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The light climate of Moscow is presented based on the long-term observations of natural illuminance of the Earth's surface, illuminance of differently oriented vertical surfaces, and factors influencing their variability, which have been performed at the Moscow State University Meteorological Observatory. The obtained normals are sufficient for any practical applications in most cases. An issue analyzed in the study is conditions that lead to a decrease in illumination below the critical values for which the use of combined or artificial lighting of premises is required. It is shown that illumination can be forecasted based on low-level clouds predicted using a general weather forecast. [ABSTRACT FROM AUTHOR]

Climate-change driven increases in temperature and precipitation are leading to increased discharge of freshwater and terrestrial material to Arctic coastal ecosystems. These inputs bring sediments, nutrients and organic matter (OM) across the land-ocean interface with a range of implications for coastal ecosystems and biogeochemical cycling. To investigate responses to terrestrial inputs, physicochemical conditions were characterized in a river- and glacier-influenced Arctic fjord system (Isfjorden, Svalbard) from May to August in 2018 and 2019. Our observations revealed a pervasive freshwater footprint in the inner fjord arms, the geochemical properties of which varied spatially and seasonally as the melt season progressed. In June, during the spring freshet, rivers were a source of dissolved organic carbon (DOC; with concentrations up to 1410 μmol L–1). In August, permafrost and glacial-fed meltwater was a source of inorganic nutrients including NO2 + NO3, with concentrations 12-fold higher in the rivers than in the fjord. While marine OM dominated in May following the spring phytoplankton bloom, terrestrial OM was present throughout Isfjorden in June and August. Results suggest that enhanced land-ocean connectivity could lead to profound changes in the biogeochemistry and ecology of Svalbard fjords. Given the anticipated warming and associated increases in precipitation, permafrost thaw and freshwater discharge, our results highlight the need for more detailed seasonal field sampling in small Arctic catchments and receiving aquatic systems.

Introduction. This study investigated the ecology of two boreal-montane bryophytes, the saxicolous moss Bartramia halleriana and the epixylic liverwort Crossocalyx hellerianus, at the edge of their ranges, in oceanic deciduous woodland in Wales. Methods. The study site comprises two adjoining woodlands, Allt Penyrhiw-iar (canopy trees ca. 100 yr old) and Allt Rhyd Groes (ca. 200 yr), Carmarthenshire, UK. The distribution and abundance of B. halleriana and C. hellerianus was surveyed, relevés sampled to record habitat and community composition, and sporophyte frequency and stage of development measured. Light climate of B. halleriana was investigated via hemispherical photography, and abundance of large rotten logs used as a measure of habitat quality for C. hellerianus. Results. Four subpopulations of B. halleriana occured, comprising 21 individual-equivalents (occupied 1 m grid cells), all on mildly base-rich mudstone of north-facing rockfaces, with very little direct solar radiation and a diverse assemblage of bryophytes. Sporophytes were scarce. A total of 143 individual-equivalents (occupied logs or trees) of C. hellerianus was recorded, an exceptional population in Wales. Most of the population was within Allt Rhyd Groes, where large rotten logs were much more abundant due to greater woodland age. The liverwort mainly occupied the sides of decorticated rotten logs, amongst a sparse community composed mainly of other small liverworts and Cladonia lichens. Neither sporophytes nor perianths were found on C. hellerianus specimens. Conclusion. Further research could usefully focus on the description and measurement of favourable habitat conditions for each species and associated physiological tolerances to inform conservation decisions for both species. [ABSTRACT FROM AUTHOR]

Introduction. This study investigated the ecology of Pallavicinia lyellii in a mire habitat dominated by Molinia caerulea, including potential abundance, frequency of sexes and sporophytes, and characteristics of the light climate beneath the herbaceous canopy. Methods. Within the study site (Cors Fochno, Wales, UK), P. lyellii was surveyed in March 2020. Abundance was measured as counts of colonies, their sexual composition was recorded, and number of sporophytes determined. Photosynthetic photon flux density (PPFD) was measured beside colonies, paired with measurements taken 1 m above the colony under open sky. A hemispherical sky image was captured, and sun tracks overlaid, at a representative colony. Results. Fifty-eight colonies of P. lyellii occurred, in two populations. One population exhibited male bias, a low portion of sterile colonies and a relatively large number of sporophytes. The other exhibited female bias, a high portion of sterile colonies and very few sporophytes. Molinia caerulea blocked ca. 91% of PPFD, and light at colonies of P. lyellii was consistently low (mean PPFD = 29 μmol m−2 s−1). Hemispherical photography shows no direct sunlight may reach P. lyellii during October–February and suggests during the remainder of the year such events could be extremely limited. Conclusions. Molinia mire can support large populations of P. lyellii and will be important for its future survival in Britain. Research is needed to quantify the microhabitat favoured by the liverwort, such as relationship to light, hydrology and water chemistry, and how these can be affected by management. [ABSTRACT FROM AUTHOR]

