Photosynthetic Photon Flux Density Levels Research Articles

Photosynthesis of Chinese cabbage and radish in response to rising leaf temperature during spring

Photosynthesis in leaves, exposed to temperature change, was measured during spring to provide information for modeling of cropping systems for Chinese cabbage (Brassica campestris subsp. napus var. pekinensis) and radish (Raphanus sativus) in Jeju Island. The net photosynthetic rates (A) of Chinese cabbage and radish were depressed at temperatures above approximately 25°C (corresponding to the optimum temperature for photosynthesis), due in part to high rates of respiration above 25 or 30°C. Chinese cabbage was more sensitive to high temperature than radish. Above 25°C, stomatal conductance (g s) declined steeply with a sharp increase in the vapor pressure deficit (VPD) and transpiration rate (E), suggesting that stomatal closure could not be associated with transpiration rate, particularly in Chinese cabbage. Dramatic declines in water use efficiency (WUE), instantaneous transpiration efficiency (ITE), and carboxylation efficiency (CE) above 30°C indicated that the plants could be severely affected negatively by high temperatures above 30°C. However, compared to Chinese cabbage, radish had high light use efficiency and potential to assimilate CO2, judged by their high quantum yield (φ) and maximum photosynthetic rate (A max). Radish utilized high levels of photosynthetic photon flux and strongly acclimated to the light environment, considering their high light saturation point (Q sat) and low light compensation point (Q comp). These results suggested that rising temperature might substantially alter water availability of plants by itself and/or through effects on the rate of water loss from leaves of Chinese cabbage and radish under diurnal and/or seasonal fluctuations of temperature, and/or global temperature change. To maintain the high productivity and quality of these crop species, it may be necessary to harvest them until early June, when the daily maximum temperature is below 25°C in Jeju Island.

Effects of photosynthetic photon flux and carbon dioxide concentration on the photosynthesis and growth of grafted pepper transplants during healing and acclimatization

In the production of grafted transplants, healing and acclimatization are the most critical processes for survival. We investigated the influence of the photosynthetic photon flux (PPF) and the carbon dioxide (CO2) concentration during healing and acclimatization on the photosynthetic characteristics and growth of grafted pepper transplants to determine the optimum environmental conditions for healing and acclimatization in a healing chamber with artificial lighting source. Grafted pepper transplants were healed and acclimatized under two levels of CO2 (374 or 1,013 μmol·mol−1) and four levels of PPF (dark, 50, 98 or 147 μmol·m−2·s−1) for six days. The CO2 exchange rates of the grafted pepper transplants significantly increased with increasing PPF during healing and acclimatization. The CO2 exchange rates were higher under elevated CO2 concentrations than ambient CO2 concentration. The effect of CO2 enrichment was greater in low light intensity. The CO2 exchange rates at 50, 98 or 147 μmol·m−2·s−1 under elevated CO2 concentrations were 511, 261, and 172%, respectively, compared to the ambient CO2 concentrations. The increase of photosynthesis led to an improvement in growth. The SPAD value, dry weight and leaf area were greater under higher PPF and CO2 concentrations. PPF also influenced the anatomical structures of the leaves, and the palisade and spongy tissue cells of the leaves irradiated with higher PPF were aligned more densely, with more chloroplasts and small empty space. When compared to the tunnel in the greenhouse with natural light, healing and acclimatization under high CO2 (1,000 μmol·mol−1) and PPF (150 μmol·m−2·s−1) conditions in the healing chamber promoted the growth and graft union formation of grafted pepper transplants. The results suggested that high-quality grafted pepper seedlings could be achieved by healing and acclimatization in a healing chamber where optimal conditions such as high PPF and CO2 were maintained within the range evaluated in this experiment.

