The fraction of the global radiation incident on the upper surface of leaves of Tussilago farfara L. which is reflected, varies according to the presence or absence of hairs on the lower surface. Calculations of the radiation fluxes at 550 and 1000 nm wavelengths prove that these differences arise from a partial reflection of the radiation emerging from the lower epidermis by the hair cove

ELLER, BENNO M.

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Journal of Experimental Botany, Vol. 28, No. 105, pp. 1054-1059, August 1977Leaf Pubescence: The Significance of Lower SurfaceHairs for the Spectral Properties of the Upper SurfaceBENNO M. ELLERInstitute, of Plant Biology, University of Zurich, Zollikerstrasse 107, CH 8008 Zurich,SwitzerlandReceived 28 February 1977ABSTRACTThe fraction of the global radiation incident on the upper surface of leaves of Tussilago farfaraL. which is reflected, varies according to the presence or absence of hairs on the lower surface.Calculations of the radiation fluxes at 550 and 1000 ran wavelengths prove that these differ-ences arise from a partial reflection of the radiation emerging from the lower epidermis by thehair cover.INTRODUCTIONOne of the major problems of leaf ecology is the influence of pubescence on the radia-tion budget. A recent review by Johnson (1975) on the ecological significance of leafpubescence reveals the lack of sufficient information to lay down a definitive theoryon the interactions between leaf pubescence and radiative energy exchange. Ingeneral leaves covered with a hairy layer have a higher reflectivity and a lower ab-sorptivity compared with hairless leaves as is shown by Shull (1929), Billings andMorris (1951), Birkebak and Birkebak (1964), Sinclair and Thomas (1970),Ehleringer, Bjoerkman, and Mooney (1976), and Eller and Willi (1977). A distinctecological group among the leaves is formed by those with hairs restricted to thelower surface. Comparing the spectral properties of the upper and the lower surfaceof such leaves cannot give the real differences caused by pubescence, since thevalues for the upper surface differ markedly from that of the lower surface with thehairs removed as shown by Eller and Willi (1977). Experiments carried out recentlyby Eller and Willi (1977) with leaves of Tussilago farfara L. (coltsfoot) led to thesupposition that the hairs on the lower surface of the leaves might increase themeasured reflectivity of the upper surface. Additional research reported here veri-fied this hypothesis.MATERIALS AND METHODSThe diffuse reflectivity (r) and the diffuse transmissivity (t) for radiation incident on the uppersurface were determined by means of a spectroradiometer ISCO SR with an integrating sphereof special design and a light source as described by Eller (1972). The reflectivity is defined asthe quotient of the incident radiation and the fraction of this radiation which is measured asEUer—Spectral Properties of Hairy Leaves 1055the radiation coming from the surface. The transmissivity is the quotient of the incident radia-tion and that fraction of this radiation which emerges from the reverse side of the leaf. The ab-sorptivity (a) is calculated from the equationa=l-(r (1)Measurements were made for wavelengths from 400 to 1350 nm. For comparison the hairs onthe lower surface were removed by tearing them off with making tape. After the measurementswere carried out the epidermis of the lower surface was inspected with a scanning electronmicroscope to ascertain that the epidermis had not been damaged.RESULTSIn Fig. 1 the spectral properties of the upper surface are shown for a hairy and ahairless lower surface. Within the wavelength range from 400 to 700 nm the valuesfor the absorptivity differ only in the region of the green where absorptivity is ata relative minimum (Gates, Keegan, Schleter, and Weidner, 1965). For wavelengthsfrom 700 to 1150 nm removing the hairs alters the reflectivity as well as the trans-missivity. No effect is produced upon the absorptivity since these changes are of thesame magnitude and opposite in direction. For wavelengths beyond 1150 nm a slightincrease of the absorptivity is observed for the leaf with hairs on the lower surface.400 500 1350Wavelength (nm)FIG. 1. Spectral properties of the upper surface of an adult leaf of Tutsilago farfara L. withhairs on the lower surface ( ) and the hairs removed ( ).191056 EUer—Spectral Properties of Hairy LeavesI t can be assumed that the hairs on the lower surface act as a reflecting layer. In thepresence of hairs the transmitted radiation emerging from the lower epidermis ispartially reflected back to the leaf and the remainder is the transmitted radiation.Some minor part of this radiation might be absorbed by the hairs. The radiation re-flected, back into the leaf makes its way once more through the different leaf tissues.Depending on whether or not the absorptivity at a specific wavelength is high or low,a major or a minor part of this reflected radiation is absorbed while the rest emergesfrom the upper epidermis. The latter part of the radiation will increase the amount ofthe radiation coming from the upper surface and which is measured as the so-calledreflected radiation. The directly reflected radiation from the upper surface cannot bedetected