Incident Photosynthetically Active Radiation Research Articles

Comparing solar radiation interception and use efficiency for the energy crops giant reed (Arundo donax L.) and sweet sorghum (Sorghum bicolor L. Moench)

While giant reed (Arundo donax L.) seems exquisitely suited to produce biomass for energy, its production ecology is still largely unexplored. In this study we have investigated giant reed taking the well investigated sweet sorghum (Sorghum bicolor L. Moench) as the reference crop. The aim was to elucidate whether the productivity of giant reed is attributable to its ability in intercepting solar radiation during the growing season, to its radiation-use efficiency or to a combination of both factors. For both crops we periodically measured leaf area index (LAI), and aboveground dry matter accumulated. Measurements were taken in the years 2009, 2010 and 2011 in the alluvial plain of the Po Valley, Northern Italy. The light extinction coefficient (k) for giant reed was estimated in 2011 on the basis of the fraction of non-intercepted photosynthetically active radiation (PAR) and LAI. The k was 0.29 for giant reed. Such a low k value, associated with unusually high LAI, indicates optimal canopy architecture for exploiting incident PAR. We assumed k=0.60 for sweet sorghum from literature. The radiation-use efficiency (RUE) was determined by the linear relationship between cumulated aboveground biomass and cumulated intercepted PAR. The RUE was 5.74gMJ−1 intercepted PAR for giant reed and 3.48gMJ−1 for sweet sorghum. Such remarkable RUE for giant reed is attributable to an optimal canopy structure coupled with high LAI. However, after a period of very high RUE, we observed an abrupt “summer slump” of RUE for giant reed from the first decade of August onwards. Consequently, giant reed used the intercepted PAR ineffectively during a substantial part of the growing season, from August to October. On the contrary, RUE for sweet sorghum remained fairly constant throughout the growing season. Our findings will aid modellers in simulating giant reed in similar high-yielding, non-limiting conditions.

Seven years of carbon dioxide exchange over a degraded grassland and a cropland with maize ecosystems in a semiarid area of China

Based on eddy covariance measurement over a degraded grassland and a cropland with maize (Zea mays) ecosystems from 2003 to 2009, carbon exchange processes and their responses to environmental factors in different temporal scales were analyzed in semiarid of China. Accounting for carbon export and import, NBP (net biome production) of cropland with maize ranged from 54.3 to 100.6g Cm−2yr−1. NBP remained positive indicating a carbon net loss from this ecosystem although NEE (net ecosystem exchange) was negative in most of years. Due to negligible carbon import and export, NBP of degraded grassland ecosystem was equal to NEE, with an average value of 138.4g Cm−2yr−1. The grassland ecosystem behaved as carbon source during the whole period. PPFD (incident photosynthetic active radiation) was the main driver for diurnal variation of NEE during growing season in most years. NDVI (normal difference vegetation index) was in accordance with seasonal patterns of NEE especially for cropland with maize ecosystem. Soil temperature at a depth of 5cm was also a main driver for seasonal variation of NEE at the degraded grassland ecosystem in normal precipitation years (2003 and 2005). Annual peak NDVI (NDVImax) was significantly correlated with annual NEE and GPP (gross primary productivity). The amount of growing season precipitation was more responsible for annual variation of NEE. The increasing number of precipitation event (>1mmday−1) was associated with increasing annual carbon uptake. Drought in the early growing period is more critical to carbon dynamics of degraded grassland ecosystem while drought in the middle of growing season was more critical for cropland with maize ecosystem.

Fruit, yield, and vegetative growth responses to photosynthetically active radiation during oil synthesis in olive trees