We performed an annual synchronous sampling in two oligotrophic shallow lakes to assess the influence of chromophoric dissolved organic matter (CDOM) on the underwater light climate, and its potential effects on the nano and microphytoplankton community structure. Lake Escondido showed higher CDOM concentration and light attenuation with a spectral composition of underwater light shifted towards green-yellow light, while Lake Morenito presented clearer waters and a dominance of green light. Temporal dynamics of CDOM absorption at 440 nm were consistently explained by differences in cumulative precipitation. Mixotrophic cryptophytes and chrysophytes dominated the phytoplankton of both lakes, although the prevalence of each algal group was different between lakes. The dominance of these groups was largely explained by differences in spectral composition of underwater light, estimated as the ratio between Kd(RED) and Kd(GREEN) [Kd(R)/Kd(G) ratio]. Cryptophytes prevailed in Lake Morenito and their biomass showed a positive strong relationship with Kd(R)/Kd(G) ratio. Chrysophyte biomass was comparatively more important in Lake Escondido showing an opposite relationship with the Kd(R)/Kd(G) ratio. These results underscore that higher relative green light availability allowed the dominance of cryptophytes, while changes in light spectral composition driven by CDOM allowed coexistence. We suggest that nano and microphytoplankton community structure in these lakes could be driven by changes in spectral composition of underwater light shaped by differences in CDOM, ultimately determined by precipitation/hydrological patterns. [ABSTRACT FROM AUTHOR]

Abstract: The Western Antarctic Peninsula is strongly affected by stratospheric ozone depletion, leading to higher UVB radiation (UVBR) on the Earth surface. It is furthermore experiencing the fastest rates of global warming worldwide, resulting in an increased sediment run-off from glacial melting, altering the underwater light climate. Very little is known of how Antarctic organisms can cope with this rapidly changing environment. Seaweeds play an essential role within the Antarctic coastal ecosystems, building highly complex and productive underwater communities. The unicellular spores are the most sensitive stage in their life-cycle, forming the bottle-neck for successful recruitment. To supplement the very rare field experiments on seaweed propagules, three ecologically important Antarctic seaweeds (Adenocystis utricularis, Himantothallus grandifolius, Iridaea cordata) were investigated. The germination of spores after exposure in the field to different water depths to three light treatments (PAR; PAR+UVA; PAR+UVA+UVB) was recorded. In parallel, spores were exposed to the same treatments under artificial radiation in the laboratory for different periods. Germination of the intertidal species A. utricularis was not affected by the treatments. In spores of I. cordata and H. grandifolius depth was a major factor for successful germination. High PAR fluxes at 1 and 2m water depth inhibited germination significantly. UVR further lowered germination in H. grandifolius while in I. cordata UVBR had a negative impact only in the laboratory experiment. The results show that already the unicellular life stage expresses strong species-specific susceptibility to changes in the radiation climate. Not only UVR but also the high PAR fluxes in the field are important factors in determining the upper distribution limit of Antarctic seaweeds and laboratory experiments show stronger UVB effects as studies under natural radiation. [Copyright &y& Elsevier]