Effects of Light Quality and Intensity on the Carbon Dioxide Exchange Rate, Growth, and Morphogenesis of Grafted Pepper Transplants during Healing and Acclimatization

This study evaluated the influence of light quality and intensity during healing and acclimatization on the CO₂ exchange rate, growth, and morphogenesis of grafted pepper (Capsicum annuum L.) transplants, using a system for the continuous measurement of the CO₂ exchange rate. C. annuum L. ‘Nokkwang’ and ‘Tantan’ were used as scions and rootstocks, respectively. Before grafting, the transplants were grown for four weeks in a growth chamber with artificial light, where the temperature was set at 25/18℃ (light/dark period) and the light period was 14 hours·d -1 . The grafted pepper transplants were then healed and acclimatized under different light quality conditions using fluorescent lamps (control) and red, blue, and red + blue light-emitting diodes (LEDs). All the transplants were irradiated for 12 hours per day, for six days, at a photosynthetic photon flux (PPF) of 50, 100, or 180 μmol·m -2 ·s -1 . The higher PPF levels increased the CO₂ exchange rate during the healing and acclimatization. A smaller increase in the CO₂ exchange rates was observed in the transplants under red LEDs. At a PPF of 180 μmol·m -2 ·s -1 , the CO₂ exchange rate of the transplants irradiated with red LEDs was lowest and it was 37% lower than those irradiated with fluorescent lamps. The CO₂ exchange rates of transplants irradiated with blue LEDs was the highest and 20% higher than those irradiated under fluorescent lamps. The graft take was not affected by the light quality. The grafted pepper transplants irradiated with red LEDs had a lower SPAD value, leaf dry weight, and dry matter content. The transplants irradiated with blue LEDs had longer shoot length and heavier stem fresh weight than those irradiated with the other treatments. Leaves irradiated with the red LED had the smallest leaf area and showed leaf epinasty. In addition, the palisade and spongy cells of the pepper leaves were dysplastic and exhibited hyperplasia. Grafted pepper transplants treated with red + blue LEDs showed similar growth and morphology to those transplants irradiated with fluorescent lamps. These results suggest that high-quality grafted pepper transplants can be obtained by healing and acclimatization under a combination of blue and red lights at a high PPF level.

Growth, leaf photosynthesis and canopy light use efficiency under differing irradiance and soil N supplies in the forage grass Brachiaria decumbens Stapf

AbstractIrradiance and soil nitrogen effects on growth, net photosynthesis and radiation use efficiency (RUE) of Brachiaria decumbens were investigated in fertilized and non‐fertilized stands. Three levels of photosynthetic photon flux (PPF: S0 = 100%, S1 = 50% and S2 = 30%) and two N supplies, with (N+) and without (N−), were used. Forage biomass and nutrient accumulation, specific leaf area (SLA), leaf area index (LAI), fractional intercepted photosynthetic photon flux (fPPF), leaf photosynthetic response to light and efficiency of radiation use at leaf (A/Q) and canopy (RUE) levels were measured. Shade effects were mostly independent of soil N. Final yield was decreased by 34% (S1) and 57% (S2). Shade increased SLA (25–46%), so maximum LAI (2·4–3·3) was similar among light regimes. In N− stands, reductions in leaf biomass (14%), SLA (17%) and LAI (27%) were recorded, although forage yield was similar between soil N conditions. Under shade, peaks of A were comparable to those at full light, so A/Q was higher around midday. Derived parameters of the A‐PPF curves were similar between S0 and S2. A maximum fPPF = 0·8 (S0N+, S1N+) was recorded at LAI = 3–4. Under limited sunlight, relatively high RUE (1·6–2·8 g MJ−1) were observed over both soil N conditions. We concluded that B. decumbens had a high plasticity to shade, thus explaining its success under silvopastoral systems.

Using combined measurements for comparison of light induction of stomatal conductance, electron transport rate and CO2 fixation in woody and fern species adapted to different light regimes

We aimed to understand the relation of photosynthetic rate (A) with g(s) and electron transport rate (ETR) in species of great taxonomic range and light adaptation capability during photosynthetic light induction. We studied three woody species (Alnus formosana, Ardisia crenata and Ardisia cornudentata) and four fern species (Pyrrosia lingus, Asplenium antiquum, Diplazium donianum and Archangiopteris somai) with different light adaptation capabilities. Pot-grown materials received 100 and/or 10% sunlight according to their light adaptation capabilities. At least 4 months after light acclimation, CO(2) and H(2)O exchange and chlorophyll fluorescence were measured simultaneously by equipment in the laboratory. In plants adapted or acclimated to low light, dark-adapted leaves exposed to 500 or 2000 µmol m(-2) s(-1) photosynthetic photon flux (PPF) for 30 min showed low gross photosynthetic rate (P(g)) and short time required to reach 90% of maximum P(g) (). At the initiation of illumination, two broad-leaved understory shrubs and the four ferns, especially ferns adapted to heavy shade, showed higher stomatal conductance (g(s)) than pioneer tree species; materials with higher g(s) had short at both 500 and 2000 µmol m(-2) s(-1) PPF. With 500 or 2000 µmol m(-2) s(-1) PPF, the g(s) for the three woody species increased from 2 to 30 min after the start of illumination, but little change in the g(s) of the four ferns. Thus, P(g) and g(s) were not correlated for all material measured at the same PPF and induction time. However, P(g) was positively correlated with ETR, even though CO(2) assimilation may be influenced by stomatal, biochemical and photoinhibitory limitations. In addition, was closely related to time required to reach 90% maximal ETR for all materials and with two levels of PPF combined. Thus, ETR is a good indicator for estimating the light induction of photosynthetic rate of species, across a wide taxonomic range and light adaptation and acclimation capability.

