Measurements Of Photosynthetic Photon Flux Density Research Articles

Measuring method for photosynthetic photon flux density using a digital camera

The paper presents a method for measuring photosynthetic photon flux density (PPFD) using a commercially available digital camera. PPFD is one of the critically important measurements to control photosynthetic reaction of plants. Conventionally a dedicated quantum sensor is used to measure PPFD. A quantum sensor, however, can measure PPFD of a target point at once. Thus considerable cost and labor would be consumed if the quantum sensors were introduced for managing photosynthetic reaction of numerous plants in a large greenhouse for growing commercial crops. In order to solve the problem, we propose a method for measuring PPFD using a commercially available digital camera. The method estimates spectral irradiance of daylight, fluctuating hour by hour according to weather condition changes, on the basis of the color information of a reference object in an image captured by a digital camera. Then PPFD is derived from the estimated spectral irradiance. For practical use, plural reference objects are placed in a plant community and captured as an image. PPFDs at the positions of the reference objects in the captured image are simultaneously measured. We believe that proposed method is useful in real cultivation. We conducted experiments in outdoor environment. During the experiments, a Nikon digital SLR camera D300 and a white plaster ball as a reference object were used. We finally confirmed that the average error rate of PPFD measurements by this method was 2.58%, and the maximum was within 5% which is equivalent to the accuracy of the quantum sensor on the market.

Photosynthetic Photon Flux Density Distribution Inside Photovoltaic Greenhouses, Numerical Simulation, and Experimental Results

<abstract> <b><i>Abstract. </i></b> A methodology for estimating the photosynthetic photon flux density (PPFD) distribution inside a photovoltaic greenhouse using a combination of a commercial lighting simulation software and available meteorological data was investigated in this study. Numerical results were compared with those measured inside an experimental greenhouse partially covered with photovoltaic panels. The PPFD distribution inside the greenhouse was calculated on the basis of outside measurements of PPFD at the site of the experimental greenhouse and data acquired at a nearby meteorological station. Daily average percent differences were in all cases lower than 19.0%. Assuming that such differences are acceptable for agronomic purposes, on a monthly basis the numerical model demonstrated a good capability to predict the shading effect inside the photovoltaic greenhouse. Hourly analysis showed higher fluctuation of measured versus calculated data, meaning that the model was able to predict the qualitative behavior of the distribution and its average value but underestimated the fluctuation of values.

Functional relationships to estimate Morphogenetically Active Radiation (MAR) from PAR and solar broadband irradiance measurements: The case of a sorghum crop

Light quality plays a key role in higher plants’ morphogenesis. In most plant models, light is considered as a consumable resource and plants are assumed blind to light signals. However, prior to any effort for modelling photomorphogenetic mechanisms, it is necessary to characterise the spatial distribution of the Morphogenetically Active Radiation ( MAR) over and within plant canopies. Measurements of local photosynthetic photon flux density ( PPFD) and broadband irradiance ( Es) are easy to carry out by using small sensors. Thus, the distribution of the MAR within a canopy can be estimated whenever the functional relationships between these measurements and the photon flux within any spectral band are known. The objective of this work was to determine these functional relationships from the light spectra received above and at various positions around a target plant within a growing sorghum crop. The MAR components considered in this work are related either to photon flux densities in various wavebands between 330 and 950 nm or to the ratio between two photon flux densities. A part of the photon flux-related variables is strictly included in the PAR band and might be estimated from PPFD measurements using linear relationships. The other variables are related to both PPFD and Es by multiple linear relationships. The phytochrome photoequilibrium and the red to far-red ratios were related to the relative transmitted PPFD and to the ratio PPFD/ Es within the canopy using a non-linear model. Models were validated against an independent set of data. We demonstrate that the MAR components within a sorghum crop can be accurately estimated with the functional relationships presented in this paper.

Optimal photosynthetic use of light by tropical tree crowns achieved by adjustment of individual leaf angles and nitrogen content.