Abstract: The optical properties and light climate in the deep and extremely acid Lake Caviahue have been studied in order to better understand its characteristics and the possible influence upon the phytoplankton community. The absorption coefficients for the dissolved fraction were maximal in the ultraviolet (UV) region and the water absorption spectra showed a shoulder around 300nm, which was attributed to the concentration of Fe(III). No radiation was detected in the water column below 360nm. The depth of the 1% incident radiation was dependent of wavelength, showing its maximum of 13.3m at 565nm, compared to 1.7m and 4.8m at 400nm and 700nm, respectively. Phytoplankton biomass was low and showed an almost constant profile with depth despite the relative darkness of the water column. Optical climate of Lake Caviahue is not typical of high elevation lakes but is more similar to low elevation shallow lakes of the Andean region. The chemical composition of the water, mainly Fe oxidation state and concentration, is the responsible for the high attenuation of the UV radiation (UVR). Living organisms are protected of UVR because Lake Caviahue waters are a shield against UV-B. [Copyright &y& Elsevier]

The North-Patagonian Andean lakes of Argentina are high light, low nutrient environments that exhibit development of deep chlorophyll maxima (DCM) at the metalimnetic layer during summer stratification, at approximately 1 % of surface PAR irradiance. We examined whether the position of DCM changes as a consequence of long-time (global warming: glacial clay input) and short-time (eruption: volcanic ashes) events. We performed different field studies: (1) an interlacustrine analysis of six lakes from different basins, including data of the 2011 volcanic eruption, which caused an unexpected variation in water transparency; and (2) an intralacustrine analysis in which we compared different stations along a transparency gradient in Lake Mascardi caused by glacial clay input at one end of the gradient. In these analyses, we documented changes in DCM depth and its relationship with different parameters. DCM development was not related with thermocline depth or nutrient distribution. In all cases, the only significant variables were Kd 320 nm and Kd PAR. Our study showed that suspended particles (glacial clay and volcanic ashes) can play a crucial role in transparent lakes, affecting lake features such as the phototrophic biomass distribution along the water column. Suspended solid inputs from either glacial clay or volcanic ashes produce a comparable effect, provoking a decrease in light and, consequently, an upper location of the DCM. Thus, the DCM position is highly sensitive to global changes, such as increased temperatures causing glacier recession or to regional changes caused by volcanic eruptions. [ABSTRACT FROM AUTHOR]

Investigations on periphyton along an eutrophication gradient (NO = 0.23−0.96 mg L; PO = 0.16−0.86 mg L) of Ganga River indicated that benthic algal biomass decreased with increasing concentrations of nutrients and dissolved organic carbon (DOC). Periphyton biomass showed negative relationship ( R = 0.98; p < 0.0001) with DOC and positive relationship ( R = 0.96; p < 0.0001) with Secchi depth. Sites with high DOC showed dominance of cyanophycean Phormidium uncinatum. The study shows that the rising concentration of DOC over time may alter the light climate and consequently the fate of benthic primary producers in Ganga River. [ABSTRACT FROM AUTHOR]

Abstract: Canopy gap recruitment in beech (Fagus sylvatica L.) old-growth forests is influenced by the interplay of the canopy gap disturbance regime, site factors, and stand history. Persistent seedling and sapling banks of beech and other shade tolerant species often play a major role in canopy gap closure. To gain more insight into interactions between medium-sized gaps and pre-gap ground vegetation patterns, we studied beech regeneration over a 10-year period on a site rich in wild garlic (Allium ursinum L.) in a Dinaric beech old-growth forest. In 1999 we selected the five largest newly created gaps (∼200m2) and systematically established 13 permanent plots (1×2m) in each gap. In 1999, 2004, and 2009, we sampled tree species regeneration (density, cover, and growth), ground vegetation (composition and cover), and light climate. Beech regeneration slowly increased in density and cover despite an overall decrease in light (11.0–5.7%). The same was not true for maple. While beech was present in all regeneration classes, dominant maple seedlings rarely reached 50cm. Beech also had a significantly greater height increment. Regeneration of both species was negatively affected by Allium cover, low light levels, and ungulate browsing. Gap partitioning was observed among shade tolerant Allium, which preferred microsites under closed canopy and low light levels, beech regeneration, which preferred gap periphery, and other less shade tolerant herbs, which favoured gap centres. The slow development of the regeneration indicated the importance of several gap releases. The high maple seedling density and less abundant beech advance regeneration in this study site compared to other beech sites suggests that Allium dominated managed forests might present an opportunity for growing less shade-tolerant species, if managed with a gap-oriented silvicultural system (e.g. irregular shelterwood). [Copyright &y& Elsevier]