Using combined measurements of gas exchange and chlorophyll fluorescence to investigate the photosynthetic light responses of plant species adapted to different light regimes

One broad-leaved pioneer tree, Alnus formosana, two broad-leaved understory shrubs, Ardisia crenata and Ardisia cornudentata, and four ferns with different light adaptation capabilities (ranked from high to low, Pyrrosia lingus, Asplenium antiquum, Diplazium donianum, Archangiopteris somai) were used to elucidate the light responses of photosynthetic rate and electron transport rate (ETR). Pot-grown materials received up to 3 levels of light intensity, i.e., 100%, 50% and 10% sunlight. Both gas exchange and chlorophyll (Chl) fluorescence were measured simultaneously by an equipment under constant temperature and 7 levels (0-2,000 μmol m-2 s-1) of photosynthetic photon flux density (PPFD). Plants adapted to-or acclimated to high light always had higher light-saturation point and maximal photosynthetic rate. Even materials had a broad range of photosynthetic capacity [maximal photosynthetic rate ranging from 2 to 23 μmol(CO2) m-2 s-1], the ratio of ETR to gross photosynthetic rate (P G) was close for A. formosana and the 4 fern species when measured under constant temperature, but the PPFD varied. In addition, P. lingus and A. formosana grown under 100% sunlight and measured at different seasonal temperatures (15, 20, 25, and 30°C) showed increased ETR/P G ratio with increasing temperature and could be fitted by first- and second-order equations, respectively. With this equation, estimated and measured P G were closely correlated (r 2 = 0.916 and r 2 = 0.964 for P. lingus and A. formosana, respectively, p<0.001). These equations contain only the 2 easily obtained dynamic indicators, ETR and leaf temperature. Therefore, for some species with near ETR/P G ratio in differential levels of PPFD, these equations could be used to simulate dynamic variation of leaf scale photosynthetic rate under different temperature and PPFD conditions.

Quantification of Carbon Assimilation of Plants in Simulated and In Situ Interiorscapes

Interiorscape plants have many documented benefits, but their potential for carbon sequestration is not clear. This study was undertaken to quantify the amount of carbon assimilation under growth chamber conditions designed to mimic the photosynthetic photon flux (PPF) levels and temperatures of typical indoor environments and to quantify the amount of carbon assimilation in situ in a representative interiorscape composed of a variety of plant species and sizes. Quantitative data were obtained in 1) growth chambers with a typical range of PPF levels encountered indoors (≈10, 20, and 30 μmol·m−2·s−1); and 2) in situ conditions in an interiorscape. Under growth chamber conditions, most species exhibited positive dry mass accumulation and carbon sequestration but Sanseveria and Dracaena ‘Janet Craig’ exhibited consistent dry mass loss throughout the 10 weeks under simulated conditions. Carbon content was lower in herbaceous species (e.g., Scindapsus aureus, 38% of dry mass) compared with woody ones (e.g., Ficus benjamina, 43%). PPF-saturated net photosynthetic rates of plants were low, ranging from 3.4 to 7.0 μmol·m−2·s−1, whereas their light compensation points ranged from 8 to 78 μmol·m−2·s−1. In situ, plants exhibited varying dry mass gain, largely dependent on size. In general, a large plant and/or species with a higher amount of woody tissue in their above- or belowground organs (e.g., 4.6 m high arboreal plant) sequestered more carbon than small and/or herbaceous species. This study is the first to provide quantitative data of carbon sequestration in interiorscape environments.

