The measurement of photosynthetically active radiation

Solar Enert'v, 1973,Vol. 15,pp.83-87. PergamonPress. PrintedinGreatBritain

THE

MEASUREMENT

OF

ACTIVE

PHOTOSYNTHETICALLY

RADIATION K. J. McCREE*

(Received 10 May 1971)

Abstract--In crop ecology, the two most popular definitions of photosynthetically active radiation are the irradiance (radiant power flux density) in the waveband 400 to 700 nm, and the quantum flux density in the same waveband. Instruments calibrated in either of these two units are available. Calculations show that the quantum flux measurement is less subject to the systematic error caused by the spectral response not matching the action spectrum for photosynthesis in an "average crop plant" (I I) than is the irradiance measurement. The range of errors is __-6and _+16 per cent, respectively, for the 9 natural and artificial light sources examined. The imperfections of the instruments themselves are not included. Resum6- Dans l'dcologie des r~coltes, les deux d6finitions pr~f6r~es de la radiation accompagnee de photosynth6se sont I'irradiation (densit6 du flux d'dnergie radiante) pour les Iongueurs d'ondes de 400 ~ 700 nm, et la densit6 du flux quantique pour les m~mes Iongueurs d'ondes. On peut se procurer des appareils calibres dans l'une ou I'autre de ces deux unitds. Les calculs montrent que la mesure du flux quantique est moins sensible que la mesure de l'irradiation. ~ rerreur syst~matique due au d6saccord entre la r~ponse spectrale et la region spectrale r~elle, off se produit la photosynth6se dans les plantes cultiv6es. Les erreurs sont de -+6% et +-16% respectivement pour les neufs sources de lumi6re artificielle et naturelle dtudi6e. Il n'est pas tenu compte des imperfections des instruments eux-m~mes. R e s u m e n - E n la ecologia de los cultivos, las dos definiciones mils populares de la radiaci6n fotosintetica-

mente activa son la irradiancia (densidad del flujo de energia radiante) en la banda de ondas de 400 a 700 ram, y la densidad del flujo cufintico en ia misma banda de ondas. Se cuenta con instrumentos calibrados en una u otra de estas unidades. Los cfi.lculosindican que la medici6n del flujo cufintico presenta menos susceptibilidad que la medici6n de la irradiancia al error sistemfitico causado por el hecho de que la respuesta espectral no iguala al espectro de acci6n para la fotosintesis en una "planta de cultivo promedio" (11). El campo de errores correspondiente alas 9 fuentes de lux natural y artificial examinadas es de --_6%y -+16% respectivamente. No se incluyendetalles de las imperfecciones que presentan los instrumentos mismos. INTRODUCTION ACTION s p e c t r a are b a s i c to a n y d i s c u s s i o n o f the biological effects o f solar r a d i a t i o n . B e c a u s e the p r i m a r y act of the light in p h o t o b i o l o g y is p h o t o c h e m i c a l , a n d photoc h e m i s t r y is initiated b y a b s o r b e d q u a n t a , we in this field are m o r e i n t e r e s t e d in q u a n t u m fluxes t h a n in e n e r g y fluxes [ 1]. S i n c e b o t h the e n e r g y c a r r i e d b y a q u a n t u m a n d the a b s o r p t i o n of that q u a n t u m b y a p h o t o c h e m i c a l l y a c t i v e p i g m e n t are w a v e l e n g t h d e p e n d e n t , the " b i o l o g i c a l a c t i o n " p e r u n i t of e n e r g y i n c i d e n t m u s t d e p e n d o n the s p e c t r a l d i s t r i b u t i o n o f the light. T h i s p a p e r will d i s c u s s the p r a c t i c a l i m p l i c a t i o n s o f this b a s i c fact, t a k i n g as a n e x a m p l e o n e o f the m o s t i m p o r t a n t biological r e s p o n s e s in ecology-photosynthesis. C r o p ecologists are c e r t a i n l y a w a r e that, w h e n m e a s u r i n g the rates o f photos y n t h e s i s of l e a v e s as a f u n c t i o n o f light, they are d e a l i n g with s p e c t r a l l y - s e n s i t i v e effects. N e v e r t h e l e s s , t h e i r t r a i n i n g s e l d o m e q u i p s t h e m to j u d g e the i m p o r t a n c e o f this for t h e i r o w n e x p e r i m e n t s . T h e e l a b o r a t e d i s c u s s i o n s of the p h y s i c i s t s go o v e r the h e a d s of m o s t ecologists, to w h o m light is o n l y o n e o f m a n y v a r i a b l e s to be m e a s u r e d . S p e c t r a l m e a s u r e m e n t s are g e n e r a l l y o u t o f the q u e s t i o n in a field e x p e r i m e n t . T h e p r o b l e m s are *Department of Biology, Texas A&M University, College Station. Texas 77843. U .S .A. 83