Abstract: The single-tree selection system is an important option for management of Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.) forests because it provides continuous cover, requires low investments for tending, and promotes natural regeneration as well as high stand resistance and elasticity. It is often regarded as a very conservative system that usually results in only minor spatiotemporal changes in forest structure and composition. We studied management history, structural changes, regeneration dynamics, and light climate of a traditional single-tree farmer selection silver fir-Norway spruce forest (site typology Bazzanio-Abietetum). Stand structure was analyzed on five 0.25ha permanent plots in 1994, 2001, and 2008. Regeneration density and height growth, forest floor vegetation, and light climate were also assessed on 1.5×1.5m regeneration subplots in 2001 and 2008. Tree cores extracted from dominant trees from both species in two plots were used for reconstructing stand history and age structure of the canopy layer. We documented the forest response to three types of selection management regimes: excessive, normal, and conservative. Excessive management with harvest intensity significantly above the increment was documented until the late 1950s, including two peaks of heavy fellings (diameter limit cut) in the 1880s and 1930s, which favoured establishment of Norway spruce and released regeneration. The period that followed was characterized by normal selection management, but was nevertheless marked by a decline of silver fir as a result of air pollution and several droughts. This led to sanitary fellings that were carried out from the late 1970s to the early 1990s. In the last two decades conservative management followed, which led to suppression and decline of regeneration, especially of Norway spruce, and loss of selection structure. Although we recorded lower regeneration potential of silver fir compared with Norway spruce within the seedling category, silver fir outcompeted Norway spruce within the small-sized tree category (1cm

Abstract: During a summer period we studied the vertical variation of in vivo and chlorophyll a specific phytoplankton absorption spectra in relation to the underwater light climate of ten deep North Patagonian Andean lakes of Argentina. The lakes were thermally stratified, and the underwater light climate was characterized by extended euphotic zones which included highly illuminated epilimnetic layers (both UVR and PAR) and metalimnia exposed to dim blue-green light. Most of the lakes presented the development of Deep Chlorophyll Maxima (DCM) at the metalimnetic layers, near 1% of surface PAR irradiance. Analyzing the fourth-derivative plots of in vivo phytoplankton absorption spectra [d IV a ph(λ)], we were able to identify several maxima absorption values attributed to different pigments. Considering lakes with DCM, a significant positive linear relationship was found between d IV a ph (495–500nm) normalized by chlorophyll a and downward irradiance. Indeed, a negative significant relationship was found between d IV a ph (495–500nm) normalized by chlorophyll a and diffuse PAR attenuation coefficients. These results point out an increase in the relative concentration of different carotenoids at surface layers indicating the role of photoprotection of these pigments. On the other hand, significant negative linear relationships were found between fourth-derivative spectra normalized by chlorophyll a at 650, 590–595, 560–565 and 520–525nm and downward irradiance. These results indicated an increase in the relative concentration of photosynthetic accessory pigments at deep layers of the euphotic zone. Furthermore, we found a decrease in depth of specific absorption spectra at 440, 670nm and in the ratio a ph* (440nm) to a ph* (670nm). This pattern was associated with the package effect concept. The increase in relative photosynthetic accessory pigment concentrations and the decrease in values of specific absorption spectra at the bottom of the euphotic zone were attributed to changes in phytoplankton communities between surface and deep layers. These outcomes pointed out that the underwater light climate and temperature water structure are, like in marine systems, very important factors governing the distribution of phytoplanktonic organisms. In addition, the possession of specific photosynthetic accessory pigments suggests that dominant species in the DCM are well adapted to these dim blue-green light scenarios. [Copyright &y& Elsevier]

The ‘Vallée de Mai’ on the island of Praslin in the Seychelles group is a World Heritage Site because of its unique forest vegetation dominated by the palm Lodoicea maldivica. However, despite its great conservation importance, the stand structure, species diversity and regeneration of this forest have not previously been studied. Moreover, the conservation value of the reserve is currently threatened by invading alien plants and limited regeneration of some native species. The species composition of the forest was studied in six permanent plots (1985 and 1998) and along seven trail transects (1997). To explain potential variation in tree regeneration, the light climate within all plots was investigated using hemispherical photographs. The results reveal little change in forest structure, species diversity and regeneration in the permanent plots over the 13-year period, but some regeneration of most woody species, presence of some alien species and a relatively high species turnover. We found no significant influence of local light climate upon forest structure, species diversity or regeneration. Although the stand characteristics seem to be relatively stable, the permanent plots and transects should be monitored in order to detect future changes in stand structure and to optimize the protection of this unique forest reserve. [ABSTRACT FROM AUTHOR]

