Incident Photosynthetic Photon Flux Density Research Articles

Topography-mediated light environment regulates intra-specific seasonal and diurnal patterns of photosynthetic plasticity and plant ecophysiological adaptation strategies.

Due to substantial topographic variations in the Himalaya, incident solar radiation in the forest canopy is highly unequal. This results in significant environmental differences at finer scales and may lead to considerable differences in photosynthetic productivity in montane forests. Therefore, local-scale ecophysiological investigations, may be more effective and instructive than landscape-level inventories and models. We investigated leaf ecophysiological differences and related adaptations between two Quercus semecarpifolia forests in aspect-mediated, significantly varying light regimes in the same mountain catchment. Seasonal and diurnal rates of photosynthesis (A) were significantly higher in south aspect (S) than the north (N). Although temperature was a key contributor to seasonal fluctuations in photosynthetic physiology, photoperiod significantly determined intraspecific variations in seasonal and diurnal plasticity of leaf ecophysiological traits between the two topography-mediated light environments. The regression model for A as a function of stomatal conductivity (gsw) explained the critical role of gsw in triggering photosynthetic plasticity as an adaptive function against varying environmental stresses due to seasonal solar differences. We also examined, modifications in chlorophyll content between the two light regimes across seasons to determine the chlorophyll adaptation strategy. The N aspect had higher leaf chl a, b, and chl a + b and a lower chl-allocation ratio (a/b) than S, which helped to optimize the required light reception in the photoreaction centers for improved photosynthetic performance. The leaf light response curves for A and gsw were observed against varying incident photosynthetic photon flux densities (0-2000mol.m2s-1PPFD) for both aspects. We found that the same species developed significantly distinct light response strategies and photosynthetic capacities in S than in N for the given magnitudes of PPFD. Such acquired ecophysiological adaptations owing to varying light environments may provide significant clues for understanding the impact of future climate change on Himalayan tree species.

Global terrestrial nitrogen uptake and nitrogen use efficiency

Abstract Plant biomass production (BP), nitrogen uptake (Nup) and their ratio, and nitrogen use efficiency (NUE) must be quantified to understand how nitrogen (N) cycling constrains terrestrial carbon (C) uptake. But the controls of key plant processes determining Nup and NUE, including BP, C and N allocation, tissue C:N ratios and N resorption efficiency (NRE), remain poorly known. We compiled measurements from 804 forest and grassland sites and derived regression models for each of these processes with growth temperature, vapour pressure deficit, stand age, soil C:N ratio, fAPAR (remotely sensed fraction of photosynthetically active radiation absorbed by green vegetation) and growing‐season average daily incident photosynthetic photon flux density (gPPFD; effectively the seasonal concentration of light availability, which increases polewards) as predictors. An empirical model for leaf N was based on optimal photosynthetic capacity (a function of gPPFD and climate) and observed leaf mass per area. The models were used to produce global maps of Nup and NUE. Global BP was estimated as 72 Pg C/year; Nup as 950 Tg N/year; and NUE as 76 g C/g N. Forest BP was found to increase with growth temperature and fAPAR and to decrease with stand age, soil C:N ratio and gPPFD. Forest NUE is controlled primarily by climate through its effect on C allocation—especially to leaves, being richer in N than other tissues. NUE is greater in colder climates, where N is less readily available, because below‐ground allocation is increased. NUE is also greater in drier climates because leaf allocation is reduced. NRE is enhanced (further promoting NUE) in both cold and dry climates. Synthesis. These findings can provide observationally based benchmarks for model representations of C–N cycle coupling. State‐of‐the‐art vegetation models in the TRENDY ensemble showed variable performance against these benchmarks, and models including coupled C–N cycling produced relatively poor simulations of Nup and NUE.