Theory for optimal allocation of foliar nitrogen (ONA) predicts that both nitrogen concentration and photosynthetic capacity will scale linearly with gradients of insolation within plant canopies. ONA is expected to allow plants to efficiently use both light and nitrogen. However, empirical data generally do not exhibit perfect ONA, and light-use optimization per se is little explored. The aim was to examine to what degree partitioning of nitrogen or light is optimized in the crowns of three tropical canopy tree species. Instantaneous photosynthetic photon flux density (PPFD) incident on the adaxial surface of individual leaves was measured along vertical PPFD gradients in tree canopies at a frequency of 0.5 Hz over 9-17 d, and summed to obtain the average daily integral of PPFD for each leaf to characterize its insolation regime. Also measured were leaf N per area (N(area)), leaf mass per area (LMA), the cosine of leaf inclination and the parameters of the photosynthetic light response curve [photosynthetic capacity (A(max)), dark respiration (R(d)), apparent quantum yield (phi) and curvature (theta)]. The instantaneous PPFD measurements and light response curves were used to estimate leaf daily photosynthesis (A(daily)) for each leaf. Leaf N(area) and A(max) changed as a hyperbolic asymptotic function of the PPFD regime, not the linear relationship predicted by ONA. Despite this suboptimal nitrogen partitioning among leaves, A(daily) did increase linearly with PPFD regime through co-ordinated adjustments in both leaf angle and physiology along canopy gradients in insolation, exhibiting a strong convergence among the three species. The results suggest that canopy tree leaves in this tropical forest optimize photosynthetic use of PPFD rather than N per se. Tropical tree canopies then can be considered simple 'big-leaves' in which all constituent 'small leaves' use PPFD with the same photosynthetic efficiency.

Estimation of absorbed PAR across Scandinavia from satellite measurements: Part I: Incident PAR

The primary productivity of a plant community can be modeled as the product of the amount of photosynthetically active radiation (PAR) absorbed by the canopy and a light use efficiency factor, where the amount of absorbed PAR (APAR) is the product of the fractional absorption and the amount of incident PAR. By implementing a method, PARcalc, using atmospheric data from the Moderate Resolution Imaging Spectroradiometer (MODIS), incident PAR is estimated in this study. In addition, since many PAR datasets are generated by converting shortwave radiation into PAR, the ratio of PAR to shortwave radiation was also investigated. PARcalc models the photosynthetic photon flux density (PPFD) as a product of atmospheric transmittance, the cosine of the Sun zenith angle, and the solar constant. The atmospheric transmittance includes the attenuation of radiation by Rayleigh and aerosol scattering, and absorption by water and ozone. A cloud transmittance factor which is primarily a function of the cloud optical thickness is added in order to cope with cloudy conditions. The model was implemented at two sites in Sweden, Asa and Norunda, where in situ measurements of PPFD were made during the spring and summer of 2004. Modeled time-series were evaluated against the measurements, and daily sums of PPFD were calculated by fitting of a sine function in combination with linear interpolation of the instantaneous estimates from sunrise to sunset. This gave correlation coefficients at Norunda and Asa of 0.80 and 0.77, respectively, when comparing modeled and measured daily insolation. The average relative errors were 24% and 25%. Corresponding figures for five day averages were 0.91 and 0.86; and 9.3% and 11.9%. Instantaneous estimates of PPFD were modeled with correlation coefficients of 0 88–0 93 and average relative errors from 17.0%. These numbers were acquired when using measured values for determining cloudiness; the corresponding figures when the method is fully implemented using satellite data are 0.84 to 0.71 and 24.9%, respectively. The ratio of PAR to shortwave radiation was measured at Norunda 1 Jan to 31 Oct 2004 and was found to vary between 0.27 and 0.48 on a daily basis with an average of 0.43 for the whole period.

