UV Chapter 5 Flux

Photon flux

Photon Flux Density

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Design and management of PFALs

Toyoki Kozai, ... Kazutaka Ohshima, in Plant Factory (Second Edition), 2020

25.5.1 PPFD distribution

The PPFD distribution on the culture panel can be well simulated by PFAL-D&M (LS). Fig. 25.11 shows a simple simulated result of the two-dimensional PPFD distribution on the cultivation panel with and without a light reflector set above the light sources. The results show that the average PPFD is increased by 38% using a white reflector, compared with the one without a reflector.

Figure 25.11. An output example of PFAL-D for light environment improvement by use of a light reflector.

Fig. 25.12 shows a simple simulated result of the three-dimensional PPFD distribution over model crisp lettuce heads planted in a grid-like fashion on the cultivation panel. Using this software, the ratio of light energy received by plants to that the light emitted by the light source can be estimated. Thus, the software tool is useful for designing the planting density, vertical distribution of PPFD, etc. More and detailed examples are provided in Kozai et al. (2016).

Figure 25.12. A simple simulated result using PFAL-D of the three-dimensional PPFD distribution on crisp lettuce heads planted.

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CANOPY STRUCTURE AND LIGHT INTERCEPTION

P.S. NOBEL, S.P. LONG, in Techniques in Bioproductivity and Photosynthesis (Second Edition), 1985

4.2.4 Measurement of light in canopies

Chapter 3 describes the measurement of photon flux density (Q) and radiant energy flux density (I). Most instruments utilise point or circular sensors, which are appropriate where there is little horizontal variability in light quantity, as occurs in the open. However, the heterogeneity of leaf distribution in canopies leads to marked small scale variation in Q, and so to determine Q at any height in the canopy we need to make measurements at a number of locations and then obtain an average. This may be achieved by: 1) positioning an array of sensors at one height in the canopy; 2) moving a single sensor through a length of canopy; or 3) using a horizontal line or tube sensor whose output is the spatial average over the sensor length. Because of its simplicity, the latter technique is the most widely used, although it provides no indication of spatial variability.

Line photon sensors (e.g. the one-metre cosine and spectrally corrected LI-COR LI–191 rod with its photoelectric detector; LI-COR Inc., Lincoln, Nebraska, U.S.A.) measure the average photon flux density falling on a line placed horizontally in the canopy. Tube solarimeters (e.g. the one-metre Delta-T TSL glass tube enclosing a strip thermophile of alternating black and white surfaces; Delta-T Devices, Cambridge, U.K.) measure the average solar energy flux density. Instruments of the latter type are generally cheaper and relatively simple to construct14.

However, filters must be used in combination with tube solarimeters when the photosynthetically active part of the spectrum is determined (the infra-red component of sunlight readily penetrates canopies and will account for most of the radiant energy near the canopy base). The appropriate sensor length depends on plant spacing; for example, in a dense canopy of a fine-leaved grass, one metre would be excessive, while in a citrus plantation with 3 metre spacing of plants a one metre sensor would be too short, although a series of readings along a line could overcome this inadequacy.

To determine the foliar absorption coefficient (k) or the absorption of Q, more than one sensor location must be used. Typically, a conventional photon sensor is placed facing up just above the top of the canopy to determine Q0, and a line or tube sensor is inserted in the canopy to determine Q there. When the line or tube sensor is placed at the base of the canopy, the photon flux density intercepted by the whole canopy (Qa) is:

(4.2)Qa=Q0−Qb

where Qb is the photon flux density incident in a horizontal plane at the base of the canopy. Such a measurement tells us the absorption at one time. Usually, Qa is integrated over a day or even a growing season to record the cumulative photon flux densities.

Most measurements of k and Q are based on the ambient (incident) photon flux density intercepted. We could equally well base our discussion strictly on absorbed photons, where Q0 would represent the downward photon flux density minus that reflected from the canopy (the latter determined by a photon sensor facing downward placed just above the canopy). Similarly, Qb in Eqn. 4.2 then becomes the photon flux density downward at the base of the canopy minus that reflected upward by the soil, i.e. the photon flux density absorbed by the soil. Such a change to a strictly absorbed photon basis causes the foliar absorption coefficient k to become slightly smaller compared to when the ambient levels of Q are used. The smaller k is appropriate in most studies of the quantum efficiency of CO2 fixed or O2 released, which are based on absorbed photons, although most studies of canopy structure and most photosynthetic response curves relating CO2 uptake to ambient conditions are based on intercepted photon flux density.