Photosynthetic characteristics of the invasive weed Chromolaena odorata and other co‐occurring species in KwaZulu‐Natal

AbstractIn this study, we tested the hypothesis that photosynthetic characteristics contributed to the success and spread of the invasive, C3 perennial weed, Chromolaena odorata (L.) R. M. King & H. Rob. Measurements of gas exchange, chlorophyll fluorescence and water relations parameters were taken in summer and winter, as well as in sun and shade plants. Typically, diurnal CO2 exchange increased from dawn to a mid‐morning maximum between 10:00 h and 11:00 h, and thereafter decreased gradually for the rest of the day. Light response curves indicated saturation of CO2 uptake and electron transport rate (ETR) through Photosystem II (PSII) at a photosynthetic photon flux density (PPFD) ≥1800 μmol m−2 s−1. Maximal CO2 exchange was 15.64 ± 0.87 (±SE) in summer and 13.15 ± 0.26 μmol m−2 s−1 in winter. Leaf conductance (g), CO2 uptake (A) and transpiration (E) followed trends similar to those for PPFD. Leaf water potential (Ψ) and water use efficiency (WUE) declined from the predawn maximum values to a minimum at midday and thereafter recovered at dusk. Diurnal trends in actual quantum yield (ΦPSII) and maximum quantum yield (Fv/Fm) decreased from dawn to a minimum at midday, followed by complete recovery at dusk. The relationship between A and g, and A and ETR was linear. Trends in A, ΦPSII and ETR through PSII were tightly coupled to those of incident PPFD. Efficient light utilisation is achieved through modifying plastic traits such as leaf size, specific leaf area and chlorophyll content. Chromolaena is a facultative shade‐tolerant weed that exhibits superior light utilisation to maximise carbon gain, while efficient photoprotective mechanisms minimise photoinhibition and photodamage to the photosystems. These adaptive architectural and physiological strategies may probably confer competitive advantage over other species.

Gradually increasing light intensity during the growth period increases dry weight production compared to constant or gradually decreasing light intensity in lettuce

The objective of this research was to investigate the effects of gradually increasing or decreasing photosynthetic photon flux density (PPFD) during cultivation compared to a constant PPFD on biomass production. Lettuce plants (Lactuca sativa L. ‘Expertise’) were grown in climate rooms in which every three days the PPFD was increased by 16 µmol m−2 s−1 (from 140 to 300 µmol m−2 s−1 from day 0 to 30), decreased (from 300 to 140 µmol m−2 s−1), or kept constant (221 µmol m−2 s−1), while the total light integral at the end of the cultivation period (30 d) was the same for all three treatments. Gradually increasing PPFD resulted in a 16 or 13% increase in total plant dry weight compared to treatments with decreasing or constant PPFD, respectively. This increase was explained by a higher light interception mainly because, in this treatment, most of the light was provided at the end of the cultivation period when the leaf area index was high. Consequently, the light use efficiency based on incident PPFD was highest when PPFD gradually increased, even though the light use efficiency based on intercepted PPFD was highest when PPFD gradually decreased during cultivation. Despite the higher shoot dry weight when PPFD gradually increased, shoot fresh weight was not significantly affected by the light treatments. This difference in response between fresh and dry weight resulted from a higher shoot dry matter content when PPFD gradually increased. Our results show that gradually increasing PPFD had a positive effect on dry weight accumulation and increased dry matter content, but did not affect the shelf life. So, although vertical farms enable growers to keep all conditions constant, some dynamic variation of conditions might be needed for optimizing the light use efficiency.

Light use efficiency of lettuce cultivation in vertical farms compared with greenhouse and field

AbstractVertical farming is a relatively new fresh fruit and vegetable production system, where lamps (mostly light emitting diodes [LED]) are the sole light source. A high light use efficiency (LUEinc), defined as shoot dry weight per incident photosynthetic photon flux density (PPFD; g mol−1) integral, is crucial for the economic viability of vertical farming. Very different values for LUEinc have been reported in the literature and it is not clear whether LUEinc is higher in vertical farming than in greenhouse or open field cultivation. Values of LUEinc of lettuce grown in a vertical farm (53 studies), greenhouse (13 studies) and open field (8 studies) were collected from literature, as well as relevant cultivation aspects such as lettuce weight at harvest, cultivation period (plant age at harvest), daily light integral, cumulative daily light integral for the whole cultivation period, planting density and CO2 concentration. The average LUEinc for lettuce grown in a vertical farm was 0.55 g mol−1 which was higher than 0.39 g mol−1 for greenhouse‐grown lettuce. Both were substantially higher than for field‐grown lettuce (0.23 g mol−1). The maximum measured LUEinc for lettuce grown in a vertical farm (1.63 g mol−1) is close to the published maximum theoretical value, which ranges from 1.26 to 1.81 g mol−1. Since all environmental factors can be fully controlled, vertical farming has the capability to achieve the theoretical maximum LUEinc. Using the highest reported LUEinc based on shoot fresh weight (44 g mol−1 at 200 μmol m−2 s−1 PPFD and 16 h photoperiod), it is estimated that each layer of a vertical farm can potentially produce annually up to 700 kg of lettuce per m2 at 500 μmol m−2 s−1 of continuous light.