Photosynthetic characteristics of ornamental passion flowers grown under different light intensities

Responses of leaf gas exchange, fluorescence emission, chlorophyll concentration, and morpho-anatomical features to changes in photosynthetic photon flux density (PPFD) were studied in three wild ornamental species of Passiflora L. to select sun and shade species for landscaping projects. Artificial shade was obtained with different shading nylon nets, under field conditions, which allowed the reduction of 25, 50, and 75% of global radiation, along with a control treatment under full sunlight. For Passiflora morifolia the highest mean values of light-saturated net photosynthetic rate (P Nmax) and light compensation point (LCP) were observed at 50 and 25% shade, respectively, while the highest values of dark respiration rate (R D) and apparent quantum yield (α) were observed at 75% shade. For Passiflora suberosa litoralis the highest value of P max was observed at full sunlight. The highest mean values for P max, R D, and LCP for Passiflora palmeri var. sublanceolata were obtained at 25% shade. The highest values of net photosynthetic rate (P N) for P. morifolia, P. palmeri var. sublanceolata, and P. suberosa litoralis were 21.09, 16.15, and 12.36 μmol(CO2) m−2 s−1, observed at 50 and 75% shade and full sunlight, respectively. The values of the minimal chlorophyll fluorescence (F0) were significantly different in P. suberosa litoralis and P. palmeri var. sublanceolata, increasing with the increase of the irradiance. In contrast, the values of maximum photochemical efficiency of PSII (Fv/Fm) were significantly different only in P. suberosa litoralis, being higher at 75%, progressively reducing with the increase of PPFD levels. The total concentration of chlorophyll (Chl) was higher in shaded plants than in the ones cultivated in full sunlight. On the other hand, the values of Chl a/b ratio were reduced in shaded plants. A significant effect of shade levels on leaf area (LA) and specific leaf area (SLA) was found for the three species, whose highest mean values were observed at 75% shade. The thickness of foliar tissues was significantly higher for the three species at full sunlight and 25% shade. These results suggested that P. morifolia and P. palmeri var. sublanceolata appeared to be adapted to moderate shade conditions. P. suberosa litoralis presented higher plasticity to greater variation of the irradiance levels, while the photoinhibition was one of the limiting factors for this species at full sunlight.

Relationships between photosystem II efficiency and photochemical reflectance index under different levels of illumination: comparison among species grown at high- and low elevations through different seasons

Previously, we found a significant association between photosystem II efficiency (ΦPSII) and photochemical reflectance index (PRI) measured at predawn among different species at different elevations and throughout several seasons. However, this relationship has not been evaluated under varied levels of illumination. Here, we used the Taiwan species Pinus taiwanensis (a conifer distributed at 750–3,000 m a.s.l.), Stranvaesia niitakayamensis (an evergreen tree, 1,700–3,100 m) and two Miscanthus spp. (perennial C4 Gramineae, coastline–3,200 m) to elucidate the ΦPSII–PRI relationship. We studied six levels of photosynthetic photon flux density (PPFD) (0, 200, 400, 800, 1,200 and 2,000 μmol m−2 s−1) over several growth seasons at high (2,600 m a.s.l.) and low (800 m a.s.l.) elevation sites. In comparing the same species or genus, ΦPSII and PRI were closely correlated in darkness or under the same level of PPFD, with data obtained from different seasons and elevations pooled for regression analysis. Because both the intercept and slope of the ΦPSII–PRI equation showed a negative curvilinear correlation with PPFD, we could fit an empirical regression model, ΦPSII = c + d·ln(PPFD) + e·[ln(PPFD)]2 + f·PRI + g·PRI·ln(PPFD) + h·PRI·[ln(PPFD)]2, for multiple regression analysis. Using this model, we found a close correlation between the estimated and measured ΦPSII (r 2 = 0.842−0.937, P < 0.001) for all four species examined and for mango (Mangifera indica) measured under both artificial illumination and sunlight (data from Weng et al. 2010). This empirical regression model could simulate both seasonal and diurnal variations of leaf-scale photosynthetic efficiency at high and low elevations.