Maximizing productivity in super high density and intensive olive orchards requires proper management of illumination of the canopy walls and their interior. Currently, this is difficult to achieve due to the limited knowledge about the responses to incident photosynthetically active radiation (PAR) of yield determinants and components. We determined the response functions for PAR during the oil synthesis phase of yield components (fruit dry weight and oil concentration) of fruit at a height of 2m on the canopy periphery by applying several radiation levels (3, 20, 40, and 70% of incident PAR) to the north side (S hemisphere) of well-illuminated trees. The experiment was initiated after endocarp hardening as fruit number had already been established at that time. This avoided possible confounding effects due compensation between fruit number and size. Absence of differential fruit fall in response to treatments and of changes in (endocarp+seed) dry weight after application of treatment confirmed the achievement of this objective. Fruit dry weight, oil concentration, and, consequently, yield increased linearly with mean daily PAR receipt up to a threshold of 15mol PARm−2d−1 (i.e., 40% of PAR). In treatments with irradiance levels below this threshold the fruit became the priority sinks for assimilates, although their growth rate and oil concentration were reduced. Increments in length of non-fruiting branches and of trunk cross-sectional areas were substantially reduced in response to shading. We conclude that manipulation of PAR levels during the oil synthesis phase can reduce final fruit dry weight and oil concentration, confirms the existence of upper thresholds to PAR responses for these variables, and provides evidence that fruit growth has priority in the partitioning of photosynthate over vegetative growth under low to moderate levels of PAR.

Produtividade biológica de genótipos de tomateiro em sistema hidropônico no outono/inverno

O crescimento da planta de tomateiro depende de numerosos fatores, entre os quais se podem mencionar o genótipo, a radiação solar global incidente, a temperatura, a nutrição, o suprimento de água e a concentração de CO2, que atuam conjuntamente em complexa interação. Este trabalho teve o objetivo de avaliar a produtividade biológica de sete genótipos de tomateiro em cultivo hidropônico, no ciclo de outono/inverno caracterizado por baixa disponibilidade de radiação solar global, enfocando aspectos vegetativos e produtivos, a partição dos fotoassimilados e a eficiência no uso da radiação fotossinteticamente ativa (PAR), a fim de definir o(s) mais promissor(es) para o cultivo em casa de vegetação no sul do Rio Grande do Sul. O delineamento experimental utilizado foi em blocos casualizados com 7 tratamentos (genótipos) e 3 repetições, com 6 plantas por parcela. Dos genótipos utilizados, cinco pertencem ao grupo dos minitomates: Tomate Cereja Pendente Yubi Feltrin® (hábito determinado), Cereja Vermelho ISLA®, Minitomate Pêra Amarelo TOP SEED®, Grape (não comercial), Flavor Top (não comercial). Além desses, estudaram-se os genótipos Tomate Santa Cruz Kada Gigante TOP SEED® e Tomate Gaúcho TOP SEED®. As avaliações foram produtividade (g m-2) de matéria fresca e seca de hastes, folhas e frutos; área foliar; altura e compacidade de planta; radiação solar global e PAR incidentes. Os genótipos mais promissores para o cultivo em épocas de baixa disponibilidade de radiação solar são Kada Gigante, do tipo Santa Cruz, e Cereja Yubi de crescimento determinado, do grupo dos minitomates, por apresentarem produtividade de frutos superiores e uma alta eficiência no uso da PAR.

Photosynthesis Assessment in Microphytobenthos Using Conventional and Imaging Pulse Amplitude Modulation Fluorometry

Imaging pulse amplitude modulated (Imaging-PAM) fluorometry is a breakthrough in the study of spatial heterogeneity of photosynthetic assemblages. However, Imaging and conventional PAM uses a different technology, making comparisons between these techniques doubtful. Thereby, photosynthetic processes were comparatively assessed using conventional (Junior PAM and PAM 101) and Imaging-PAM on intertidal microphytobenthos (MPB; mud and sand) and on cork oak leaves. Lower values of α (initial slope of the rETR, relative photosynthetic electron transport rate) vs E (incident photosynthetic active radiation) curve), ETR(max) (maximum relative ETR), E(k) (light saturation parameter) and F(v)/F(m) (maximum quantum efficiency of photosystem II of dark-adapted samples) were obtained using the Imaging-PAM. The level of discrepancy between conventional and Imaging-PAM systems was dependent on the type of sample, being more pronounced for MPB muddy sediments. This may be explained by differences in the depth integration of the fluorescence signal related to the thickness of the photosynthetic layer and in the light attenuation coefficients of downwelling irradiance. An additional relevant parameter is the taxonomic composition of the MPB, as cyanobacteria present in sandy sediments rendered different results with red and blue excitation light fluorometers. These findings emphasize the caution needed when interpreting chlorophyll fluorescence data of MPB communities.

