Spectral Properties of Plants
David M. Gates, Harry J. Keegan, John C. Schleter, and Victor R. Weidner
The spectral properties of plant leaves and stems have been obtained for ultraviolet, visible, and infraredfrequencies. The spectral reflectance, transmittance, and absorptance for certain plants is given. Themechanism by which radiant energy interacts with a leaf is discussed, including the presence of plantpigments. Examples are given concerning the amount of absorbed solar radiation for clear sky andovercast conditions. The spectral properties of desert plants are compared with those of more mesicplants. The evolution of the spectral properties of plant leaves during the early growing season is givenas well as the colorimetric behavior during the autumn.
Plants depend upon radiant energy for the energynecessary to carry on photosynthesis and other physi-ological processes. The green plant has been calledthe converter of solar energy. In the presence of sun-light it synthesizes complex organic compounds such assugars, fats, proteins, etc., from simple inorganic com-pounds such as water, carbon dioxide, minerals, salts,etc. The interaction of plants with radiant energy is ofinterest to the botanist, forester, geographer, bio-physicist, biochemist, ecologist, hydrologist, agrono-mist, photogrammist, and others. Naturally, theperspective will be dramatically different as viewed bythese various scientists. The biochemical perspectivedealing with the photochemistry of plant pigments maybe the most complex and has been treated extensivelyin the scientific literature, see French,' and Calvin andAndroes.2 The quantum chemistry of photosynthesisand the photosynthetic process will not be treated inthis article. The primary purpose here is to describethe interaction of radiant energy with the plant leaf asseen from the classical optics viewpoint.
The leaf of a plant is the primary photosynthesizingorgan with photosynthesis occurring in the chloroplastswhere the chlorophyll pigment is located. The crosssection of a leaf is shown in Fig. 1(a), where the chloro-plasts are readily seen located along the walls of theparenchyma cells comprising the mesophyll or middle
D. M. Gates is with the National Bureau of Standards, Boulder,Colorado. H. J. Keegan, J. C. Schleter, and V. R. Weidner arewith the National Bureau of Standards, Washington, D.C.
Received 17 August 1964.This work was supported in part by the Advanced Research
Projects Agency, DOD.
section of the leaf. The parenchyma cells are filledwith cell sap and protoplasm. The cell structure ofleaves is strongly variable depending upon species andenvironmental conditions during growth. Most leaveshave a distinct layer of long palisade parenchyma cellsin the upper part of the mesophyll and more irregular-shaped, loosely arranged spongy parenchyma cells in thelower part of the mesophyll. The palisade cells tend toform in the portion of the mesophyll toward the sidefrom which the light enters the leaf. In most hori-zontal leaves the palisade cells will be toward the uppersurface, except in leaves which grow nearly vertical inwhich case the palisade cells may form from both sides.In some leaves the elongated palisade cells will be en-tirely absent and only spongy parenchyma will existwithin the mesophyll.
The cellular structure of the leaf is large comparedto the wavelengths of light. Typical cell dimensionswill be 15 y X 15 A X 60 y for palisade cells and 18M X 15 A X 20 , for spongy parenchyma cells. Theepidermal cells are of the same order of dimension as thespongy parenchyma cells, and these have a thin waxycuticle overlay which is highly variable in thicknessbut often is only 3-lu to 5-,u thick. Clements 3 hasgiven an excellent discussion of the physical dimensionsand relationships of leaf structure. The chloroplastssuspended within the cellular protoplasm are generally5 ,u to 8 ,u in diameter and about 1 ,4 in width. As manyas 50 chloroplasts may be present in each parenchymacell. Within the chloroplast are long slender strandscalled grana within which the chlorophyll is located.The grana may be 0.5 /i in length and 0.05 ju in diameter.Clearly, the grana are of the dimension of the wave-length of light and may produce a considerable scat-tering of light entering the chloroplast. The chloro-plasts are generally more abundant towards the upperside of the leaf in the palisade cells and hence accountfor the darker appearance of the upper leaf surfacecompared with the lower lighter surface.