To avoid systematic errors, sensors must be matched or cross-calibrated. Line and tube sensors generally are directionally sensitive, and so are not appropriate for measurement above the canopy. Directional sensitivity is less important within the canopy, because photon flux densities there tend to be more diffuse, i.e., come from all angles.

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Plant responses to light

Haijie Dou, Genhua Niu, in Plant Factory (Second Edition), 2020

9.2.2 Plant response to light intensity, photoperiod, and daily light integral

In a PFAL, light intensity (PPFD) usually is kept constant, unlike natural sunlight, which varies diurnally as well as seasonably in intensity, duration, and spectral composition. PPFD and photoperiod are two important light conditions in regulating plant growth, development, and nutritional values.

The daily light integral (DLI) represents the total photosynthetic photon flux radiated by a light source in 1 day, and usually has a linear relationship with crop yield in a PFAL. For leafy greens, PPFD in commercial PFALs are in the range of 150–250 μmol m−2 s−1 with a photoperiod of 16 h per day, corresponding to DLI of 8.64–14.4 mol m−2 d−1. High DLIs are favorable for improving yield and accumulating secondary metabolites in horticultural crops (Schnitzler and Habegger, 2004; Chang et al., 2008). However, higher DLI increases production cost by increasing capital cost (light fixture numbers) and operational cost (electrical consumption). Therefore, for PFAL, a minimum DLI should be targeted to reduce costs while still achieving a reasonably high yield and quality.

In a study by Zhang et al. (2018) to determine the optimal PPFD in a PFAL, lettuce plants were grown under a range of PPFD of 150, 200, 250, or 300 μmol m−2 s−1, provided by fluorescent lamp with red to blue light ratio (R:B) of 1.8 or two types of LED with R:B ratio of 1.2 or 2.2 at two photoperiods (12 h or 16 h per day). Based on yield, quality (nitrate, vitamin C, soluble sugar, and protein and anthocyanin content), and energy consumption, PPFD between 200 and 250 μmol m−2 s−1 with photoperiod of 16 h per day under LED light with R:B ratio of 2.2 was the best. By continuously measuring the photosynthetic rates for 48 h of lettuce leaves 2 weeks after transplanting, the plants under PPFD of 300 μmol m−2 s−1 had constantly lower net photosynthetic rates than those under 250 μmol m−2 s−1. PPFD at 300 μmol m−2 s−1 did not have any advantage in enhancing quality traits except for leaf nitrate content, compared to PPFD of 250 μmol m−2 s−1. Leaf nitrate content can be lowered by preharvest continuous lighting (Bian et al., 2016) and by lowering nitrogen level in the nutrient solution.

In our previous study, the effects of five DLI levels of 9.3, 11.5, 12.9, 16.5, and 17.8 mol m−2 d−1 with a 16 h photoperiod were tested on yield and nutritional values of sweet basil (Ocimum basilicum) under an indoor controlled environment (Dou et al., 2018). The results indicated that higher DLIs of 12.9, 16.5, or 17.8 mol m−2 d−1 led to higher net photosynthetic rate, larger and thicker leaves, and the shoot fresh weight was 54.2%, 78.6%, and 77.9% higher than that at DLI of 9.3 mol m−2 d−1, respectively. However, no differences in yield were observed among DLIs of 12.9, 16.5, or 17.8 mol m−2 d−1. The amounts of total anthocyanin, phenolics, and flavonoids per plant were also positively correlated to DLIs (Fig. 9.3). We also conducted another study on sweet basil with a fixed DLI of 12.9 mol m−2 d−1, created by varying photoperiod (12, 14, 16, 18, or 20 h/d) and PPFD (298, 256, 224, 199, or 179 μmol m−2 s−1). Except for leaf area, we did not find any differences in yield, photosynthesis, chlorophyll concentration, and other growth parameters, while nutritional quality traits were not determined in this study.