Athletic Field Paint Impacts Light Spectral Quality and Turfgrass Photosynthesis

ABSTRACTAthletic field paints are applied to turf surfaces with little or no acute injury. However, field managers notice chronic declines in turfgrass health after repeated applications. This study examines athletic field paint impacts on spectral quality and associated turfgrass photosynthesis. Growth chamber experiments evaluated effects of red and white athletic field paint as well as one, two, three, and four repeated weekly applications on total canopy photosynthesis (TCP) of perennial ryegrass (Lolium perenne L.). Paint treatments were applied weekly for 6 wk with TCP recorded 24 h after each application using a gas exchange system. Spectroradiometry experiments evaluated reflection, absorption, and transmission of light at various wavelengths based on paint color, dilution, and thickness. Over a six week period all treatments reduced TCP based on color (P ≤ 0.0001) and dilution (P ≤ 0.0001). Red no‐dilution paint produced a 75% reduction in TCP over 6 wk while white 1:1 diluted paint only produced a 19% reduction. Spectroradiometry data suggests this is likely due to reductions in photosynthetically active radiation (PAR) with red paint absorbing 51% of incident PAR while transmitting and reflecting 6 and 43%, respectively. White paint transmitted 5% of PAR while reflecting 95%. Alterations in light spectral quality resulting from athletic field paint applications can impact PAR, which may result in reduced turfgrass health.

Thinning effects on the net ecosystem carbon exchange of a Sitka spruce forest are temperature-dependent

Commercial forest plantations need to be actively managed, through tree removal, in order to improve wood quality, maintain productivity and provide an economic return, although this could compromise an important role for forests in carbon sequestration and greenhouse gas mitigation. The impact of forest thinning on net primary productivity (NPP) and net ecosystem exchange (NEE) was assessed using a combination of biometric and eddy covariance (EC) techniques. Two thinning operations were performed in close succession, which reduced the basal area of the stand by 17% and 11% and removed a timber volume of 48m3ha−1 and 50m3ha−1, respectively. Annual rates of NPP ranged from 13.24 (±3.96) to 18.94 (±4.88)tCha−1 and 13.22 (±3.72) to 17.77 (±5.30)tCha−1 for the pre- and post-thinning periods, respectively. Estimates of NEE varied between 8.44 (±1.34) to 8.87 (±1.48)tCha−1 and 6.75 (±1.19) to 10.33 (±1.41)tCha−1 in the pre- and post-thinning periods. Forest thinning did not have a significant impact on carbon stocks or fluxes when pre-thinning (2002–2006) and post-thinning (2007–2009) estimates of NPP and NEE were compared, however the range of inter-annual variability in NEE increased after thinning. The partitioning of annual NEE carbon budgets into gross primary productivity (GPP) and ecosystem respiration (Reco) together with an analysis of key physiological parameters suggested that the impacts of forest thinning are largely dependent on temperature. An expected decrease in GPP after the initial thinning in 2007 was not observed due, in part, to the higher mean annual air temperatures and incident photosynthetic active radiation (PAR) and a compensatory increase in photosynthesis by the remaining trees. A continual decline in Reco, was observed in the years subsequent to the first thinning and was attributed to both biomass removal and climatic factors.Inter-annual variations in climate had a significant impact on NEE, GPP and Reco. Annual mean air temperature, total precipitation and total incident PAR were all shown to influence the processes driving CO2 exchange. Overall, these results suggest that the impacts of the thinning practices, as implemented in this study, are dependent on climate and under similar conditions are unlikely, in the short-term, to compromise a role for forest ecosystems in carbon sequestration and greenhouse gas mitigation.