Influence of seedling treatment of calcium on some enzyme activity in walnut under salinity

Levels of peroxidase (POD) and catalase (CAT) enzyme activity were determined in 2 years-old walnut plants which were grown in environmental conditions characterized by high temperatures. It’s also high photosynthetic photon flux density levels and subjected to a different salinity levels for 60 days period. The present study, through an experimental campaign, was carried out on both normally irrigated and salt-stressed plants with CaCl2 in order to comprehend the plant response to salt stress and its ability to recover. Leaf samples were collected and analyzed for POD and CAT enzymes activities before the experimental period. The levels of CAT and POD activity increased in parallel with the severity of salt levels in leaves of tolerant genotypes walnut especially the plants which treated by Ca+2. This increase extended, after 6 days of salt stress usage. However, in all the treatments, POD and CAT activity was reduced significantly from 6 to 10 days after beginning the salt stress. The increase of antioxidant enzymes activity indicates its potential role in salt tolerant salt in promising walnut genotypes. The amount of antioxidant compounds increases in plant during the salt stress. Key words: Enzyme activity, peroxidase, catalase, salinity, walnut.

Photosynthesis of Lolium perenne L. at low temperatures under low irradiances

To develop mechanistic models for winter survival of grasses under climate change, more knowledge is needed of photosynthetic activity at low irradiance and temperature. Photosynthetic activity of small stands of Lolium perenne was continuously studied for up to two weeks under low photosynthetic photon flux density (PPFD) in the air temperature range from −3 °C to +9 °C. The photosynthetic rate of plants growing at 2, 6 and 9 °C was similar at 120 μmol m −2 s −1 PPFD but the rate increased with increasing PPFD at all temperatures, particularly 9 °C. Light saturation of photosynthesis was reached at approx. 300 μmol m −2 s −1 at 2 and 6 °C, and at approx. 600 μmol m −2 s −1 at 9 °C. At 300 μmol m −2 s −1, the CO 2 exchange rate (CER) at 2 °C was approx. 60% of that at 9 °C. When the temperature increased gradually from 0 °C at start to +5 °C at the end of the 8-h photoperiod, CER decreased by about 20% compared with that at constant 5 °C. Changing the temperature from constant +5 °C to a diurnal variation between −2 and +2 °C gradually decreased daytime CER to approx. 10% after five days, partly due to leaf area losses (∼50% loss) when the poorly acclimatised plants were exposed to frost. At start of the photoperiod at −2 °C CER was negative, but became slightly positive when temperature exceeded −1 °C after two hours. Total daily photosynthesis was negative due to night-time respiration in this treatment. Soil heating to avoid freezing when the diurnal air temperature fluctuated between −3 and +3 °C had no effect on CER at this low PPFD level (150 μmol m −2 s −1). In contrast to the −2/+2 °C treatment, total daily photosynthesis was slightly positive in the −3/+3 °C treatment, where the plants were better acclimatised to frost. Increasing the CO 2 concentration from 350 to 600 μmol mol −1 had no effect on CER at 2 and 6 °C, but increased it by 20% at 9 °C. The data indicate that the minimum temperature for photosynthesis in the cultivar studied is about −4 °C. The results can be applied in different photosynthesis models.

Light Intensity Affects Growth, Photosynthetic Capability, and Total Flavonoid Accumulation of Anoectochilus Plants

Anoectochilus formosanus, a medicinal plant used to treat hypertension, lung disease, and liver disease, was grown to maximize biomass and secondary metabolite production in a controlled environment under four levels of photosynthetic photon flux (PPF), namely, 10, 30, 60, or 90 μmol·m−2·s−1, that is L10, L30, L60, and L90 treatments, respectively. On Day 45, all growth values were greatest for the L30 plants. Dry weight was lowest for the L10 plants. Leaf area, stem length, and fresh weight were lowest for the L90 plants. The chlorophyll concentration was highest in the L10 treatment and decreased with increasing PPF. Electron transport ratios of leaves were highest in the L30 treatment and lowest in the L90 for the second leaf (counted down from the apex) and in the L10 for the third leaf. An increase in light intensity from 10 to 60 μmol·m−2·s−1 increased the superoxide dismutase activity and was associated with an increase in the total flavonoid concentration. The total flavonoid concentration (mg·g−1 DW) was greatest in the L60 and lowest in the L90. However, the total flavonoid content (mg/plant) was highest in the L30 plants as a result of great biomass. The results indicated that A. formosanus is a typical shade plant suitable to grow under low light intensity at PPF of 30 to 50 μmol·m−2·s−1 for both growth and production of total flavonoid. A light intensity of 90 μmol·m−2·s−1 induced stress on plant growth and reduced photosynthetic capability and the flavonoid accumulation.