Radiation use efficiency and instantaneous photosynthesis at different growth stages of wheat (Triticum aestivum L.) in semi arid ecosystem of Central Punjab, India

The instantaneous canopy net photosynthetic rate (Ac) and photosynthetic radiation use efficiency (PhRUE) were investigated diurnally at different growth stages of wheat during two seasons 2009-2010 and 2010-11. Diurnal changes in Ac were synchronized with changes in incident photosynthetic photon flux density (PPFDo) throughout the experimental period during both the seasons. Regardless of growth stages, high Ac values were always observed around noon, when there was a high photosynthetic photon flux density (PPFD). In addition, the highest AC values were noticed at heading stage, when compared to other growth and development stages of wheat. With respect to PhRUE, the highest values occurred in the active tillering stage and decreased gradually upto soft dough stage during both the seasons. PhRUE was lowest after heading stage for wheat crop in both seasons, corresponding to decrease in photosynthetically active leaf area, another explanation for the decrease in RUE is a decrease in crop growth rate. Regarding diurnal changes in PhRUE, study revealed that it was always higher during early morning and evening time of the day at all the stages of the crop. The seasonal PhRUE of wheat was observed to be 1.21 g MJ-1 and 1.27 g MJ-1, respectively during two seasons.

Photosynthetic Physiology of Blue, Green, and Red Light: Light Intensity Effects and Underlying Mechanisms.

Red and blue light are traditionally believed to have a higher quantum yield of CO2 assimilation (QY, moles of CO2 assimilated per mole of photons) than green light, because green light is absorbed less efficiently. However, because of its lower absorptance, green light can penetrate deeper and excite chlorophyll deeper in leaves. We hypothesized that, at high photosynthetic photon flux density (PPFD), green light may achieve higher QY and net CO2 assimilation rate (An) than red or blue light, because of its more uniform absorption throughtout leaves. To test the interactive effects of PPFD and light spectrum on photosynthesis, we measured leaf An of “Green Tower” lettuce (Lactuca sativa) under red, blue, and green light, and combinations of those at PPFDs from 30 to 1,300 μmol⋅m–2⋅s–1. The electron transport rates (J) and the maximum Rubisco carboxylation rate (Vc,max) at low (200 μmol⋅m–2⋅s–1) and high PPFD (1,000 μmol⋅m–2⋅s–1) were estimated from photosynthetic CO2 response curves. Both QYm,inc (maximum QY on incident PPFD basis) and J at low PPFD were higher under red light than under blue and green light. Factoring in light absorption, QYm,abs (the maximum QY on absorbed PPFD basis) under green and red light were both higher than under blue light, indicating that the low QYm,inc under green light was due to lower absorptance, while absorbed blue photons were used inherently least efficiently. At high PPFD, the QYinc [gross CO2 assimilation (Ag)/incident PPFD] and J under red and green light were similar, and higher than under blue light, confirming our hypothesis. Vc,max may not limit photosynthesis at a PPFD of 200 μmol m–2 s–1 and was largely unaffected by light spectrum at 1,000 μmol⋅m–2⋅s–1. Ag and J under different spectra were positively correlated, suggesting that the interactive effect between light spectrum and PPFD on photosynthesis was due to effects on J. No interaction between the three colors of light was detected. In summary, at low PPFD, green light had the lowest photosynthetic efficiency because of its low absorptance. Contrary, at high PPFD, QYinc under green light was among the highest, likely resulting from more uniform distribution of green light in leaves.