Influence of light environment on yield determinants and components in large olive hedgerows following mechanical pruning in the subtropics of the Southern Hemisphere

Olive cultivation has expanded considerably beyond the Mediterranean Basin to include new areas in the Southern Hemisphere. This study evaluated the response functions of yield determinants and components to photosynthetically active radiation (PAR) in a large olive hedgerow (cv. ‘Arbequina’) in subtropical Argentina, and assessed whether mechanical pruning altered these relationships. Mechanical pruning was applied at the end of the winter dormant period to the east side and top of hedgerows oriented North–South, while the other side was left unpruned. The PAR distribution and leaf density at different positions within the hedgerow were measured over the two subsequent growing seasons with inflorescence density, fruit set, fruit density, individual fruit dry weight, and oil content (%) being assessed the second growing season. Hedgerow size increased substantially during the first season, especially on the pruned side and top, reducing transmitted PAR in much of the canopy to less than 20% of full Sun incident PAR values above the orchard. The relationship between leaf density (and thus number of axillary buds) and PAR was bilinear for the canopy positions within the unpruned side with a threshold of about 15% of full Sun PAR, above which leaf density did not increase. No relationships between leaf density and PAR were found within the pruned side and top. In contrast, inflorescence density exhibited a threshold of 18% of PAR for canopy positions within both the pruned and unpruned sides, although no relationship was apparent for the pruned top. Neither fruit set nor yield component relationships with PAR were affected by pruning. Fruit set had a linear response to PAR for the observed range of PAR values within the hedgerow (approx. 0–50% of full Sun). Individual fruit dry weight and oil content (%) also showed linear responses to PAR. The results suggest that oil production in large olive hedgerows (height×width) is limited by low solar radiation within the canopy, and that substantial vegetative growth triggered by mechanical pruning may be an important contributor to this limitation under current management practices in subtropical climates.

Effects of invasive colonial tunicates and a native sponge on the growth, survival, and light attenuation of eelgrass (Zostera marina)

We examined the effects of invasive colonial tunicates (golden star, Botryllus schlosseri; violet Botrylloides violaceus), and the native breadcrumb sponge (Halichondria panicea) on the growth, survival, and light attenuation of eelgrass (Zostera marina). Eelgrass shoot growth and survival were higher for unfouled shoots than for fouled shoots, and dependent on fouling species identity. Growth was lowest for shoots with violet tunicate fouling, and survival was lowest for shoots with sponge fouling. A large proportion (0.20-0.38) of fouled shoots marked for growth measurements were not found during retrieval compared to unfouled plants (0.08), suggesting that fouling led to premature breaking away of shoots and blades. Transmission of incident photosynthetically active radiation (PAR) through fouled blades decreased exponentially with increasing biomass of most fouling species. Relatively low biomass (1-1.5 dry mg cm -2 of blade) of the fouling organisms reduced light transmission by up to 98 % compared to unfouled blades. The proportion of incident PAR attenuated by the sponge and orange and burgundy morphs of the violet tunicate increased hyperbolically across their biomass, and reached a plateau at ~0.5 dry mg cm-2 of blade. The golden star tunicate and cream morph of the violet tunicate attenuated incident PAR linearly across their biomass. For the range of biomass examined, all fouling species attenuated 65-95 % of incident PAR prior to it reaching the blade. The reduction in light transmission was likely the causal mechanism underlying reduced growth of fouled shoots. Fouling of eelgrass by the invasive colonial tunicates and the native sponge will have numerous ecological consequences, including reduced productivity and coverage of eelgrass beds.

Light interception principally drives the understory response to boxelder invasion in riparian forests

Since several decades, American boxelder (Acer negundo) is replacing white willow (Salix alba) riparian forests along southern European rivers. This study aims to evaluate the consequences of boxelder invasion on understory community in riparian areas. We determined the understory species richness, composition and biomass in boxelder and white willow stands located in three riparian forests, representative of three rivers with distinct hydrological regimes. We investigated correlation of these variables to soil moisture and particle size, main soil nutrient stocks, potential nitrification and denitrification, tree canopy cover and photosynthetic active radiation (PAR) at the ground level. A greenhouse experiment was then conducted to identify the causal factors responsible for changes in the understory. The effect of soil type, PAR level and water level on the growth and the biomass production of Urtica dioica were examined. A lower plant species richness and biomass, and a modification of community composition were observed for boxelder understory in all sites, regardless of their environmental characteristics. The strongest modification that follows boxelder invasion was the decline in U. dioica, the dominant species of the white willow forest understory. These differences were mainly correlated with a lower incident PAR under boxelder canopy. The greenhouse experiment identified PAR level as the main factor responsible for the changes in U. dioica stem number and biomass. Our results indicate that adult boxelder acts as an ecosystem engineer that decreases light availability. The opportunistic invasion by boxelder leads to important understory changes, which could alter riparian ecosystem functioning.