Figure 9.3. Correlation between amount of total anthocyanin per plant (A), amount of total phenolic per plant, and amount of total flavonoid per plant (B) with DLIs of green basil "Improved Genovese Compact" grown for 21 days at different DLIs in an indoor controlled environment (Dou et al., 2018).

Plant growth responds almost linearly to increasing PPFD under a controlled environment, and the photosynthetic efficiency decreases when a light saturation point is reached. The light saturation point is species specific and is dependent on environmental conditions. For example, there was a linear increase in both leaf fresh weight and dry weight in kale (Brassica oleracea) and spinach (Spinacia oleracea) as the PPFD increased from 125 to 620 μmol m−2 s−1 in a growth chamber, while the concentrations of Ca, Cu, K, and Mn in kale plants all decreased at high PPFD due to dilution effects resulting from increased leaf fresh weight (Lefsrud et al., 2006). However, the shoot and root growth and anthocyanin content of "Kudo" perilla (Perilla frutescens) were decreased under PPFD of 500 μmol m−2 s−1 compared to 300 μmol m−2 s−1, while the total polyphenol content was increased (Hwang et al., 2014). For leafy greens and seedlings in PFALs, PPFDs are kept in the low linear dose–response range of up to 250 μmol m−2 s−1 as indicated by our research results (Dou et al., 2018; Zhang et al., 2018; Yan et al., 2019).

Growing plants under a low PPFD for a long photoperiod at the same DLI could reduce the capital costs of a plant factory due to decreased number of light fixtures and requirements for cooling. A number of studies showed that long photoperiod generally increased plant biomass accumulation due to leaf expansion and chlorophyll increase than plants exposed to a high PPFD for a short photoperiod at the same DLI (Adams and Langton, 2005). However, many sensitive species developed physiological disorders such as leaf chlorosis and chlorophyll degradation under an extended photoperiod (Langton et al., 2003; Kang et al., 2013). For instance, the growth and yields of tomato (Solanum lycopersicum) and sweet pepper (Capsium annuum) plants were decreased, and light injury symptoms were observed in tomato, eggplant (S. melongena), potato (S. tuberosum), radish (Raphanus sativus), and cucumber (Cucumis sativus) plants under continuous lighting (24 h photoperiod) (Sysoeva et al., 2010). The reduced growth and yield under extended photoperiod are caused by the leaf inability to export accumulated photosynthates out of the leaf or the destruction of chloroplasts due to some photooxidative stress caused by a long photoperiod (Demers et al., 1998; Ali et al., 2009).

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Scale-up of microalgae-based processes

Niels-Henrik Norsker, in Handbook of Microalgae-Based Processes and Products, 2020

32.2.4.4 Enhancing light flux into photobioreactors

There are various approaches to increase the photon flux density (PFD) of the daylight falling on a photobioreactor. A flat panel reactor may be tilted toward the sun to optimize light capture, either in a manually adjustable position or with tracking devices (Qiang et al., 1996, 1998a). It is not yet clear whether increased productivity justifies the extra expenses and complications with automatic tilting as opposed to stationary vertical panels.

Another strategy to obtain volumetrically specific irradiation is to concentrate the daylight, for example, with linear Fresnel lenses combined with solar tracking systems (Masojídek et al., 2003, 2008; Zijffers et al., 2008). The concentrated beam irradiation from linear Fresnel lenses may then be projected directly on to tubes of the reactor (Masojídek et al., 2003, 2008) or distributed to the cultures via light prisms (Zijffers et al., 2008) or fibers (Ogbonna and Tanaka, 2000). In an experimental system with 9 m2 linear Fresnel lenses, established at Nove Hrady (48.5°N, 114.5°E) in the Czech Republic, noon tube surface light intensity was up to 3.5 times that of the ambient horizontal light intensity. Optimal biomass concentration (Arthrospira) was judged to be 1.2 g dw L− 1 (Masojídek et al., 2003). Arthrospira appears to be able to adapt to the high noon irradiation and recover from quenching on a diel base. It was shown that Arthrospira platensis can grow under highly concentrated irradiation without being irreversibly photoinhibited. Photosynthetic efficiency of the culture was rather low. A cost of a 65-L experimental system, including 9 m2 of linear Fresnel lenses, 24 m of glass tubes, frame and tracking system for the tubes, pump, degasser, heat exchanger, sensors, and control (pH, light, temperature) was indicated to total €8700 (Masojídek et al., 2003).