Photosynthetic light‐use efficiency of rice leaves under fluctuating incident light

AbstractThe responsiveness of photosynthesis to continuously fluctuating light intensities can provide an estimate of the net assimilation rate (A) and photosynthetic light‐use efficiency (PLUE), which cannot be measured under steady‐state conditions. This study was conducted to investigate whether the leaves of rice (Oryza sativa L.) ‘Koshihikari’ and ‘Akitakomachi’ can efficiently utilize light under constantly fluctuating incident photosynthetic photon flux density (PPFD) and also to determine the effect of leaf N concentration on the acclimation of photosynthesis to changing PPFD. To this end, the instantaneous A and continually fluctuating incident PPFD were measured for 16 youngest fully expanded leaves of individual plants from dawn to dusk at the vegetative and reproductive phases under open‐air field conditions using two portable photosynthesis systems. The results revealed that (a) daily PLUE and calculated as the slope of the linear regression of A for values of PPFD between the light compensation point and 200 μmol m−2 s−1 can be improved with leaf N; (b) rice leaves do not utilize light efficiently under constantly fluctuating light conditions; and (c) the stomatal conductance per unit PPFD decreases under higher daily incident PPFD and diurnal fluctuation. Overall, our findings will help in refining crop photosynthesis models and would assist with the selection of high‐PLUE lines in breeding programs.

Characterization of the Spectrum of Solar Irradiance under Different Crop Protection Coverings in Mediterranean Conditions and Effect on the Interception of Photosynthetically Active Radiation

Plants use visible light and part of adjacent ultraviolet and near infrared regions for photosynthesis. Crop protection coverings enable plant cultivation in areas or seasons not suitable open field. However, the use of covering materials is a detriment to solar irradiance, which may decrease the photosynthetic rate. Here, the effect of two different covering materials, tempered glass and white polyethylene mesh, on solar irradiance was compared to open field (control) under real farming conditions. Relative irradiance (RI) and photosynthetic photon flux density (PPFD) were recorded along 380-780 nm wavelength spectrums in the two conditions at 10:00 h and 13:00 h. Also the efficiency of Capsicum peppers in capturing solar irradiance was evaluated in leaves as the reflectance of both RI and PPFD under the mentioned growing conditions. Low differences in RI among the three conditions were found, and the lowest values corresponded to glasshouse conditions. Differences were more obvious in PPFD and, compared to open field, both mesh greenhouse and glasshouse conditions provoked remarkable decreases in all the spectral bands, 50-55% and 75-80% respectively. Covering materials also differed on the ratio of reflected PPFD and incident PPFD. Glasshouse plants displayed the highest reflectance at both 10:00 h and 13:00 h (0.05-0.20), followed by mesh greenhouse (0.05-0.10), suggesting that glasshouse conditions might decrease the photosynthesis rate due to both PPFD decrease and reflectance, although the effect of polyethylene mesh should not be disregarded as it also decreases considerably PPFD. Our results have important implications for the physiology and the productivity of crops under different covering materials.

Measurement of Diurnal Variation in Needle PRI and Shoot Photosynthesis in a Boreal Forest

The photochemical reflectance index (PRI) is calculated from vegetation narrowband reflectance in two bands in the visible part of the spectrum. Variations in PRI are associated with changes in the xanthophyll cycle pigments which regulate the light use efficiency of vegetation. Correlations have been found between remotely-sensed PRI and various photosynthetic productivity parameters at the scales from leaves to landscapes. Environmental satellites can provide only an instantaneous value of this index at the time of overpass. The diurnal course of needle (leaf) PRI needs to be known in order to link the instantaneous values robustly with photosynthetic parameters at time scales exceeding one day. This information is not currently available in the scientific literature. Here we present the daily cycle of Scots pine needle and canopy PRI in a southern boreal forest zone in the presence of direct solar radiation during the peak growing season of two consecutive years. We found the PRI of the needles which are exposed to direct radiation to have a distinct diurnal cycle with constant or slightly increasing values before noon and a daily minimum in the afternoon. The cycle in needle PRI was not correlated with that in the incident photosynthetic photon flux density (PPFD). However, when PPFD was above 1000 μmol m−2 s−1, approximately between 8 a.m. and 5 p.m., needle PRI was correlated with the light use efficiency (LUE), measured with shoot chambers. The timing of the minimum needle PRI coincided with the minimum canopy value, as measured by an independent sensor above the canopy, but the correlation between the two variables was not significant. Our field results corroborate the applicability of needle PRI in monitoring the daily variation in LUE. However, to apply this to remote sensing of seasonal photosynthetic productivity, the daily cycle of leaf PRI needs to be known for the specific vegetation type.