84

K.J. McCREE

usually resolved by choosing the cheapest commercially-available instrument (ecology still being a "shoe-string" science), and by quoting the "light intensity" in whatever units the instrument purports to measure. Obviously, some compromise is needed, and a rational one is closest to being reached in the special field of research dealing with the primary productivity of field crops. Here, "photosynthetically active radiation" is often defined as the irradiance (radiant power flux density) in the waveband 400 to 700 nm, as originally suggested by Gabrielsen [2]. It can be measured with sufficient accuracy for the purpose (--+10 per cent) with pyranometers fitted with hemispherical glass filters [3, 4]. Another compromise has appeared r e c e n t l y - t h e quantum flux density in the waveband 400 to 700 nm. Tanner[5] originally suggested measuring the absorbed quantum flux density (quantum flux divergence within the crop), since the data available at that time showed that the yield of photosynthesis per quantum absorbed by leaves was independent of wavelength, within this waveband. In other papers on quantum flux instruments [6-10], the distinction between absorbed and incident flux measurements has been lost. Although absorbed flux measurements are better in principle, they are probably too elaborate to become popular. This paper will examine these two current definitions of "photosynthetically active radiation", in the light of some new, comprehensive measurements of the action spectrum for photosynthesis in crop plants[11]. The discussion will be based on the spectral response of an "average plant", established in these experiments.

ACTION SPECTRUM FOR PHOTOSYNTHESIS Almost all of the published action spectra were obtained by photobiologists primarily interested in the activities of the many different pigments found in the photosynthesizing cells of different organisms. To obtain as wide a range of pigments as possible, they used algae as experimental material. In crop plants, there are fewer pigments, and they all take part in photosynthesis, by transferring their energy to the few special chlorophyll molecules which initiate the photosynthetic reactions. Consequently, the action spectrum for photosynthesis in crop plants (Fig. 1) does not resemble the familiar double-peaked absorption spectrum of chlorophyll. Since the pigment molecules do not always transfer their energy with 100 per cent efficiency, the action spectrum does show some structure, depending on the species of plant, and on the conditions under which it is grown. Nevertheless, in a survey of 22 species of crop plant, grown and tested under a wide variety of conditions, we found very few wide excursions from the average curve shown in Fig. 1. We feel that this is a fair representation of the action spectrum for photosynthesis in "normal green leaves". The data have been plotted in two forms, photosynthesis per quantum absorbed (quantum yield) and photosynthesis per unit of energy incident (action). In neither form does the curve resemble a rectangle bounded by the wavelengths 400 and 700 nm. Consequently, the rate of photosynthesis of an "average" leaf in "white" light will not be proportional to the integrated flux of quanta or energy in the 400 to 700 nm waveband. The ratio of the photosynthetic rate to the integrated light flux (photosynthetic efficacy of the light) will vary with the spectral composition of the light. The amount of variation can be calculated from the curve of Fig. 1, if the spectral energy distribution of the light is known. I shall now present some examples of this type of calculation.

The measurement of photosynthetically active radiation

I

l

I

'

I

85

I

1.0

\ I.O

E E

cr

_o

0

] 400

500

600

Wovelength,

700 nm

Fig. I. Action spectrum for photosynthesis in an "average field crop leaf". The data[i !] have been plotted in two ways. "Relative action" is the rate of uptake of carbon dioxide in monochromatic light divided by the incident radiant power flux density (spectral irradiance). "Relative quantum yield" is the rate of uptake of carbon dioxide divided by the absorbed quantum flux density. The curves have been normalized to a maximum of 1.0. U S E O F A C T I O N S P E C T R U M IN L I G H T C A L C U L A T I O N S *

If the photosynthetic rate of unit area of leaf, per unit of incident monochromatic radiant power flux, is Px, and the spectral radiant power flux density (spectral irradiance) of the "white" light is lx, then the photosynthetic rate per unit leaf area in white light is 760

P = f PJ~dh. 360

For the white light, the radiant power flux density (irradiance) is 700

I = f i~dh 400

*The following S .1. units are recommended: X. in nanometers (nm), I in watts m -2, P in moles CO2 sec -1 m -2, Q in einsteins sec-' m -2.