To achieve high volumetric productivity and high photosynthetic efficiency at the same time, biomass concentration and mixing must be carefully optimized.

As was shown in a series of works at Algal Biotechnology Group around the turn of the millennium, it is possible to utilize high light intensity with high productivity and high photosynthetic efficiency. The work with continuously dual-side illuminated, laboratory flat panel Spirulina culture is a most impressive demonstration of the reach of this approach (Qiang et al., 1998b). An area productivity of 15 g dw m− 2 h− 1 was obtained at a combined dual-side irradiation of 8 mmol m− 2 s− 1, corresponding to a photosynthetic efficiency of about 6% (PAR, photosynthetically active radiation) (see Fig. 32.5). The daylight equivalent photosynthetic efficiency is about 2.5%, which in itself is not a lot, but at a PFD corresponding to about four times the maximum direct solar PFD it is impressive. At a PFD that corresponds to the maximum direct solar PFD (2000 μm m− 2 s− 1), productivity of about 11 g dw m− 2 h− 1 was obtained, corresponding to a PAR efficiency of 15% (Qiang et al., 1998b). Equivalent solar efficiency would be 6.3%.

Fig. 32.5. Photosynthetic efficiency and biomass productivity of Spirulina platensis in a dual-side illuminated flat panel photobioreactor at varying flux density. The flux density is the sum of the flux density from the two sides. PE (%) is based on the photosynthetic active radiation (PAR) (Qiang et al., 1998b).

Translating these data to outdoor photobioreactor conditions, however, would result in exaggerated productivity projections.

One obstacle in reproducing this scheme under outdoor conditions is the principle to adjust the biomass concentration carefully to suit the irradiation. This is difficult to achieve under outdoor conditions because of the diel variation in irradiation, not even under cloudless conditions, and even less with changing cloud cover and the resulting dynamical changing light regime.

A general problem with irradiation of photobioreactors is that the light is rapidly absorbed in the algal biomass, depending on biomass concentration and light absorption coefficient. There is an inverse relationship between photosynthetic efficiency of the absorbed light and light intensity; and with high light intensity, sustained exposure may even lead to irreversible photoinhibition.

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LED advancements for plant-factory artificial lighting

Cary A. Mitchell, Fatemeh Sheibani, in Plant Factory (Second Edition), 2020

10.8 Sorting out the spectral contributions of LED wavebands

LEDs are more or less "monochromatic," with maximum photon flux density (PFD) for a given waveband occurring at very narrow-spectrum peak absorbance, and with minimal PFD from a spreading-wavelength base on either side of the peak wavelength. At least for the PAR range, each waveband is defined by that low-PPFD, spreading base (typically in 100-nm increments). However, the effectiveness of a given-waveband LED depends to a large extent upon the peak-wavelength output of that LED (typically 1 nm), where the great majority of photon energy resides. Thus, the efficacy of a given LED to drive or regulate a given plant photo-response typically is defined by its peak energy output and PFD. Although plant physiological processes evolved under broad-band solar radiation and are extremely complex and interactive, using a controlled blend of monochromatic LEDS for sole-source plant-growth lighting allows the grower to manipulate and predictively control plant growth in terms of yield, productivity, and quality. The waveband output of a given LED is determined by the primary composition of diode components, with peak wavelength specificity determined by differential doping of diode alloy composition. Thus, desired plant growth and development outcomes are potentially possible using narrow-spectrum LEDs that may not be possible under broadband sunlight!

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Challenges for the next-generation PFALs

Toyoki Kozai, Genhua Niu, in Plant Factory (Second Edition), 2020

32.2.2 Using green LEDs

The photosynthetic rate of a leaf is directly related to PPFD, not to the PAR energy flux density (Chapters 9–13Chapter 9Chapter 10Chapter 11Chapter 12Chapter 13). It is often said that, in a PFAL, red light (wavelength: 600–700 nm) is most effective for promoting photosynthesis of plants but some blue light (400–500 nm) is necessary mainly for photomorphological and/or phytochemical reasons for a given amount of electric energy consumption. This is because the light energy contained in one photon is inversely proportional to the wavelength. Therefore, the light energy per photon is 1.2 (= 600/500) to 1.75 (= 700/400) times higher in a blue photon than in a red photon.