Photosynthetic photon flux density levels affect morphology and bromatology in Cichorium endivia L. var. latifolia grown in a hydroponic system

Abstract Photosynthetic photon flux density (PPFD) in suboptimal or super-optimal levels can modify the biomass accumulation, bromatological composition and appearance of crops. This study was conducted in a hydroponic system to determine whether PPFD could change physiological and bromatological aspects of escarole (Cichorium endivia L. var. latifolia). Three PPFD levels (100%, 70% and 50% of PPFD) were investigated to determine crop’s responses to this factor. Plants of escarole grown under 50% of incident PPFD had converted 0.88 g of dry matter per mol of incident photosynthetically active radiation (42% more efficient compared to unshaded environment), had 26% more lipid content, higher levels of chlorophyll a, b and total (12%, 14% and 13%, respectively), 42% less acid detergent fiber, 34% less cellulose, 47% less lignin, lesser leaf thickness and better visual features. The attenuation of PPFD in sites or seasons where there is high light intensity is an effective method to improve morphological, bromatological and visual characteristics of escarole.

Different genotypes of Dalbergia sissoo trees modified microclimate dynamics differently on understory crop cowpea (Vigna unguiculata) as assessed through ecophysiological and spectral traits in agroforestry system

Microclimate being a crucial and challenging issue for the growth, survivability and ecological relevance of understory crops in agroforestry system, major aim of the present study was to determine that how the micro-environment was modulated by the canopies of different genotypes of a same tree species. Field experiments were conducted at Central Agroforestry Research Institute, Jhansi, during kharif (rainy) season located in a semi-arid region of Central India. Cowpea [Vigna unguiculata (L.) Walp. variety Gomti] was grown under three genotypes namely Bundel-2 (PT-2), Bundel-6 (PT-6) and one local of Dalbergia sissoo Roxb. tree. Cowpea was also grown in adjacent open field (treeless area) for comparison. Microclimate variables as incident photosynthetic photon flux density (PPFD), air temperature, leaf temperature, canopy temperature depression and soil surface temperature were monitored for understanding microclimate dynamics with reference to the understory crop. Several ecophysiological and leaf spectral traits were also evaluated for assessing the efficiency of the understory crop keeping relevance with microclimate modulation and their use as determinants. Light interception was remarkably different under the canopies of three different genotypes of D. sissoo trees. Intercepted PPFD (iPPFD) was higher by PT-2 and PT-6 than the local. Higher iPPFD was related to the higher leaf area index (LAI) of the corresponding tree genotypes indicating the larger canopy of the improved genotypes captured more light than the local one and thus a strong linear correlation was obtained between iPPFD and LAI. Microclimate variables such as air temperature, canopy temperature, canopy temperature depression, soil surface temperature and relative humidity (RH) were conspicuously different in the areas under the different tree genotypes in the field. There was clear vertical gradient of temperature at the top, middle and bottom layer of the tree canopies. Similarly, RH of the spaces in between the tree-rows showed a vertical gradient. Vertical profile of temperature was different depending on the tree genotypes and RH was much higher inside canopy than in the open field. Intensity of shade varied depending upon the tree genotypes and it was strongly associated with the LAI and iPPFD. Deep shade (50–60%) was observed under the PT-2 and PT-6, whereas moderate shade (up to 30%) was observed under the local genotype. Grain yield of cowpea was relatively less under the PT-2 and PT-6.Thus yield was found associated with the iPPFD by the respective tree canopies and the modulation of the microclimate dynamics. Various ecophysiological traits namely CO2 assimilation, electron transport rate across PS-II (ETR), transpiration, stomatal conductance, reflectance based leaf spectral traits like CCI, NDVI and PRI were also observed to be associated with differential responses of the understory crop. Our results highlighted about the relevance and significance of the ecophysiological and spectral traits evaluated as indicators towards better understanding of the microclimate modulation by the tree canopies of different genotypes and efficiency of the understory crop in agroforestry. Overall, these results demonstrated that the tree canopies of a same species played critical role not only in controlling the efficiency of understory crop, but also in modulating the microclimate which has significance for various ecosystem services perspectives.