Estimation of Photosynthetic Photon Flux Density from 368-nm Spectral Irradiance*

The estimation of photosynthetically active radiation across the earth’s surface is needed to model plant productivity and future impacts of ultraviolet-B (UV-B) radiation on plant productivity. While radiation in the ultraviolet and visible wavelengths is routinely measured using broadband and multifilter rotating shadowband radiometers in the United States Department of Agriculture (USDA) UV-B Climate Monitoring Network, measurements of photosynthetically active radiation have not been made prior to 2002. Three models were developed to estimate the photosynthetic photon flux density (PPFD) from measurements of the diffuse and global spectral irradiance at 368 nm made by multifilter rotating shadowband radiometers. The models were developed from measurements made at half-hour intervals between March and November 2002 at five USDA UV-B Climate Monitoring Network locations in the United States between 348 and 438N latitude and 778 and 1128W longitude. The model that best estimated the PPFD included global and diffuse 368-nm irradiance, solar zenith angle, sky brightness, and aerosol transmittance and had a residual error of 79 mmol m22 s 21 PPFD. The best two-variable model for estimating PPFD utilized measured global and diffuse 368-nm irradiance, with a root-mean-squared error of 85 mmol m22 s 21. Since errors of the order of 80 mmol m22 s 21 represent biologically significant amounts of dry matter production in crops and the 9% model error is approximately twice that of the nominal measurement error for the PPFD sensor, PPFD measurements should be made and not modeled wherever possible.

Radiation regime and canopy architecture in a boreal aspen forest

This study was part of the Boreal Ecosystem-Atmosphere Study (BOREAS). It took place in a mature aspen forest in Prince Albert National Park, Saskatchewan, Canada. The aspen trees were 21.5 m high with a 2–3 m high hazelnut understory. The objectives were: (1) to compare the radiation regime beneath the overstory before and after leaf emergence; (2) to infer the structural characteristics of the aspen canopy leaf inclination and clumping; (3) to determine the seasonal course of the leaf area index ( L) for both the overstory and understory. Above-stand radiation measurements were made on a 39m walk-up tower, and understory radiation measurements were made on a tram which moved horizontally back and forth at 0.10 m s −1 on a pair of steel cables 65m in length suspended 4 m above the ground. In addition, several LI-COR LAI-2000 Plant Canopy Analyzers were used to determine the effective leaf area index and the zenith angle dependent extinction coefficient (G(θ)) for both the aspen and the hazelnut throughout the growing season. These measurements were supplemented with destructive sampling of the hazelnut at the peak of the growing season. Before leaf emergence, the ratios of below- to above-aspen solar radiation ( S), photosynthetic photon flux density (PPFD) and net radiation ( R n) during most of the day were 0.58, 0.55 and 0.47, respectively. By midsummer, these ratios had fallen to 0.33, 0.26 and 0.26, respectively. The aspen G(θ) was relatively invariant with θ, within ±0.05 of 0.5 throughout the growing season, indicating a spherical distribution of leaf inclination angles (i.e. the leaves were randomly inclined). The hazelnut G(θ) has a cosine response with respect to θ, which was consistent with the generally planophile leaf distribution for hazelnut. Using canopy gap size distribution theories developed by Chen and Black (1992b, Agric. For. Meteorol., 60: 249–266) and Chen and Cihlar (1995a, Appl. Opt., 34: 6211–6222) based on Miller and Norman (1971, Agron. J., 63: 735–738), the foliage clumping index (Ω) of the aspen canopy was derived from high-frequency tram measurements of PPFD. The aspen Ω was fairly constant with θ, but showed a small seasonal variation, with a minimum value of 0.70 in the midsummer. The hazelnut Ω was found to be 0.98 determined using the L from the destructive sampling, indicating no clumping. After corrections for clumping and the wood area index (α), the seasonal course of L near the tower for both aspen and hazelnut was determined, with maximum L of 2.4 and 3.3 for the aspen and hazelnut, respectively.