January 1965 / Vol. 4, No. 1 / APPLIED OPTICS 11
Leaf anatomies typically have a great deal of openstructure in the form of intercellular spaces, whichcontain moisture-saturated air. The materials of theleaf which are important from the standpoint of lightand radiation are: cellulose of the cell walls, watercontaining solutes (ions, small and large molecules suchas protein and nucleic acid) within the cells, and inter-cellular air spaces and pigments within the chloro-plasts. The pigments generally found in chloroplastsare chlorophyll (65%), carotenes (6%), and xantho-phylls (29%) although the percentage distribution ishighly variable. Chlorophyll a and chlorophyll bare most frequent in higher plants, but altogether aboutten forms have been identified, each with a uniqueabsorption spectrum. The role in photosynthesis ofpigments other than chlorophyll has been questioned,but there is evidence that energy transfer can takeplace from accessory pigments to chlorophyll, seeFrench and Young4 who have given a review of thespectra of photosynthetic pigments.
Radiant energy interacts with the leaf structure byabsorption and by scattering. The energy absorbedselectively at certain wavelengths by chlorophyll willbe converted into heat or fluorescence, and convertedphotochemically into stored energy in the form oforganic compounds through photosynthesis. The ab-sorption spectra of chlorophyll a, chlorophyll b, a-carotene, lutein (xanthophyll), and liquid water areshown in Fig. 1 (b). Chlorophyll a is found in allphotosynthesizing plants and chlorophyll b in mostplants but not all. It should be noticed in Fig. 1(b)that the predominant pigments absorb in the sameregion, in the vicinity of 445 mg (22,500 cm-') in theblue, but only chlorophyll absorbs in the red in thevicinity of 645 m (15,500 cm-'). Liquid waterabsorbs strongly in the far infrared at wavelengthsgreater than 2.0 A (5000 cm-') and only weakly atshorter wavelengths. The materials comprising leavesare moderately transparent in the green, around 540mut (18,500 cm-'), and highly transparent in the nearinfrared from 700 mu (14,300 cm-') to nearly 2.0 (5000 cm-').
Plants are magnificently adapted to the radiationenvironment in which they live. Figure 1 (c) shows thespectral distribution of the energy of sunlight, cloudlight, sky light, and light transmitted through vege-tation plotted as a function of wavenumber in cm-'.The wavenumber is the reciprocal of the wavelengthand is proportional to the frequency of the radiation.The advantage of a wavenumber plot rather than awavelength plot is that the full frequency span of thesolar radiation distribution is readily accommodatedby the graph without the long wavelength tail beingcut off. The infrared part of the spectrum is not givena disproportionate amount of graph. The ultravioletportion of the spectrum is terminated by the atmo-spheric absorption and hence the graph need not extendindefinitely on the high-frequency end. The energyscale used in Fig. 1(c) is energy per unit area per unit
time per wavenumber increment. It will be noticedthe solar energy distribution has its peak at 1.0 A inthe infrared and approximately 50% of the energy re-ceived from the sun is in the infrared beyond 0.7 (14,300 cm-').
Previous Spectral Measurements
A relatively small amount of research has been doneon the spectral properties of plants and most of thatwork has concerned itself with the visible and very nearinfrared portions of the spectrum. Only a few pri-mary references will be mentioned here and these willcontain extensive reference to earlier work. Rabideauet al.5 using an Ulbricht integrating sphere gratingspectrophotometer reported on the absorption and re-flection spectra of leaves and chloroplasts. Some of theearliest work was done by Shull6 and by McNicholas.7
Clark8 reviews most of the earlier work pertaining tothe photographic region of the infrared. Krinov9
reviews a large volume of data collected by himself andthe Russian workers including field data pertaining tostands of vegetation, and a more recent paper byKleshnin and Shulgin 0 is important. Interestingpapers concerned with some of the ecological implica-tions of plant reflectivity were by Billings and Morris"and by Obaton. 12 The visible spectral property ofleaves was also investigated by Moss and Loomis."'The infrared specular reflection spectrum from 1.5 uto 25 u of many plants was reported by Gates andTantraporn. 14 They showed that the far-infraredreflectance was generally less than 5% for 650 angle ofincidence and less than 3% for 20 incidence angle.At far-infrared wavelengths the upper surface reflectsmore than the lower, old leaves more than young, andshade leaves more than sun leaves. In each instancethe inverse is true in the visible and near infrared.
Recently, Kuiper" has studied the action spectrum ofstomatal movement and shown that maximum openingwas caused by light of 432 myu and 675 mu. Stomataremained closed for light of 525 mu to 580 mu. Theaction spectrum showed photosynthesis within guardcell chloroplasts to be responsible for maintenance ofstomatal opening. The action spectrum of stomatalmovement is very similar to the action spectrum ofphotophosphorylation by ATP-formation within theguard cells.
Plant pigments fluoresce, a