Relationships between light environment and subsurface accumulation during the daytime in the red-tide dinoflagellate Karenia mikimotoi

Diurnal vertical migration of the red-tide dinoflagellate Karenia mikimotoi and environmental conditions were measured in Hiroshima Bay and the Yatsushiro Sea in July of 2012 and 2015, respectively. The uppermost daytime depths for K. mikimotoi were 1.3–6.0 m and did not correspond to the pycnocline; most cells reached the bottom each night. Although the incident photosynthetic photon flux density (PPFD) varied among days, average PPFDs at the mean cell distribution depths were similar (59–65 µmol m−2 s−1) between compensation and saturation PPFDs for photosynthesis. Total daily PPFD at the mean depth was between the growth threshold and that for maximum growth rates. Among visible wavelengths, the photon flux density of green and yellow light (500–600 nm) at the mean depth was major (about 61% of the total), and the PFD of blue (400–500 nm) was always relatively minor (about 20%). Laboratory results identified green light as sufficient for K. mikimotoi growth, yielding growth rates equivalent to those under blue light. This study suggests that the vertical cell distribution depth during the daytime is regulated by underwater light transmission and that green light is valuable for growth and photosynthesis of K. mikimotoi.

The effect of sunlight interception by sooty mold on chlorophyll content and photosynthesis in orange leaves (Citrus sinensis L.)

Sooty molds are a lineage of follicolous fungi that cover the upper surface of leaves with black mycelia. Sooty molds do not infect plants, but grow on surfaces where honeydew deposits accumulate. It causes a reduction of incident sunlight by physical obstruction and in some species it interferes with photosynthesis. However, there are no studies proving that light interception by the sooty mold mycelia affects photosynthesis in orange plants. The aim of this study was to experimentally evaluate changes in the interception of sunlight caused by the black coating of sooty mold formed on orange leaves and to investigate its effects on the leaf chlorophyll content, stomatal conductance and photosynthetic rate. To facilitate the measurements, orange leaves with and without sooty mold colonies were selected. On a clear day, the sooty mold mycelia intercepted between 44 and 74 % of the total incident photosynthetic photon flux density (PPFD). However, even on leaves covered by the sooty mold mycelia, the measured PPFD was sufficient to saturate maximum net photosynthesis rate (Amax) for much of the day. No differences were found in Amax or leaf conductance, but there were increases in chlorophyll content and quantum yield in leaves infested by sooty mold, revealing a clear acclimation response. This study is the first to experimentally assess the direct effects of sunlight interception by sooty mold on chlorophyll content and net photosynthesis in orange leaves.

A Consideration for the Light Environmental Modeling under Tropical Rainforest Canopies

Abstract. Photosynthetic Active Radiation (PAR) is the most important light source for plant photosynthesis. It is known that most of PAR from solar radiation is well absorbed by the surface. The canopy is the surface in forest region, consists an aboveground portion of plant community and formed by plant crowns. On the other hand, incident solar radiation is fluctuating at all times because of fluctuating sky conditions. Therefore, qualitative light environmental measurements in forest are recommended to execute under stable cloudy condition. In fact, it is quite a few opportunities to do under this sky condition. It means that the diffuse light condition without the direct light is only suitable for this measurement. In this study, we challenged the characterization the forest light environment as its representativeness under no consideration of sky conditions through analysis huge quantities of instantaneous data which obtained under the different sky conditions. All examined data were obtained under the different sky conditions at the tropical rainforest canopy as one of the typical fluctuating sky conditions regions. An incident PAR is transmitted and scattered by different forest layers at different heights. Various PAR data were measured with quantum units as Photosynthetic Photon Flux Density (PPFD) at different forest heights by the quantum sensors. By comparing PPFDs at different heights with an incident PPFD, relative PPFDs were calculated, which indicate the degree of PPFD decrease from the canopy top to lower levels. As the results of these considerations, daily averaging is confirmed to be cancelled sky fluctuating influences.