Spatial and temporal variation in the Bight environment of developing Scots pine stands: the basis for a quick and efficient method of characterizing Bight

Spatial and temporal variations in photosynthetic photon flux density (PPFD) at 1 m above the forest floor were measured under and between dominant pines and in 21- to 52-m2 gaps under developing Scots pine (Pinussylvestris L.) stands in southern Finland. For each stand, PPFD data were collected continuously every 10-s and then averaged for every 5-min period during 4-6 days under completely clear and overcast sky conditions. Measurements were made during 102 consecutive days within two different areas in each of 10 different Scots pine stands ranging in age from 7 to 105 years. Mean daily percent PPFD in the understory varied from 7.5% in the 20-year-old stands to 38% in the 7-year-old stands, and did not differ significantly between completely clear and overcast sky conditions. Mean daily percent PPFD did not vary significantly between the different sensor locations within closed pine stands, indicating that at high latitudes mean daily percent PPFD does not vary much spatially under closed Scots pine forests. Measurements of PPFD made at different periods during the day under clear sky conditions, one of the main approaches used in the literature, were not representative of the whole-day conditions. Two different results suggest that PPFD can be better characterized under completely overcast sky conditions. First, the 5-min percent PPFD in the understory was fairly constant between 06:00 and 20:00 for all stand ages under completely overcast sky conditions. Second, there was an almost perfect 1:1 relationship of the mean daily percent PPFD measured at 40 different locations in the 10 stands between clear and overcast sky conditions. These two results suggest that an instantaneous measure of percent PPFD obtained under completely overcast sky conditions is representative of the mean daily percent PPFD for both sky conditions. These results provide the basis for a quick and efficient method of estimating the mean daily percent PPFD in the understory of closed forests.

Long-term study of solar radiation regimes in a tropical wet forest using quantum sensors and hemispherical photography

Daily photosynthetic photon flux density (PPFD) was monitored for 1 year in the understorey of a tropical wet forest along a transect extending from a treefall gap to the closed canopy at La Selva Biological Station, Costa Rica. Quantum sensors attached to data loggers were operated continuously. Hemispherical photographs were taken monthly above the sensors. An additional quantum sensor was used to monitor PPFD outside the canopy. Sensor measurements show large differences between gap and closed-canopy locations and high daily and seasonal variability at each sensor location, with notable increases in solar radiation in gap stations during September and April. The photographs demonstrate that seasonal variation in PPFD results primarily from shifts in the solar angle relative to canopy openings and secondarily from variation in PPFD levels outside the canopy (cloudiness). Photographs also demonstrate an overall decrease in PPFD with time, for the gap stations, owing to vegetation regrowth. Analyses reveal excellent agreement between sensor PPFD measurements and estimates from hemispherical photographs. Long-term monitoring of PPFD enables calibration of hemispherical photography to permit estimation of PPFD with a high degree of reliability.

Random Stand Deficiency and Replanting Delay Effects on Soybean Yield, Yield Components, Canopy, and Morphological Responses