PEROXIDASE, GUAIACOL PEROXIDASE AND ASCORBATE PEROXIDASE ACTIVITY ACCUMULATION IN LEAVES AND ROOTS OF WALNUT TREES IN RESPONSE TO DROUGHT STRESS

Peroxidase isozymes (POX, POD and APX) activity were determined in 1-year-old walnut plants grown in environmental conditions characterized by high temperatures and high photosynthetic photon flux density levels and subjected to withholding irrigation for a 20-day period. These mechanisms have been investigated over both irrigated and drought-stressed plants in order to investigate the plant response under stressed conditions and its ability to recover. Before and during the experimental period, leaf and root samples were collected and their POD was analyzed. Significant increases of POD especially APX isozyme activity were also observed during the progressive increment of drought stress in both leaf and root tissues of tolerant genotypes at the seventh day after the beginning of the drought period. The differences in drought tolerance may be closely related with efficient photo protective systems as well as the increased capacity of the anti oxidative system to scavenge reactive oxygen species.

Measurement of Leaf Vapor Conductance of Cucumber Transplants in the Greenhouse with Minimal Invasion

We estimated leaf vapor conductance (gl) of cucumber grafted transplants under greenhouse growing conditions. Fifty-six transplants were placed on a bench in the greenhouse. The transpiration rate (Tr) of the canopy was estimated by weighing the 16 transplants in the center using an electronic balance. The total vapor diffusion resistance (Rl+b) from inside the leaf to the atmosphere was estimated based on the vapor diffusion model, which incorporates the absolute humidity near the leaf surface and that inside the leaf as well as Tr. Next, gl was estimated from Rl+b and the resistance of leaf boundary layer evaluated with a model leaf. The Tr in the afternoon tended to be larger than that in the morning at the same photosynthetic photon flux (PPF) level. By contrast, the gl in the afternoon tended to be smaller than that in the morning at the same PPF level. The decrease of gl in the afternoon seems to be induced by the excessive transpiration resulting from an increase of vapor pressure deficit at the leaf surface.

Effects of Simulated Dark Shipping on Photosynthetic Status and Post-shipping Performance in Phalaenopsis Sogo Yukidian ‘V3’

Phalaenopsis plants are routinely shipped long distances in total darkness. To determine how these long dark periods affect photosynthetic status in Phalaenopsis Sogo Yukidian ‘V3’, changes of net CO2 uptake, photosystem II (PS II) efficiency, and abscisic acid (ABA) concentration after a long-term simulated dark shipping were investigated. Net CO2 uptake rate, malate concentration, and titratable acidity in potted Phalaenopsis Sogo Yukidian ‘V3’ decreased after a 21-day simulated dark shipping at 20 °C, but recovered gradually with time after shipping. It took 6 to 9 days to recover to a normal photosynthetic status after shipping. The value of Fv/Fm was little affected by shipping. Therefore, net CO2 uptake rate would be a better indicator for estimating the recovery time after shipping. After shipping, fresh weight loss, leaf ABA concentration, and number of yellowed leaves of bare-root plants were higher than those of potted plants, and increased with longer durations (7, 14, and 21 days) of the simulated dark period. The spiking (the emergence of flowering stems) date was delayed when plants were stored in a bare-root condition. The concentration of ABA in leaves rose in the first 3 days after simulated shipping and then decreased within the next 3 to 8 days. Plants that received photosynthetic photon flux (PPF) at 399 μmol·m−2·s−1 after shipping had lower PS II efficiency and reduced net CO2 uptake rate than those given less PPF levels. We recommend a post-shipping acclimation for 6 to 9 days with gradual light increase (34–72–140–200 μmol·m−2·s−1 PPF) or maintaining a light level of 140 μmol·m−2·s−1 PPF for Phalaenopsis to achieve a better photosynthetic status after prolonged dark storage.