In 1998, we analysed regeneration success in four 20 year old medium-sized (600–800 m2) and four small experimental gaps (100–200 m2) in subalpine Norway spruce forest in Triglav National Park, Slovenia. We assessed the influence of site (depressions, slopes), gap size, and position within gap (centre, gap south, and north edge) on regeneration success and interactions between regeneration density, height, height increment, direct and diffuse light, ground vegetation, soil depth, thickness of humus horizons, presence of woody debris, and microrelief. Seedling density was higher in depressions, where more seedlings developed in central positions of gaps in both gap sizes, and in microsites with deeper soils, less competition from ground vegetation and convex microsites. On slopes, a higher seedling density was found in small gaps, and at the northern edge of gaps regardless of gap size. Here, seedling density was negatively influenced by diffuse radiation and positively by soil depth. The results suggest that regeneration should be initiated from medium to large gaps in depressions and small gaps on slopes. Development of established seedlings was enhanced by higher radiation levels on both sites, therefore favourable extension of the gaps would be lateral to northwards. [ABSTRACT FROM AUTHOR]

Large fluvial lakes are understudied with respect to their underwaterlight climates. Fluvial lakes pose unique challenges for photobiologistsinterested in the interactions amongst light climate, nutrients and microbialcommunity structure and biodiversity. This is because fluvial lakes are typifiedby highly dynamic flow regimes often incorporating different inflows anddischarges each characterized by their own unique physico-chemical composition.These compositional characteristics include the concentrations of chromophoricdissolved organic matter (CDOM), suspended solids, and pigments such aschlorophyll. Together these factors contribute to the distribution andcomposition of the water masses that make up fluvial lakes. These water masses,in turn, flow over lakebeds that are typically complex in their morphometry andfeature extensive macrophyte beds, further enhancing the habitat heterogeneityof these ecosystems. We here report on the spectral attenuation of ultravioletradiation (UVR = 280–400 nm) and photosyntheticallyactive radiation (PAR = 400–700 nm) in the three mainwater masses of Lake Saint-Pierre and evaluate the relative contribution ofCDOM, and particulate organic material to UVR attenuation. We demonstrate thatUVR penetrates 18 to 30% of the water column (1% penetration depth) in the LakeSaint-Pierre ecosystem, and show how the underwater spectral UVR varies withinthe three water masses. [ABSTRACT FROM AUTHOR]

1. The major components of the underwater light field (ULF: vertical attenuation, absorption, scattering and suspensoids including plankton fractions) of Las Madres Lake, a small wind-sheltered, oligohumic lake in Central Spain, were investigated daily over a period of 3 months at the onset of vernal circulation. 2. Gilvin, arising mostly from the decomposition of reeds in the littoral in autumn, was the main component of vertical attenuation, and its variability explained the highest fraction of absorption variability. Tripton appeared to be the main factor responsible for scattering, and might have resulted from dust deposition from the surrounding mining land. The plankton community played a minor role in attenuation, absorption or scattering throughout the investigation period. 3. Vertical and horizontal mixing dynamics may control the ULF to a certain extent, as most optical properties changed within different mixing periods and poor advective exchanges may have resulted in uneven distribution of water colour in this small lake. 4. Time series analysis showed that most autocorrelations were shorter than a week, inherent properties (absorption, scattering) being delayed longer than apparent properties (attenuation, transparency) as a result of their lower dependence on solar irradiance. A 2-day lag was observed in cross-correlations between either gilvin and absorption or tripton and scattering. When different mixing periods at early circulation were considered, however, ULF components changed their relationships and delays with suspensoids and dissolved substances over such periods, probably tracking the dynamics of their controlling factors. 5. Our study, and others at daily, weekly, seasonal, interannual and long-term scales, demonstrates that ULF is a system upon which different processes are operating at different time scales. Contrary to expectations, however, the variability in the ULF does not increase with time scale and depends partly upon the... [ABSTRACT FROM AUTHOR]