A combined rain shelter and free‐air CO2 enrichment system to study climate change impacts on plants in the field

Summary1. There is a clear need for field experiments to estimate the effects of global climate change like decreasing precipitation and rising atmospheric CO2 concentration. Adequate methods for controlled manipulations of these environmental parameters under field conditions are scarce, particularly in regard to multi‐factor experiments. Here, we describe a new flexible rain shelter system, which can be assembled manually and easily be combined with further experimental treatments in field studies.2. Frames of tents with a ground area of 12 m × 20 m were assembled on a field site after the sowing of the maize and sorghum crop and after the equipment for free‐air CO2 enrichment has been set up. The tents were equipped with transparent tarpaulins, which were installed on the frames only in cases of high amounts of precipitations forecast. In autumn, the entire experimental equipment was removed from the field site.3. The rain shelter tents were operated in the growing seasons 2008 and 2010 for 9 and 20 days with 54 mm and 176 mm of precipitation excluded, respectively. In the months with rain shelters in operation, pronounced reductions in precipitation were achieved (2008: 39·5%, 2010: 58·6%). The tent frames did not affect temperature or CO2 concentration, but slightly decreased incident photosynthetic active radiation (PAR) by 6·6%. In times with tarpaulins installed, PAR decreased by 24·1%. Comparing times without and with tarpaulins installed, the fraction of time in which 1‐min mean CO2 concentration was within ±20% limits of the setpoint was decreased from 99·7% to 97·8% in 2008 and from 99·0% to 96·7% in 2010, respectively.4. The rain shelter tents provide a suitable and versatile tool for excluding precipitation from larger areas in the field without relevant disturbances to the soil and aerial environment except a slight decrease in incident radiation, which can be accounted for i.e. in the evaluation of plant growth data. Furthermore, they can be easily combined with further experimental treatments like free‐air CO2 enrichment.

Reduction of apple tree height ( Malus domestica Borkh) cv. Ultra Red Gala/MM111 does not decrease fruit yield and quality

In Chile, like in other countries, a high percentage of apple ( Malus domestica Borkh) orchards are grafted on vigorous or semi-vigorous rootstocks. The need to decrease the amount of labor involved and increase efficiency has motivated this study on the effect of reducing the height of cv. Ultra Red Gala/MM111 trees in a commercial orchard in the Maule Region of Chile. Apple trees were planted in 2003, and their tree heights were adjusted to 2.5, 3.0 and 3.6 m prior to blooming in 2006. The reduction of plant height from 3.6 m to 2.5 m determined a significant reduction in canopy volume (26% and 29% in 2007/2008 and 2008/2009, respectively), but no differences were detected in leaf area index (LAI) and the photosynthetic active radiation (PAR) transmitted through the canopy. However, a higher proportion of the canopies of the shorter trees had over 30% of incident PAR, the threshold for the production of good fruit quality and flower-bud differentiation. There were no differences in fruit production among plant heights, accumulating 131 tonnes ha −1 in the three growing seasons. Over the three seasons, the shortest plants (2.5 m) required 19, 57 and 42% less labor time at harvest, respectively, than the plants maintained at a height of 3.6 m. There were no evident differences in fruit quality among the treatments; flesh firmness varied between 73.0 and 74.0 N, soluble solids between 11.6 and 12.6°Brix, mean weight between 180 and 200 g, and the percentage of red coloring exceeded 59% of the Premium fruit in the three growing seasons. Our results suggest that it is possible to manage plants with reduced height on semi-vigorous rootstocks and thus reduce the time necessary for harvesting, without affecting fruit yield and quality.