It is well known that a thin layer of chlorophylls suspended in a liquid absorbs red and blue photons very well but absorbs little or no green photons. It is also known that a single green leaf absorbs about 50%, reflects about 20%, and transmits about 30% of green photons. These figures are for a thin layer of chlorophylls and a single leaf, respectively, not the whole plant community.

When plants are densely populated, most green photons transmitted by the upper leaves will be received by the lower leaves. In a PFAL, green photons reflected upward by the leaves are again reflected back downward by the light reflector to the plant community and are received by the leaves. As a result, most green photons emitted by the LEDs are received by the plant community vertically more evenly than red and green photons, because almost all red and blue light photons are absorbed by the upper leaves, and thus are neither transmitted to the lower leaves nor reflected upward (Fig. 32.2; Kozai, 2012).

Figure 32.2. Schematic diagram showing the percentages of green light energy transmitted and reflected by upper, middle, and lower horizontal green leaves. Total green light energy absorbed by upper, middle, and lower leaves is also given. Red and blue light energy is absorbed by the upper leaf, and the middle and lower leaves receive almost no red or blue light energy. It is assumed that the percentages of reflection, transmission, and absorption of a horizontal leaf for green light energy are 20%, 30%, and 50%, respectively.

Owing to the recent widespread application of green LEDs to traffic signals, etc. and of white LEDs for office and home lighting, the cost performance of green and white LEDs has improved considerably, and so green and white LEDs with 25%–40% green light could be useful in the densely populated plant community in a PFAL. Recent studies indicate different effects of green light on plant growth and development (e.g., Kim et al., 2004; Johkan et al., 2012).

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Nature and Source of Light for Plant Factory

Yong Xu, in Plant Factory Using Artificial Light, 2019

2.1.4.1 Light Intensity

The light intensity in horticulture is a measure of the photosynthetic photon flux density (PPFD) and quantified as μmol photons m–2 s–1, that is also simplified to μmol m–2 s–1 in the range of photosynthetically active radiation (PAR), which designates the radiation spectrum between 400 and 700 nm that higher plants are able to use in the process of photosynthesis [12]. As was mentioned before, there is no direct conversion between lux and μmol m–2 s–1for different light sources and this is especially important in the design of artificial lighting in plant factories or supplemental lighting in greenhouses using narrow-band LEDs.

The importance of light intensity in promoting plant growth and development is very obvious and there have been many experiments that showed, under a certain extent, higher light intensity leads to higher plant yields [13, 14]. On the other hand, plants always have the effect of respiration and photosynthesis can only be carried out after the intensity of illumination has been increased to a certain degree. Photosynthesis and respiration are independent and interrelated processes and the balance of them is the compensation point of a plant's photosynthesis (see Fig. 2.1.5). When the intensity of light is lower than that point, respiration is stronger than photosynthesis; when the intensity of light goes higher, photosynthesis is stronger than respiration and increases almost linearly with the intensity of light. But as the intensity of light increases to a certain amount, the rate of photosynthesis would reach a saturation point and starts to decrease after that. Therefore, the best intensity of light would be in between the compensation and saturation points.

Fig. 2.1.5. Change of photosynthesis rate with the intensity of light.

On the other hand, the unit μmol m–2 s–1 for PPFD involves a macro-quantity, the Avogadro's constant. That is to say, this is a mix of both macro and micro (the photon) quantities. In order to investigate the interaction between photons and the molecules in plants (such as chlorophylls) more precisely, we can convert this mixed macro-quantity into an entire micro-quantity by introducing the area of (nm)2. The conversion between μmol m–2 s–1 and phtons nm–2 s–1 is as follows.

(2.1.5)1μmolm−2s−1=6.022×1023×10−6×1092=0.6022nm−2s−1

That is to say, for every micro-mole of photons per square meter per second plants receive, there are 0.6 photons per second every square nanometer. As the size of nanometer is similar to that of the chlorophylls, by using the unit of photons nm–2 s–1, we can easily estimate in average how many photons a chlorophyll would perceive at different PPFDs. Therefore, the unit of photons nm−2 s−1 would be more meaningful in measuring the interaction between photons and the molecules, such as the chlorophylls, in plants.