• Canopy meteorology and physiology are crucial for greenhouse production. • Tomato leaflet photosynthetic rate were predicted by coupling models of light and thermal microclimate inside a 3D greenhouse structure. • Detailed leaf temperature and stomatal conductance related photosynthesis rate were simulated at the leaflet level. • Photosynthetic limitation analysis showed that leaf temperature is a key factor limiting the net photosynthesis rate under cloudy conditions. In order to determine the effects of leaf temperature, gas exchange, and photosynthesis on plant growth and productivity under greenhouse conditions, predictions at a high spatial and temporal resolution are essential. In addition, simulations of light and thermal microclimate conditions are needed for the modelling of physiological processes. To the best of our knowledge, these physiological processes have not been addressed so far with respect to their spatiotemporal distribution and dynamics in Chinese greenhouse. In the present study, we developed a structural model for a Chinese Liaoshen-solar greenhouse (LSG) and a tomato functional-structural plant model (FSPM), which combined a greenhouse energy balance model with the mechanistic understanding of stomatal function and leaf photosynthesis. Photosynthetic limitation analysises were also carried out using this model. Leaf temperature and stomatal conductance related to the photosynthetic process were simulated at high resolution. Two scenarios (sunny and cloudy) were considered in the simulation and results were verified against field data. According to our findings, our model was able to predict net photosynthesis for each individual tomato leaflet more accurately and in more detail than the most commonly used approaches, which consider a constant leaf temperature of 25 °C. The present study examined the effect of different limiting factors on crop photosynthesis under external climate change conditions. Our results showed that leaf temperature is a key factor that limits the net photosynthetic rate under cloudy conditions. The modelling approach described herein provides a basis for a precise simulation of greenhouse crops, which could be used in the future to provide guidance during the production process of various plant species in solar greenhouses with different structures. [ABSTRACT FROM AUTHOR]

The photosynthetic induction state under conditions of different lightfleck frequencies or durations, or different shade periods was studied in soybean leaves in order to examine how it might limit utilization of sunflecks in leaf canopies. Induction following an increase in photon flux density (PFD) from strongly limiting to saturating PFDs exhibited two phases; a fast-inducing one, requiring about 1 mm and a stow one, requiring up to 60 mm for completion. Transfer of fully induced leaves to low light resulted in a rapid decrease in the fast-inducing component, a slower decrease in the slow-inducing component and an even slower decrease in stomatal conductance. Therefore, the decreases in extent of induction appeared to be due to biochemical factors and not to stomatal closure. Under flashing light regimes consisting of 1-s lightflecks given at different frequencies for long periods, a constant induction state was achieved, the measure of induction state increased with the frequency of the lightflecks. This constant induction state also depended on the growth conditions, with shade leaves having a higher value than those grown at high light at any particular lightfleck frequency. The measure of induction state was mostly tower in flashing fight as compared to constant light of the same mean PFD, particularly in leaves with a low light saturation point and in short lightflecks. Initial activities of ribulose-1,5-bisphosphate carboxylase (rubisco) were also higher in continuous light and were highly correlated with the measure of induction state. The rapid decrease in extent of induction of soybean leaves during shade periods is an important limitation to the ability of the leaves to respond to light increases similar to those occurring with sunflecks. At least part of the limitation on carbon assimilation during sunflecks due to photosynthetic induction is based on regulation of rubisco activity. [ABSTRACT FROM AUTHOR]

Leaves of soybean plants grown in contrasting light and nutrient availability conditions were exposed to constant and to flashing light regimes with lightflecks of different frequencies, durations and photon flux density (PFD). The lightfleck characteristics were selected to be representative of the range of variation found for sunflecks in a soybean canopy. CO2 fixation rates were measured using a fast-response gas-exchange apparatus. The net CO2 fixation due to 1-s-duration lightflecks was 1.3 times higher than predicted from steady-state measurements in constant light at the lightfleck and background PFD, This lightfleck utilization efficiency (LUE) was somewhat higher at a high than at a tow frequency of one second lightflecks. LUE in flashing tight with very short lightflecks (0.2s) and single 1 s lightflecks was as high as 2, but decreased sharply with increasing duration of lightflecks. The decrease occurred because CO2 fixation rates during lightflecks were constrained by carbon metabolism and induction limitations, and because the contribution of post-illumination CO2 fixation to total CO2 fixation decreased with increased duration of lightflecks. LUE increased with increased PFD during the lightflecks, particularly in leaves from plants grown in high-light, high-nutrient conditions. Saturation PFDs were much higher in flashing light than in constant light. Only small differences in LUE were apparent between leaves from the three growth conditions. [ABSTRACT FROM AUTHOR]