Development of a model for the estimation of photosynthetically active radiation from geostationary satellite data in a tropical environment

This paper presents a model for the estimation of photosynthetically active radiation (PAR) from geostationary satellite data. The model is aimed to estimate the monthly average hourly PAR in a tropical environment. This model represents a physical relation of PAR incident on the earth's surface and satellite-derived earth-atmospheric albedo together with the absorption and scattering coefficients of various atmospheric constituents. The earth-atmospheric albedo was obtained from the Multifunctional Transport Satellite-1R (MTSAT-1R). The absorption of PAR by water vapor, an important process for the tropics, was computed from the ambient temperature and relative humidity. The absorption of PAR by aerosols was estimated by using the visibility data and aerosol optical properties obtained from the Aerosol Robotic Network (AERONET) of NASA in this region. The total column ozone from the Ozone Monitoring Instrument onboard of AURA satellite (OMI/AURA) was used for the estimation of the absorption of PAR by ozone. The model was validated against the monthly average hourly PAR from measurements at four solar radiation measuring stations situated in the tropical environment of Thailand. The values of the monthly average hourly PAR estimated from the model and those obtained from the measurement were in good agreement, with the root mean square error (RMSE) and mean bias error (MBE) of 9.8% and 0.6%, respectively. After the validation, the model was employed to estimate the monthly average hourly PAR over Thailand using a 4-year period of data from MTSAT-1R and other ancillary surface data. Values of the monthly average hourly PAR were presented as maps showing the geographical distribution of PAR. These maps reveal the diurnal and seasonal variation of PAR over the country.

The cost of photoinhibition

Photoinhibition is an inevitable consequence of oxygenic photosynthesis. However, the concept of a 'photoinhibition-proof' plant in which photosystem II (PSII) is immune to photodamage is useful as a benchmark for considering the performances of plants with varying mixes of mechanisms which limit the extent of photodamage and which repair photodamage. Some photodamage is bound to occur, and the energy costs of repair are the direct costs of repair plus the photosynthesis foregone during repair. One mechanism permitting partial avoidance of photodamage is restriction of the number of photons incident on the photosynthetic apparatus per unit time, achieved by phototactic movement of motile algae to places with lower incident photosynthetically active radiation (PAR), by phototactic movement of plastids within cells to positions that minimize the incident PAR and by photonastic relative movements of parts of photolithotrophs attached to a substrate. The other means of avoiding photodamage is dissipating excitation of photosynthetic pigments including state transitions, non-photochemical quenching by one of the xanthophyll cycles or some other process and photochemical quenching by increased electron flow through PSII involving CO₂ and other acceptors, including the engagement of additional electron transport pathways. These mechanisms inevitably have the potential to decrease the rate of growth. As well as the decreased photosynthetic rates as a result of photodamage and the restrictions on photosynthesis imposed by the repair, avoidance, quenching and scavenging mechanisms, there are also additional energy, nitrogen and phosphorus costs of producing and operating repair, avoidance, quenching and scavenging mechanisms. A comparison is also made between the costs of photoinhibition and those of other plant functions impeded by the occurrence of oxygenic photosynthesis, i.e. the competitive inhibition of the carboxylase activity of ribulose bisphosphate carboxylase-oxygenase by oxygen via the oxygenase activity, and oxygen damage to nitrogenase in diazotrophic organisms.

The influence of coastal upwelling and a river plume on the subsurface chlorophyll maximum over the shelf of the northeastern South China Sea

Frequently observed subsurface chlorophyll maximum (SCM) contributes substantial biomass to the waters over the continental shelf of the northeastern South China Sea (NSCS) but it has not been sufficiently investigated. In this study, observations and a three-dimensional coupled physical–biological numerical model were utilized to investigate the characteristics of the SCM under strong controls of coastal upwelling circulation and the Pearl River plume in the NSCS. The model captures the observed characteristics of the SCM in the NSCS reasonably well. Both the depth and intensity of the SCM are spatially variable and regulated by the variable upwelling circulation and associated plume distribution over the complex shelf topography. In nearshore waters, the SCM shoals and weakens towards the coast as a result of the upwelling of high nutrient low chlorophyll deep water while surface productivity is enhanced. The intensity of the SCM weakens when the surface layer is covered by the river plume because of the substantial reduction of photosynthetic active radiation (PAR). Strengthening upwelling-favorable wind weakens the intensity of the SCM due to dilution by the enhanced mixing, but the SCM depth remains relatively stable in the offshore water as a result of no apparent shift of the nutricline. Changing incident PAR leads to an interactive response in chlorophyll concentration, nutricline, and the depth of the SCM.

