynthetic assimilation, followed by more gradual decreases in transpiration and Gs. However, internal pressures and flows in the two low light intensities remained similar to values recorded in full sunlight. WPD was more significantly related to pressures and flows in P. australis and T. orientalis than Gs. In B. articulata, pressures increased at low Gs values but flow rates were unaffected, as predicted by earlier models describing pore size effects on pressures and flows. The data suggest that emergent macrophytes can maintain significant internal convection even at low light intensities, and this may be beneficial for nocturnal aeration, particularly in arid climates where the atmospheric humidity at night is low.

The influence of UV-B radiation on light-dependent photosynthetic performance in Sanionia uncinata (Hedw.) Loeske in Antarctica

Photosynthetic activity of the moss Sanionia uncinata (Hedw.) Loeske was investigated on Leonie Island (67°35'S, 68°20'W, Antarctic Peninsula) in response to short-term changes of UV-B radiation. The UV-environment of natural mat formations dominated by S. uncinata was altered using filter screens. Two filter experiments were conducted in the Antarctic summers 1998 and 1999. A third filter experiment was conducted during springtime ozone depletion in October 1998. Photosynthetic activity of S. uncinata was mainly determined by photosynthetically active photon flux density (PPFD). Light response of relative electron transport rate through photosystem II (rel ETR=ΔF/Fm'×PPFD) remained unaffected by ambient summer levels of UV-B radiation. The same was found for net photosynthesis and dark respiration. In October 1998, S. uncinata was mainly metabolically inactive due to low temperatures. No significant levels of DNA-damage measured as cyclobutane pyrimidine dimers (CPDs) were induced by ambient summer levels of UV-B. Artificially enhanced UV-B radiation supplying a Setlow-DNA-dose of 8.7 kJ m−2 day−1 UV-B led to formation of 7±3 CPD (106 nucleotides)–1. It is concluded that current ambient summer levels of UV-B radiation do not affect photosynthetic activity in S. uncinata.

Leaf photosynthetic characteristics and net primary production of the polar willow (Salix polaris) in a high arctic polar semi-desert, Ny-Ålesund, Svalbard

Photosynthetic characteristics and their leaf-age dependence were examined to estimate ecophysiological effects on net primary production (NPP) of a polar willow (Salix polaris), a dominant dwarf shrub species in a polar semi-desert area of Ny-Ålesund, Svalbard. Leaves of S. polaris emerged just after snowmelt in early July in 2000; flowers were initiated within 1 week, and fruits in late July. Light-saturated rate of photosynthesis and stomatal conductance to water vapor increased rapidly to their maximum values within 1 week after leaf emergence and then decreased gradually. Depending on the leaf age, photosynthetic rates saturated at photosynthetically active photon flux density (PPFD) of 200–400 µmol·m–2·s–1, which is the light level usually available in the natural habitat. Optimum leaf temperature of photosynthesis ranged from 10 to 18°C, while air temperature in the habitat ranged from 8 to 20°C. These light and temperature responses of photosynthesis of S. polaris would be suitable for efficient carbon gain in the natural habitat characterized by highly variable light and temperature conditions. Using the photosynthetic and respiratory characteristics, biomass distribution, and meteorological data, NPP of S. polaris in the current year was estimated to be 26.1 g C·m–2. A model simulation of rising temperature conditions predicted a reduction of NPP because of a large increase in respiration. It was suggested that temperature condition and leaf phenological aspects strongly influence the carbon fixation by plants in the high arctic area studied.Key words: arctic semi-desert, climate change, net ecosystem production (NEP), net primary production (NPP), Salix polaris, photosynthesis.