Tobacco ( Nicotiana tabacum L.) plants transformed with 'antisense' rbcS to decrease the expression of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO at 20°C at 100, 300 and 750 μmol·m·s irradiance, and at 28°C at 100, 300 and 1000 μmol·m·s irradiance. (i) Measurement of photosynthesis under ambient conditions showed that the flux control coefficient of Rubisco (C) was very low (0.01-0.03) at low growth irradiance, and still fairly low (0.24-0.27) at higher irradiance. (ii) Short-term changes in the irradiance used to measure photosynthesis showed that Cincreases as incident irradiance rises, (iii) When low-light (100 μmol·m·s)-grown plants are exposed to high (750-1000 μmol·m·s) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. However, when high-light-grown leaves (750-1000 μmol·m·s) are suddenly exposed to high and saturating irradiance (1500-2000 μmol·m·s), Cremained relatively low (0.23-0.33), showing that in saturating light Rubisco only exerts partial control over the light-saturated rate of photosynthesis in 'sun' leaves; apparently additional factors are co-limiting photosynthetic performance, (iv) Growth of plants at high irradiance led to a small decrease in the percentage of total protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminated at growth irradiance avoided an inbalance between the 'light' reactions and Rubisco; this was shown by the low value of C(see above) and by measurements showing that non-photochemical quenching was low, photochemical quenching high, and NADP-malate dehydrogenase activation was low at the growth irradiance. In contrast, when a leaf adapted to low irradiance was illuminated at a higher irradiance, Rubisco exerted more control, non-photochemical quenching was higher, photochemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conclude that changes in leaf composition allow the leaf to avoid a one-sided limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions, (v) 'Antisense' plants with less Rubisco contained a higher content of insoluble (thylakoid) protein and chlorophyll, compared to total protein or structural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at which the plant had grown. We propose that N-allocation in low light is not optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in low light. [ABSTRACT FROM AUTHOR]

The accumulation of non-structural leaf carbohydrates is one of the most consistent plant responses to elevated CO2. It has been found in both fast-and slow-growing plants and is largely independent of the duration of exposure. Changes in leaf quality are thus to be expected, irrespective of other plant responses to atmospheric CO2 enrichment. However, there is no experimental evidence from tropical forests, the biome with the largest biomass carbon pool. Here we report in situ mesophyll responses of mature tropical trees to a doubling of CO2. Individually CO2-enriched leaves on 25 to 35-m-tall forest trees living at 26–35°C can be assumed to experience little sink limitation, and so, may be expected to exhibit no or very little carbohydrate accumulation. We tested this hypothesis using the leaf cup method on leaves accessible via the canopy crane of the Smithsonian Tropical Research Institute in a semi-deciduous tropical forest in Panamá. We also investigated the influence of the leaf-specific light regime, another possible environmental determinant of leaf carbon gain and mobile leaf carbohydrates. Total non-structural carbohydrates (TNC) reached a new steady state concentration after less than 4 days of exposure to twice ambient CO2 concentration. Against expectation, all four tree species investigated ( Anacardium excelsum, Cecropia longipes, C. peltata, Ficus insipida) accumulated significant amounts of TNC (+41 to +61%) under elevated CO2. The effect was stronger at the end of the daylight period (except for Ficus), but was still significant in all four species at the end of the dark period. In contrast, neither artificial nor natural shading affected leaf TNC. Taken together, these observations suggest that TNC accumulation reflects a mesophyll-bound tissue response specific to elevated CO2, presumably unrelated to sink limitations. Thus, leaves of tropical forests seem not to be an exception, and will most likely contain more non-structural carbohydrates in a CO2-rich world. [ABSTRACT FROM AUTHOR]