Deschampsia antarctica Desv. primary photochemistry performs differently in plants grown in the field and laboratory

Primary photochemistry of photosystem II (Fv/Fm) of the Antarctic hair grass Deschampsia antarctica growing in the field (Robert Island, Maritime Antarctic) and in the laboratory was studied. Laboratory plants were grown at a photosynthetic photon flux density (PPFD) of 180 μmol m−2 s−1 and an optimal temperature (13 ± 1.5°C) for net photosynthesis. Subsequently, two groups of plants were exposed to low temperature (4 ± 1.5°C day/night) under two levels of PPFD (180 and 800 μmol m−2 s−1) and a control group was kept at 13 ± 1.5°C and PPFD of 800 μmol m−2 s−1. Chlorophyll fluorescence was measured during several days in field plants and weekly in the laboratory plants. Statistically significant differences were found in Fv/Fm (=0.75–0.83), F0 and Fm values of field plants over the measurement period between days with contrasting irradiances and temperature levels, suggesting that plants in the field show high photosynthetic efficiency. Laboratory plants under controlled conditions and exposed to low temperature under two light conditions showed significantly lower Fv/Fm and Fm. Moreover, they presented significantly less chlorophyll and carotenoid content than field plants. The differences in the performance of the photosynthetic apparatus between field- and laboratory-grown plants indicate that measurements performed in ex situ plants should be interpreted with caution.

Eight years of carbon dioxide exchange above a mixed forest at Borden, Ontario

This paper summarizes results from 8 years (1996–2003) of eddy covariance-based ecosystem CO 2 exchange measurements at the Borden Forest Research Station (44°19′N, 79°56′W). The site represents a mid-latitude, 100-year-old, mixed deciduous and coniferous forest dominated by red maple, aspen and white pine. The years 1996 and 1997 were relatively cold, had a late spring and received below average photosynthetic photon flux density (PPFD). This contrasts with an early spring, warmer soil and air temperatures during 1998–1999, and with distinctly wet year of 2000 and dry years of 2001–2003. The combination of early spring, warmer air and soil temperature and relatively high level of PPFD was associated with higher net ecosystem productivity (NEP) that peaked during 1999. Photosynthetic capacity was reduced and NEP showed a mid-growing season depression during the dry years of 2001–2003. Annual average ecosystem respiration (R) determined from a light response model was ∼30% less than R derived from a logistic respiration equation, relating night time CO 2 flux and soil temperature. However these independently determined R values were well correlated indicating that the site is unaffected by fetch and spatial heterogeneity problems. Based on the combined 8 years of growing season daytime data, an air temperature of 20–25 °C and a vapor pressure deficit (VPD) of 1.3 kPa were found to be the optimal conditions for CO 2 uptake by the canopy. Over the 1996–2003 period, the forest sequestered carbon at an average rate of 140 ± 111 gC m −2 y −1. The corresponding gross ecosystem photosynthesis (GEP) and R over this period were 1116 ± 93 gC m −2 y −1 and 976 ± 68 gC m −2 y −1, respectively. The annual carbon sequestration ranged from 19 gC m −2 in 1996 to 281 gC m −2 in 1999. However, these estimates were sensitive to frictional velocity threshold ( u * TH ) used for screening data associated with poor turbulent mixing at night. Increasing u * TH from 0.2 m s −1 (based on the inflection point in the nighttime CO 2 flux vs. u * relationship) to 0.35 m s −1 (determined using a u * TH selection algorithm based on change-point detection) modified the 8-year mean NEP estimate from 140 ± 111 gC m −2 y −1 to 65 ± 120 gC m −2 y −1. Both approaches show that the Borden forest was a low to moderate sink of carbon over the 8-year period.

Relationship between allocation of absorbed light energy in PSII and photosynthetic rates of C3 and C4 plants