Assessment of carbon- and fluorescence-based primary productivity in Ariake Bay, southwestern Japan

A comparative account of primary productivity (PP), in the characteristically turbid and highly dynamic waters of Ariake Bay, measured by 13C uptake and fast repetition rate fluorometer (FRRF) was conducted to ensure compatibility between the two methods. Estimates from both methods depicted strong linearity for both short-term (r 2 > 0.90) and daily (r 2 = 0.42–0.93) measurements, except in the near-surface (∼0 m) layer. 13C-based short-term (1 h; in situ) PP estimates showed similar magnitudes and trend with the instantaneous PP measured by FRRF concurrently. Whereas, unlike short-term measurements, the daily PP estimates from both methods showed large difference, with FRRF-based time integrated daily PP resulting in 1.09–1.82 times higher than the carbon-based daily (24 h; simulated in situ) PP. This difference between daily PP estimates was mainly due to: (1) the temporal variation of water column chlorophyll a (Chl a) because of frequent moving of water mass, and (2) the dissimilarity in ambient light field conditions between the two methods. Results revealed that considering the above two environmental factors invariable over a daylength, fairly close approximation of daily PP, compared to 13C-based daily PP, could be obtained from FRRF. Hence, FRRF-based daily PP can be considered as more realistic in this highly dynamic water body like Ariake Bay where water column parameters are subjected to strong temporal variation. The relationship between Chl a-specific photosynthetic rate ( P B ) and the corresponding photosynthetically active radiation (PAR) in the water column (PAR– P B relationship) was found to be linear for FRRF and curvilinear for 13C-based measurements in the near-surface layer, for the same intensities of incident PAR, and this is thought to be the primary basis for the higher difference in PP estimates at the near-surface layer. Considering the minor variations in FRRF-based time series of PAR– P B relationships, a combined and/or instantaneous PAR– P B relationship in combination with incubation Chl a and light field condition was used to obtain fairly close estimates of daily water column integrated PP from FRRF.

Comparing indices of understory light availability between hemlock and hardwood forest patches

We evaluated whether two indices of light availability resolved differences among microsites within deeply shaded understories (<12% of above-canopy photosynthetic photon flux density (PPFD)) and also whether marked differences in forest canopy structure affected how the two indices related to direct measures of incident photosynthetically active radiation (PAR). Incident PAR was measured with gallium–arsenide–phosphide photodiodes at numerous points in two adjacent forest patches in Michigan, USA: one dominated by the evergreen conifer eastern hemlock ( Tsuga canadensis (L.) Carrière), and the other by the deciduous hardwood sugar maple ( Acer saccharum Marsh.). The two indices tested were canopy openness, measured with the LI-COR LAI-2000 plant canopy analyzer, and the percentage of above-canopy PPFD measured in the understory during overcast conditions (%PPFD). Canopy openness and %PPFD did not effectively predict the long-term mean of daily PPFD. However, both indices reliably predicted the long-term mean of daily median PPFD, an alternative standard of directly measured incident PAR that reduces the relative contribution of sunflecks. The relationships of both indices with mean daily median PPFD differed between hemlock and hardwood patches. Hence, the effect of canopy structure should be considered when using these indices to draw conclusions about differences in light availability between forest patches, particularly when narrow ranges of light availability in deep shade are important.