86

K.J. McCREE

and the quantum flux density is 7OO

Q=

8"36 × 10 -9 I" ~,l~dg. 4O0

The "photosynthetic efficacy" of the light can now be calculated as either P/I or P/Q, depending on the definition of light flux adopted. Calculations of these two ratios have been made for natural light, and for artificial light produced by some of the lamps used in photosynthesis research (Table 1). Values of In for the natural light were taken from the paper by Henderson and Hodgkiss[l 2], while values for the artificial lights were measured by us. Since we were interested only in variations of the ratio, for a single P~ curve and various in curves, the values have been normalized with respect to sun + sky radiation. Table I. Photosynthetic efficacies of various types of "white" light, calculated from the photosynthetic action spectrum of Fig. 1 and the spectral irradiance of the light. (P/! = photosynthetic rate per unit leaf area, per unit irradiance (400-700 nm); P/Q = photosynthetic rate per unit leaf area. per unit quantum flux density (400-700 nm))

P/I Natural light Sun + sky Blue sky Discharge lamps Lucalox Metalarc Mercury Fluorescent lamps Warm white Cool white Grolux WS Incandescent lamps Quartz iodine + 5 cm water

IMPLICATIONS

P/Q

1.00 0.88

i.00 0.95

1.18 1.01 1.05

1.08 1.01 1-01

1.03 0.99 1.04

1.01 0.99 1.02

1.19

1-08

FOR LIGHT MEASUREMENTS

The table shows that the ratio P/Q varies about half as much as the ratio P/I. This means that the quantum flux definition of photosynthetically-active radiation will introduce less error due to spectral differences into measurements of the photosynthetic rates of leaves than will the irradiance definition. On these grounds, the quantum flux density (400-700 nm) is the preferred definition. The values in Table 1 apply only to instruments having spectral responses which match perfectly the equal-quantum or equal-energy responses assumed in the two definitions. The imperfections of real instruments, in spectral response as well as in other important properties such as angular response, linearity, and temperature insensitivity, will introduce additional errors, which cannot be discussed here. They will

The measurement of photosynthetically active radiation

87

seldom add up to less than _+ 10 per cent. It is an unfortunate fact, well known to photometrists but not to many users of light meters, that light measurements in general are much less accurate than other physical measurements. No doubt it will be quite some time before perfect photosynthetic-light meters are in the hands of perfect crop ecologists, but we hope that this discussion of the systematic errors introduced by the definitions themselves will be of interest to both groups. A c k n o w l e d g e m e n t - T h i s research was supported in part by the National Science Foundation, and by the National Oceanographic and Atmospheric Administration. REFERENCES [I] R. E . Craig, Radiation measurement in photobiology-choice of units. Photochem. Photobiol. 3. 189 (1964). [21 E. K. Gabrielsen, Einfluss der Lichtfaktoren auf die Kohlens/iureassilimation der Laubbl~itter. Dansk Botanisk A rkiv 10 (1940). [3] K. J. McCree, A solarimeter for measuring photosynthetically active radiation. Agr. Meteorol. 3, 353 (1966). 14] G. Szeicz. Measurement of radiant energy. Br. Ecol. Soc. Easter Syrnp. on Instrumentation in Ecoloy,,ical Meas., 109-130 (1968). [5l C. B. Tanner, Concluding remarks, In: Functioning o f terrestrial ecosystems at the primar3'production le re/, U N E S C O , Paris, 509-510 (1968). [61 C. A. Federer and C. B. Tanner, Sensors for measuring light available for photosynthesis. Ecology 47.654 t 1966L 171 J. M. Norman, C. B. Tanner and G. W. Thurtell. Photosynthetic light sensor for measurements in plant canopies. A gron. J. 61,840 (1969). 18] H. G. McPherson, Photocell-filter combinations for measuring photosynthetically active radiation. Agr. Meteorol. 6. 347 (1969). [91 P. Gaastra, Radiation measurements for investigations of photosynthesis under natural conditions. In: Functioning o f terrestrial ecosystems at the primary, production level, UNESCO, Paris, 463-478 (1968). 1101 W. W. Biggs, A . R. Edison, Jerry D. Eastin, K. W. Brown. J. W. Maranville and M. D. Clegg, Photosynthesis light sensor and meter. Ecology 52, 125 ( 1971 ). [ I 11 K. J. McCree, The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agr. Meteorol. 9, 191 (1971). [121 S. T . Henderson and D. Hodgkiss, The spectral energy distribution of daylight. Br. J. Appl. Phys. 14. 125(1963).