Two C3 dicotyledonous crops and five C4 monocotyledons treated with three levels of nitrogen were used to evaluate quantitatively the relationship between the allocation of absorbed light energy in PSII and photosynthetic rates (PN) in a warm condition (25–26°C) at four to five levels [200, 400, 800, 1,200 (both C3 and C4) and 2,000 (C4 only) μmol m−2 s−1] of photosynthetic photon flux density (PPFD). For plants of the same type (C3 or C4), there was a linear positive correlation between the fraction of absorbed light energy that was utilized in PSII photochemistry (P) and PN, regardless of the broad range of their photosynthetic rates due to species-specific effect and/or nitrogen application; meanwhile, the fraction of absorbed light energy that was dissipated through non-photochemical quenching (D) showed a negative linear regression with PN for each level of PPFD. The intercept of regression lines between P and PN of C3 and C4 plants decreased, and that between D and PN increased with increasing PPFD. With P and D as the main components of energy dissipation and complementary to each other, the fraction of excess absorbed light energy (E) was unchanged by PN under the same level of PPFD. At the same level of PN, C4 plants had lower P and higher D than C3 plants, due to the fact that C4 plants with little or no photorespiration is considered a limited energy sink for electrons. Nevertheless there was a significant negative linear correlation between D and P when data from both C3 and C4 plants at varied PPFD levels was merged. The slope of regression lines between P and D was 0.85, indicating that in plants of both types, most of the unnecessary absorbed energy (ca. 85%) could dissipate through non-photochemical quenching, when P was inhibited by low PN due to species-specific effect and nitrogen limitation at all levels of illumination used in the experiment.

Growth and photosynthesis of Chinese cabbage plants grown under light-emitting diode-based light source

We compared growth and the content of sugar, protein, and photosynthetic pigments, as well as chlorophyll fluorescence parameters in 15- and 27-day-old Chinese cabbage (Brassica chinensis L.) plants grown under a high-pressure sodium (HPS) lamps or a light source built on the basis of red (650 nm) and blue (470 nm) light-emitting diodes (LEDs) with a red to blue photon ratio of 7: 1. One group of plants was grown at a photosynthetic photon flux (PPF) level of 391 ± 24 μ mol/(m2 s) (normal level); the other, at a PPF level of 107 ± 9 μ mol/(m2 s) (low light). Plants of the third group were firstly grown at the low light and then (on the 12th day) transferred to the normal level. When grown at the normal PPF level, the plants grown under LEDs didn’t differ from plants grown under HPS lamps in shoot fresh weight, but they showed a lower root fresh and dry weights and the lower content of total sugar and sugar reserves in the leaves. No differences in the pigment content and photosystem II quantum yield were found; however, a higher Chl a/b ratio in plants grown under LEDs indicates a different proportion of functional complexes in thylakoid membranes. The response to low light conditions was mostly the same in plants grown under HPS lamps and LEDs; however, LED plants showed a lower growth rate and a higher nonphotochemical fluorescence quenching. In the case of the altered PPF level during growth, the plant photosynthetic apparatus adapted to new conditions of illumination within three days. Plants grown under HPS lamps at a constant normal PPF level and those transferred to the normal PPF level on the 12th day, on the 27th day didn’t differ in shoot fresh weight, but in plants grown under LEDs, the differences were considerable. Our results show that LED-based light sources can be used for plant growing. At the same time, some specific properties of plant photosynthesis and growth under these conditions of illumination were found.

Impact of Phakopsora pachyrhizi Infection on Soybean Leaf Photosynthesis and Radiation Absorption

Yield loss in soybean [Glycine max (L.) Merr.] due to soybean rust (SBR) (caused by Phakospora pachyrhizi Syd. &amp; P. Syd.) has been associated with reduced radiation interception and reduced radiation use efficiency by green leaf area. The objective of this study was to determine the mechanisms involved in the SBR‐induced reductions in radiation use efficiency by quantifying the effect of SBR on (i) absorption of photosynthetic photon flux density (PPFD) and (ii) carbon exchange rate (CER) per unit absorbed PPFD of soybean leaves. A controlled‐environment study was conducted using a split‐plot design with three replications over time, with two disease levels as main plots and two soybean cultivars as subplots. The two disease levels were a disease‐free control and inoculation with P. pachyrhizi. Measurements included leaf CER, PPFD absorptance, and chlorophyll fluorescence. As disease severity increased, there was a small linear decline in leaf absorptance and a negative exponential decline in leaf CER. At 50% disease severity, absorptance was reduced by 10% and CER was reduced by 68% compared with the disease‐free control. The relative decline in CER was similar at low and saturating PPFD levels. Soybean rust–induced reductions in CER were mainly associated with lower efficiency of photosystem II (PSII) photochemistry and damage to PSII reaction centers. Therefore, visual indicators such as light absorptance and disease severity do not reflect the degree of impact of SBR on leaf photosynthesis.