Whole-Canopy Gas Exchange and Light Interception of Vertically TrainedVitis viniferaL. under Direct and Diffuse Light

A portable, open gas exchange system was developed that enabled the simultaneous measurement of net carbon dioxide exchange (NCE) and transpiration of eight mature Sauvignon blanc grapevines in the field. On clear and cloudy days NCE and transpiration were closely related to the modeled interception of photosynthetically active radiation (PAR). On clear days, the relationship between photosynthesis and incident PAR was poor and NCE peaked in the vertically trained, north-south orientated rows three hours before and after solar noon, resulting in a distinct bimodal pattern to vine gas exchange, which was not evident on cloudy days. Under diffuse light (cloudy) conditions, the vines appeared to be more efficient, photosynthesizing at a higher rate per calculated unit of light intercepted. This efficiency was probably due to the saturation of the exterior and shading of interior leaves under clear conditions, while under diffuse conditions the increased scattered light enhanced the irradiance received by the leaves in the interior of the canopy. Despite the improved light use efficiency under diffuse conditions, lower light intensities meant that CO₂ exchange was normally lower than that recorded during clear days. The effect of the chambers on the sap flux velocity up the vine stems was minor, which suggests that their effect on vine microclimate and resulting transpiration was minimal. There was a strong relationship between concurrent transpiration and sap flux velocity, which was not improved by the use of a time delay between transpiration and sap flux, and suggests that the vines do not rely greatly on water stored in the trunk and other vegetative organs above the ground (capacitance) for transpiration under these conditions.

Finding the winners in competition for substratum between coral polyps and epilithic algae on damaged colonies of the coral Porites lutea

The study was conducted at the fringing reef of Sesoko Island (Okinawa, Japan), in the north-western Pacific. Artificial injuries were inflicted to the upper portions of colonies of the massive coral Porites lutea . The colonies were fixed to cement blocks and placed at two reef locations of different light intensity: one with moderate light (20–30% of incident photosynthetic active radiation, PAR0) and one with low light (2–5% PAR0). After six months of the treatment, all injuries were photographed. Analysis of photographs made under light and electron microscopes showed that the coral was able to recover from the injuries at both light variants. Under moderate light, some of the injuries completely healed and some injuries recovered partially. Under low light all injuries recovered partially. These partially recovered injuries were occupied by algae at dead portions of the injuries. In moderate light, coral polyps overgrew all algal settlers and were the winners in the competition for substratum. In low light, Peyssonnelia conchicola , Corallophila apiculata , Centroceras clavulatum , Anortichium tenue , and Lobophora variegata that colonized the injured areas overgrew coral polyps and were the winners in the competitive struggle for the substratum. It was shown that overgrowing is the main mechanism of coral–algal competition for substratum in light ranging from 2 to 30% PAR0. This mechanism of overgrowth of corals by algae, and the reverse, are discussed in detail.

Incorporating diffuse photosynthetically active radiation in a single-leaf model of canopy photosynthesis for a 56-year-old Douglas-fir forest

A simple top-down model of canopy photosynthesis (P) was developed and tested in this study. The model (referred to as the Q(e)-MM model) is P = alphaQ (e) P (max)/(alphaQ ( e ) + P (max)), alpha and P (max) are quantum-use efficiency and potential P, respectively. Q (e) is given by Q (d) (0) + kQ (b) (0), where Q (d) (0) and Q (b) (0) are the diffuse and direct photosynthetically active radiation (PAR) incident on the canopy, respectively. Q (e) can be considered to be the effective incident PAR contributing to P and k is a measure of the contribution of Q (b) (0) to Q (e). When k = 1, the Q(e)-MM model becomes the regular Michaelis-Menten type model of P (referred to as the MM model). A major objective of this study was to determine how well the Q(e)-MM model could estimate P of a 56-year-old coastal Douglas-fir stand. To this end, we parameterized the Q(e)-MM model using five and half years of eddy-covariance measurements of CO(2) flux above the Douglas-fir stand. The Q(e)-MM model, with the incorporation of a function of air temperature, accounted for 74% of the variance in over 34,000 half-hourly P measurements. P estimated using the Q(e)-MM model had no systematic errors with respect to Q (d) (0). Although the Q(e)-MM model has only one more parameter than the MM model, it accounted for 30% more variance in P than the latter when total incident PAR exceeded 900 micromol m(-2) s(-1). On average, k was found to be 0.22. We show that this small value of k reflects the significant effect of the scattering of the solar beam and the fraction of light-limited sunlit leaves. We also show that the success of the Q(e)-MM model was due to the fact that a large fraction of the sunlit leaves were light-limited as a result of their orientation to the solar beam.