Seedling developmental responses to understory shade combine the effects of reductions in irradiance and changes in spectral quality. We studied the seedling development of two Southeast Asian dipterocarp trees in response to differences in irradiance (photosynthetic photon flux density, PPFD) and spectral quality (red to far-red ratio, R:FR). The two species, Hopea helferei and H. odorata, are taxonomically closely related but differ in their ecological requirements; H. helferei is more drought-tolerant and typically grows in more open habitats. Seedlings were grown in six different replicated shadehouse treatments varying in percentage of solar PPFD and R:FR. The two species differed in the influence of light variables on most seedling characters, particularly for final height, internode distance, branch/trunk internodes, stem length/mass, leaf area/stem length, petiole length, and growth/mol of photons received. Most of the characters in both taxa were primarily influenced by PPFD, but spectral quality also influenced some characters – more so for H. odorata. The latter species grew more rapidly, particularly in the low PPFD treatments, and its leaves were capable of higher photosynthesis rates. However, growth in H. helferei was not reduced in direct sunlight. The growth of this taxon may be constrained by adaptations, particularly in leaves, for drought tolerance. [ABSTRACT FROM AUTHOR]

We studied the effects of atmospheric CO enrichment (280, 420 and 560 μl CO l) and increased N deposition (0,30 and 90 kg ha year) on the spruce-forest understory species Oxalis acetosella, Homogyne alpina and Rubus hirtus. Clones of these species formed the ground cover in nine 0.7 m model ecosystems with 5-year-old Picea abies trees (leaf area index of approx 2.2). Communities grew on natural forest soil in a simulated montane climate. Independently of N deposition, the rate of light-saturated net photosynthesis of leaves grown and measured at 420 μl CO l was higher in Oxalis and in Homogyne, but was not significantly different in Rubus compared to leaves grown and measured at the pre-industrial CO concentration of 280 μl l. Remarkably, further CO enrichment to 560 μl l caused no additional increase of CO uptake. With increasing CO supply concentrations of non-structural carbohydrates in leaves increased and N concentrations decreased in all species, whereas N deposition had no significant effect on these traits. Above-ground biomass and leaf area production were not significantly affected by elevated CO in the more vigorously growing species O. acetosella and R. hirtus, but the 'slow growing' H. alpina produced almost twice as much biomass and 50% more leaf area per plant under 420 μl CO l compared to 280 μl l (again no further stimulation at 560 μl l). In contrast, increased N addition stimulated growth in Oxalis and Rubus but had no effect on Homogyne. In Oxalis (only) biomass per plant was positively correlated with microhabitat quantum flux density at low CO, but not at high CO indicating carbon saturation. On the other hand, the less shade-tolerant Homogyne profited from CO enrichment at all understory light levels facilitating its spread into more shady micro-habitats under elevated CO. These species-specific responses to CO and N deposition will affect community structure. The non-linear responses to elevated CO of several of the traits studied here suggest that the largest responses to rising atmospheric CO are under way now or have already occurred and possible future responses to further increases in CO concentration are likely to be much smaller in these understory species. [ABSTRACT FROM AUTHOR]

Vertical structure of plant stands and canopies may change under conditions of elevated CO due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands of Ricinus communis exposed to ambient (340 μl l) and elevated (610 μl l) CO. Plants in each stand were grown in a common soil as closed 'artificial ecosystems' with a ground area of 6.7 m. LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO. CO enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO at the stand-level. We suggest that, under elevated CO, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction. [ABSTRACT FROM AUTHOR]

A hypothesis that a dense stand should develop a less uniform distribution of leaf nitrogen through the canopy than an open stand to increase total canopy photosynthesis was tested with experimentally established stands of Lysimachia vulgaris L. The effect of stand density on spatial variation of photon flux density, leaf nitrogen and specific leaf weight within the canopy was examined. Stand density had little effect on the value of the light extinction coefficient, but strongly affected the distribution of leaf nitrogen per unit area within a canopy. The open stand had more uniform distribution of leaf nitrogen than the dense stand. However, different light climates between stands explained only part of the variation of leaf nitrogen in the canopy. The specific leaf weight in the canopy increased with increasing relative photon flux density and with decreasing nitrogen concentration. [ABSTRACT FROM AUTHOR]