Reviewing the carbon cycle dynamics and carbon sequestration potential of Cyperus papyrus L. wetlands in tropical Africa

Tropical papyrus wetlands have the ability to assimilate and sequester significant amounts of carbon. However, the spatial extent, productivity and carbon sink strength associated with papyrus wetlands remains poorly characterised. The objective of this study was to collate information from peer-reviewed publications and relevant government and NGO reports to better understand carbon dynamics within papyrus dominated wetlands, and to assess the processes that regulate the magnitude of the carbon sink. Papyrus wetlands were shown to exhibit high rates of photosynthetic carbon dioxide (CO2) assimilation of up to 40 μmol CO2 m−2 s−1 where the incident photosynthetic photon flux density was ≥1,000 μmol m−2 s−1, high rates of net primary production ranging between 14 and 52 g DM m−2 d−1 and represent a significant carbon sink where up to 88 t C ha−1 is stored in the aboveground and belowground components of the papyrus vegetation. Under flooded conditions significant detrital and peat deposits accumulate in excess of 1 m in depth, representing an additional carbon store in the order of 640 t C ha−1. This study also highlighted the lack of empirical data on emissions of other radiatively important trace gases such as methane and nitrous oxide and also the vulnerability of these carbon sinks to both future changes in climate, in particular periods of hydrological drawdown, and anthropogenic land use change where the papyrus vegetation is removed in favour of subsistence agricultural cropping systems.

Photosynthetic characteristics of Cymbidium ‘Red Fire’ and ‘Yokihi’ at different developmental stages

Photosynthetic characteristics of two Cymbidium hybrids, ‘Red Fire’ and ‘Yokihi’, were investigated at different developmental stages. The net CO2 assimilation rate (An) as a function of incident photosynthetic photon flux (PPF), intercellular CO2 concentration (Ci), and relative humidity (RH) in a leaf chamber head was determined in a greenhouse with 1- and 2-year-old plant leaves. The maximum net CO2 assimilation rate (Anmax) in response to PPF of 1-year-old ‘Red Fire’ was 6.11 μmol·CO2·m−2·s−1 at 700 μmol·m−2 ·s−1 PPF, whereas it was 4.9 μmol·CO2·m−2·s−1 at 500 μmol·m−2·s−1 PPF in ‘Yokihi’. Under low light intensity (below 250 μmol·m−2 ·s−1 PPF), An was more pronounced in ‘Yokihi’ than in ‘Red Fire’. In 2-year-old Cymbidium, however, the maximum An was less than 2 μmol·CO2·m−2·s−1 in both cultivars at the same CO2 condition of 600 μmol·CO2·mol−1 air. The An increased in the leaves of 2-year-old plant under an elevated CO2 condition up to 1,500 μmol·CO2·mol−1 air with increasing PPF up to about 800 μmol·m−2·s−1. The An increased with increasing Ci and showed the highest value when Ci was over 1,000 and 2,000 μmol·mol −1 air in ‘Red Fire’ and ‘Yokihi’, respectively. The carboxylation efficiency of ‘Red Fire’ and ‘Yokihi’ were 0.044 and 0.036 μmol·CO2·m−2·s−1, respectively, while An of ‘Yokihi’ was higher than that of ‘Red Fire’ at above 1,000 μmol·mol−1 air Ci. The An tended to increase with RH, increasing from 15 to 70% in both hybrids. The Anmax of ‘Yokihi’ was observed at over 70% of RH, whereas 45% of RH was sufficient enough to show Anmax in ‘Red Fire’. The results shows that the photosynthetic characteristics of Cymbidium ‘Red Fire’ and ‘Yokihi’ can be increased by controlling the PPF, CO2, and RH conditions and the optimal conditions for maximum photosynthetic efficiency vary by cultivar, developmental stage, and greenhouse conditions.