AbstractPoor soybean [Glycine max (L.) Merr.] emergence presents difficult replanting decisions. Most plant population research has involved deficient stands with even distribution rather than the more commonly encountered random gap distribution. The objective was to provide guidelines for making profitable replanting decisions. Field experiments were conducted in 1984 and 1985 to compare growth parameters and seed yield of an adapted cultivar in northern, central, and southern Illinois at 42,40, and 38° N Lat, respectively. Initial and delayed (21 d) plantings were made in rows 0.76 m apart. Two plant densities (291 550 and 202 350 plants ha−1) and five random stand deficiencies levels (0 to 60%) were established within each planting. The largest effect on seed yield was caused by random stand deficiency followed by environment and planting date. Stands with randomly imposed 0.31 m gaps totaling 40 and 60% of the plot areas resulted in 10.8 and 21.1% yield reductions, relative to uniform stands in the initial planting, respectively. Replanting delays resulted in an average relative yield loss of 9.0%. Photosynthetic photon flux density measurements were made when the initial planting reached R5 and were expressed as percent light interception (PLI). The PLI at R5 was not related to the 9.0% yield loss due to replanting delay. At the 60% stand reduction level the low plant density mean yield was 6.2% less than the high plant density mean over both plantings. Seed number per plant ranged from 63.9 in replanted full stands to 195.1 in the most deficient initial planted stands. Seed weight effects were minimal and inconsistent. The PLI decreased quadratically with increasing stand reductions from 92.5% in full stands to 80.4% in stands reduced by 60% (over all other factors). Relative yield (RY) was regressed on PLI as follows: (i) initial planting: RY = 39.65 + 1.4781 PLI (r2 = 0.941, P &lt; 0.01); and (ii) Delayed replanting: RY = 47.75 + 1.4598 PLI (r2 = 0.944 P &lt; 0.01). In most environments only stand reductions of over 50% could be justifiably replanted. The results provide guidelines for decisions of whether or not to replant, based on expected yield losses due to irregular initial stand emergence compared with expected losses due to replanting delay.

Light Variation and Carbon Gain in Rain Forest Understorey Palms

(1) The consequences of light variation for daily carbon gain in three understorey palm species are described. Detailed measurements of photosynthetic photon flux density (PPFD) were made in closed-canopy understorey, gap-edge and gap-centre sites to determine the range of light conditions. Based on these results and the responses of seedlings grown in full shade, computer simulations were used to compute total daily leaf carbon gain. (2) Median daily total PPFD in gap-edge habitats was 1-1.5 mol m-2, compared with 0.26-0-33 mol m-2 in adjacent understorey sites. Median daily PPFD in the gap centre increased with gap size. In the understorey, more than 80% of averages calculated for 5-min periods were below 10 ,umol m-2 S-1, whereas most readings in the centre of a 150-M2 gap were between 50-100 ,umol m-2 s-1 and along the gap edge 20-50 ,umol m-2 s(3) For Asterogyne martiana and Geonoma cuneata, two dwarf species, median daily total PPFD did not increase from ground level to 1-5 m. Fifty per cent of all PPFD measurements below 1-5 m were between 0 -18-0-39 mol m-2. At crown heights above 1.5 m, median daily total PPFD increased to 0.51 mol m-2. Within clones of G. congesta, daily total PPFD increased significantly with crown height; median PPFD incident on crowns 3-5-4-0 m tall was five times greater than PPFD at heights below 1.4 m. (4) On sunny days, the proportion of daily total PPFD contributed by sunflecks ranged from 10% to 78%. The relative proportion of daily PPFD contributed by sunflecks increased as the PPFD of background diffuse radiation decreased. (5) Photosynthesis of seedlings grown in full shade did not differ significantly among the three species. Light compensation points of full-shade-grown plants were between 3-4 ,umol m-2 s-1, and light-saturated photosynthetic rates measured as CO2 uptake were between 3 and 4 ,umol m-2 s-1. Light saturation occurred between 200 and 400,umol m-2 s(6) Carbon gain simulations of full-shade-grown seedlings indicate that a positive carbon gain over 24 h occurs at PPFD greater than 0.20 mol m-2. When most of the total daily PPFD was in the form of diffuse radiation, total daily carbon gain was linearly related to total daily PPFD. Daily carbon gain was not a simple function of daily total PPFD when sunflecks contributed more than 50% of the total quantum flux. On days with sunny periods, the proportion of total daily carbon gain during sunflecks ranged from 15% to 60%. (7) Seedlings of the three species grown in the gap edge had higher rates of leaf production, were taller and had larger leaves than seedlings grown in a closed-canopy understorey site. Measurements of growth and photosynthesis, and carbon-gain simulations suggest that light conditions along gap edges and within small gaps are ideal for these species.