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Fundamental Components and Points to Consider in the Design of a Plant Factory: An Example of OPU New-Generation Plant Factory

Teruo Wada, ... Toichi Ogura, in Plant Factory Using Artificial Light, 2019

6.1.2.1.4 Light Source

It is thought that light requirements of plants are 100–300 μmol m− 2 s− 1 of photosynthetic photon flux density (PPFD) for leafy vegetables, 200–600 μmol m− 2 s− 1 for fruiting vegetables, and 50–200 μmol m− 2 s− 1 for ornamental plants [4]. One of the reasons why leafy vegetables are grown in plant factories is a lower light requirement, about 200 μmol m− 2 s− 1. Though the light source in plant factories was high-intensity discharge lamps in the 1990s, fluorescent lamps had become popular in the 2000s because multilayer cultivation could be realized by close irradiation. Since 2010, LEDs, which have the property of low energy consumption, are becoming the most popular light source in the background of the rising cost of electricity. The GCN had a basic policy of using LEDs as the light source from the start of the project. LED modules produced by company P in the Netherlands were adopted in the GCN as a result of discussions with experts and experiments using LEDs provided by various companies. The evaluated factors were (a) better plant growth with far-red light, (b) lower cost, (c) simple construction with a built-in power unit, (d) higher humidity resistance (IP66 rating), and (e) higher color rendering so that plants look green with white light. LED tips of blue, red, white, and far-red are arranged in a 1.2 m module as shown in Fig. 6.1.6. Its energy consumption is 28 W. Three modules are fixed above a 60 cm wide bed and the average PPFD on the cultivation panel is about 180 μmol m− 2 s− 1. Though the addition of the far-red spectrum increases plant growth, it might cause tip-burn and succulent growth of seedlings. It is thought that the effect of adding far-red light in increasing plant growth outweighs the adverse effects, though overall evaluation continues.

Fig. 6.1.6. LED lighting module.

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Cultivation of Algae in Photobioreactors for Biodiesel Production

J. Pruvost, in Biofuels, 2011

2.2 Characterization of the Incident PFD

The light energy received by the cultivation system is represented by the hemispherical incident light flux density q, or photon flux density (PFD) as it is commonly termed in microalgae studies. For any light source, the PFD has to be expressed in the range of PAR, in most cases in the 0.4-0.7 μm bandwidth. For example, the whole solar spectrum at ground level covers the range 0.26-3 μm. The PAR range thus corresponds to almost 43% of the full solar energy spectrum.

As light is converted inside the culture volume, it is also necessary to add to PFD determination a rigorous treatment of radiative transfer inside the culture. This enables us, for example, to couple the resulting irradiance field with photosynthetic conversion of the algal suspension to simulate light-limited growth. However, this determination requires certain information. In addition to the PFD value, light source positioning with respect to the optical transparent surface of the cultivation system is important, as light penetration inside a turbid medium is affected by the incident polar angle θ of the radiation on the illuminated surface (Figure 2). Ideally, beam and diffuse components of radiation should be considered separately. By definition, the direction of a beam of radiation, which represents direct radiation received from the light source, will define the incident polar angle θ with the illuminated surface. By contrast, diffuse radiation cannot be defined by a single incident angle, but has an angular distribution over the illuminated surface (on a 2π solid angle for a plane). We note that isotropic angular distribution is usually assumed, although an anisotropic distribution should ideally be considered because of the dependency of radiative transfer inside the culture volume on the angular nature of incident diffuse PFD. Both the incident angle and the degree of collimation of the light flux can be difficult to characterize. However, in most artificial light cultivation systems, normal incidence is usually chosen as the most effective way to transfer light into the culture volume (less reflection on optical surfaces and better light penetration in the culture bulk). The PFD can also in most cases be assumed to be quasicollimated (so we can consider the PFD as beam radiation only). However, these characteristics cannot be assumed in solar technology. The sun's displacement makes the incident angle time dependent and so non-normal incidence conditions will be encountered. Sunlight can also present a large proportion of diffuse radiation due to scattering through the atmosphere or by reflection from various surfaces, such as the ground. A detailed description of the respective consequences of neglecting incidence angle and direct/diffuse distribution effects in solar cultivation systems was recently published (Pruvost et al., in press). It was shown that each assumption led to an overestimation of 10-20% in biomass productivity. When the two assumptions were combined (the simplest case of radiative transfer representation), an overestimation of up to 50% was obtained, emphasizing the relevance of an accurate consideration of the incident angle and direct/diffuse distribution in the radiative transfer modeling when applied to the solar case.