Impact of clear and cloudy sky conditions on the vertical distribution of photosynthetic CO2 uptake within a spruce canopy

Summary1. Cloud cover affects carbon exchange between biota and the atmosphere. Recent studies have demonstrated that an increase in the diffuse radiation fraction enhances the photosynthetic efficiency of canopies. Although the exact mechanism behind this effect is not clear, a more even distribution of light among leaves across the vertical profile of the canopy is considered to be the most important cause of this difference.2. To test this hypothesis, the net ecosystem production (NEP) of a Norway spruce forest (30‐year‐old) was measured under cloudy and sunny skies by the eddy covariance method. In parallel, measurements of the diurnal courses of gas exchange and chlorophyll fluorescence parameters were made in the upper sun (5th whorl; 1‐year‐old needles), middle (8th and 10th whorl; 1‐ and 2‐year‐old needles) and lower shade (15th whorl; >2‐year‐old needles) shoots.3. The higher diffuse radiation fraction during cloudy days resulted in significantly higher ecosystem carbon uptake than at corresponding incident photosynthetic photon flux density on sunny days. Our shoot‐level data show that shoots from deep within the canopy contribute substantially to the overall carbon balance during cloudy days. But, although shade‐adapted shoots had a markedly positive carbon balance over a 24‐h period on cloudy days, their performance was impaired on sunny days contributing only a marginal or even negative carbon balance from the middle and shaded parts of the canopy. The uppermost sun shoots contributed 78% of the total carbon assimilated during a sunny day, but only 43% during a cloudy day.4. In addition, afternoon depression of canopy NEP and CO2 assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO2 uptake and apparent quantum yield; however, this depression did not occur under cloudy conditions. Stomatal and non‐stomatal regulations of carbon assimilation in the afternoon are discussed.

The potential of the satellite derived green chlorophyll index for estimating midday light use efficiency in maize, coniferous forest and grassland

Light use efficiency (LUE) is an important variable in carbon cycle and climate change research. We present an investigation of remotely estimating midday LUE using the green chlorophyll index (CI green) derived from the cloud-free Moderate Resolution Imaging Spectroradiometer (MODIS) images in maize, coniferous forest and grassland. Similar temporal patterns are observed in both canopy chlorophyll content and midday LUE which indicates that the chlorophyll content in the maize canopy servers as a proxy of midday LUE ( R 2 = 0.736, p < 0.001). Therefore, the CI green, tested as a good indicator of canopy chlorophyll content ( R 2 = 0.840, p < 0.001), has been demonstrated to be a reliable candidate in providing reasonable estimates of midday LUE with determination coefficient R 2 equals to 0.820 and a root mean square error (RMSE) of 0.002 mol CO 2 per mol incident photosynthetic photon flux density (PPFD). Further validation of the prediction model derived from the maize site demonstrates that the CI green has potential to be applied in the coniferous forest and grassland ecosystems with RMSE of 0.005 and 0.004 mol CO 2 mol −1 PPFD, respectively. A comparison analysis between different vegetation types is included and these results could be helpful in the development of future LUE and terrestrial models.

Summer variability, winter dormancy: lichen activity over 3 years at Botany Bay, 77°S latitude, continental Antarctica

Lichens make up a major component of Antarctic vegetation; they are also poikilohydric and are metabolically active only when hydrated. Logistic constraints have meant that we have little idea of the length, timing or environmental conditions of activity periods of lichens. We present the results of a three-year monitoring of the activity of the lichen Umbilicaria aprina at Botany Bay (77°S latitude) in the Ross Sea region, continental Antarctica. Chlorophyll fluorescence parameters that allowed hydrated metabolic activity to be detected were recorded with a special fluorometer at 2- or 3-h intervals. Air and thallus temperatures and incident PPFD (photosynthetic photon flux density, μmol photon m−2 s−1) were also recorded at hourly intervals. Activity was extremely variable between months and years and, overall, lichen was active for 7% of the 28-month period. Spring snow cover often delayed the onset of activity. Whereas the period immediately after snow melt was often very productive, the later months, January to March, often showed low or no activity. Mean thallus temperature when active was just above zero degrees and much higher than the annual mean air temperature of −15 to −19°C. Because major snow melts occurred when incident radiation was high, the lichen was also subjected to very high PPFD when active, often more than 2,500 μmol photon m−2 s−1. The major environmental stress appeared to be high light rather than low temperatures, and the variability of early season snow fall means that prediction of activity will be very difficult.