Figure 2. Solar radiation on a microalgal cultivation system: incident angle and diffuse-beam radiations (left), evolution of solar sky path during the year in France (right).

The PFD can be measured using a cosine quantum sensor (LI-190-SA, LI-COR, Lincoln, NE) with multipoint measurements to obtain an average over the illuminated surface (Janssen et al., 2000b; Pottier et al., 2005; Sanchez Miron et al., 2003). The accuracy will closely depend on the average procedure, especially if the PFD is unevenly distributed. Actinometry could also be used for accurate characterization, as this is sensitive to all photons absorbed in the reaction volume. A detailed example of the experimental procedure in artificial light can be found in Pottier et al. (2005). In the case of sunlight, measurement is obviously also possible, but mathematical relations are also available to determine radiation conditions on a collecting surface as a function of the Earth's location, year period, and surface geometry (Duffie and Beckman, 2006). An example was recently given by Sierra et al. (2008) for a solar photobioreactor. Some commercial software packages integrating solar models are also available (METEONORM 6.0 software; www.meteonorm.com). These allow easy determination of irradiation conditions on a given surface. Such an approach is thus of particular interest in the case of solar production and was applied in Pruvost et al. (in press).

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Resource Use Efficiency

D.W. Sheriff, ... P.B. Reich, in Resource Physiology of Conifers, 1995

1 Light Use Efficiency

Light use efficiency of a leaf, ALUE, is related to the rate of carbon assimilation (A) and the photosynthetic photon flux density (PPFD) as ALUE = A/PPFD; this is sometimes called quantum yield and can be determined either on the basis of light incident on a leaf (ALiUE) or on the basis of the amount of light absorbed by a leaf (ALaUE). The latter is physiologically more meaningful because it indicates the efficiency with which energy from absorbed light is utilized. However, this is a more difficult quantity to scale up to larger physical scales because it is difficult to measure light absorption by only the foliar canopy of a stand of vegetation, and to separate component species. Most frequently, leaf area index [LAI = (leaf area)/(ground area)] or a similar measure is determined. In this case light use efficiency can usually be related only to light incident on the foliage. Changes in factors that alter light absorption (e.g., foliar [N]) will most often modify ALiUE, but will often not affect ALaUE at PPFDs below that for light saturation.

Over short time periods, ALUE will remain constant in an unchanging environment unless environmental extremes cause damage to the assimilatory system (e.g., photoinhibition). Changes in light over short time periods will cause ALUE to change little at low light levels, where carbon assimilation increases linearly with PPFD. As carbon assimilation approaches and attains light saturation, ALUE will be negatively associated with PPFD. Over longer time periods, previously exposed leaves that have become shaded may take on the characteristics of shade leaves. This will alter their responses to PPFD such that ALUE will often then be lower at high PPFD and higher at low PPFD than it was previously.

Factors in the internal and external environments of leaves affect their ALUE. Effects of changes in these environments on carbon assimilation and light use are summarized in Table I.

Table I. Effects of Changes in the Numerator or the Denominator of Components of Instantaneous RUEs of Leavesa

RUEMeans of increasing RUEEffect on NPP of leafEffect on quantitative use of resourceALUEGreater efficiency of net carbon assimilationIncreaseNilGreater absorption of intercepted lightbIncreaseIncreaseGreater light interceptionIncreaseIncreaseANUEGreater efficiency of carbon assimilationIncreaseNilLower foliar nutrient concentrationcNilReduceATEGreater efficiency of carbon assimilationIncreaseNilLower transpiration by reduced stomatal conductanceReduceReduce

aNet carbon assimilation can be affected by a range of factors, as discussed in Chapter 4 of this volume. The magnitude of these effects can vary with species, time of year, ontogeny, and phenology. Factors that affect the supply or use of the resource are given in Table II.bUsually only below the PPFD at which carbon assimilation is light saturated.cUsually only below the foliar nutrient concentration at which carbon assimilation is saturated for that nutrient.

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