separately from the radiation which leaves the epidermis after a two-foldtransmission through the leaf following a reflection at the hairs on the lower surface.As shown by Eller and Willi (1977) the spectral reflectivities of the lower surfaceare almost uniformly increased by hairs within the wave range observed. A wave-length-independent reflection power of the white hair felt can be assumed and auniform influence on the measured spectral reflectivity of the upper surface mightbe suggested. This is evidently not the case as is shown by Fig. 1. The reasons whyno changes can be detected on the upper surface 'reflectivity' up to a wavelength ofabout 700 nm are (1) because the values for the transmitted radiation within thatwave range are very low and (2) that the small amount of radiation which could bereflected by the hair cover on the lower surface is absorbed on its way back throughthe leaf. Furthermore, fractions below 0-5% of the incident radiation cannot bedetected by the equipment used for the measurements. If the amount of the trans-mitted radiation increases, more radiation is reflected by the hair cover and thusthe changes become measurable. Alterations in the spectral properties which arederived from these radiation measurements can then be observed, as can be seen forthe wavelengths at about 550 nm and beyond 1150 nm. The fraction of the radiationtransmitted by a hairless leaf reflected by the hair cover can be estimated by check-ing the differences for the transmission in the presence and the absence of lowersurface hairs.At wavelengths from 700 to 1150 nm no changes in the absorptivity are detectablesince the absorptivities of the leaf are very low. Hence nearly the whole radiationreflected by the hairs on the lower surface emerges from the upper epidermis andincreases the values for the reflectivities of the upper surface.MODEL OF RADIATION FLUXES AND CONCLUSIONSA model of the radiation fluxes at and through the leaf is shown in the lower part ofFig. 2. The incident radiation (/) is partly reflected (R$) at the upper surface withthe reflectivity ry measured without hairs on the lower surface. The remainder (7L)of the incident radiation penetrates into the leaf and is partly absorbed (Ay) withthe absorptivity a^ measured without hairs on the lower surface and the rest (7H)forms the input into the hair layer. If the hairs are removed this radiation 7H is theradiation Tj$ transmitted by the leaf. For the intact leaf with hairs, this radiation issplit up into the radiation (/LL) reflected back to the leaf and the radiation transmit-ted (TH)- The radiation ZLL penetrates into the leaf and on its way through the leafEUer—Spectral Properties of Hairy LeavesI (XL.105710000-5 n8-5 54-513-5Hai^sWavelengthLeaf / H V /FIG. 2. Leaf model for TussUago farfara L. with hairs on the lower surface (right) and thehairs removed (left). Radiation fluxes at 660 and 1000 nm wavelength. I, incident radiation;J?N. reflected radiation without hairs; /L> radiation penetrating into the leaf; As, absorbed andTn, transmitted radiation without hairs; / H , radiation emerging from the lower epidermis;I I I , radiation reflected by the hair cover; TH, radiation transmitted by the hairy leaf; Ri, totalof the reflected and AT, total of the absorbed radiation; i?H. additional radiation emerging fromthe upper epidermis; AH, absorbed fraction of the reflected radiation ZLL.tissues a part (^4H) is absorbed with the absorptivity value OL measured for the hair-less lower surface. The remainder forms the radiation Bs emerging from the uppersurface. The absorbed and the reflected radiation for the leaf with hairs are Ai andJ?T respectively.Only the spectral properties and the radiation fluxes of both surfaces with hairsor hairless are measurable. The values of 7LL, A&, and Rs. are determinable only bycalculations. To verify the influence of the hair-covered lower surface upon themeasured reflectivity of the upper surface, calculations of the splitting of the inci-dent radiation into the different radiation fluxes can be made. The wavelengths atwhich such calculations should be performed are 550 and 1000 nm respectively. At550 nm the absorptivity of the leaf is high and a difference between its values with ahairy or a hairless lower surface is detectable. At 1000 nm no differences are presentand the absorptivity of the leaf is low. The equations to calculate the values notmeasurable areAs = I - {Ex + TJS) (2)Ti,-Ts (3)1058 EUer—Spectral Properties of Hairy Leaves(4)(5)A? = As + An (6)By = _RN + £ H (7)AT = I - (RT + Ts). (8)As shown in a detailed review of the spectral properties of juvenile and adult leavesof Tussilago farfara L. by Eller and Willi (1977) the values for «N and «L at thewavelengths selected for the calculations are nearly equal and the value of ON canbe taken for ax, to carry out the calculations. The values for the reflected radiationand for the reflectivity expressed as percentages are the same if a value of 100 istaken for the incident radiation. The same is valid for the transmitted radiation andthe transmissivity and also for the absorbed radiation and the absorptivity.On the left of Fig. 2 the measured values for the leaf without hairs on the lowersurface are shown. Substituting these values in the equations (2) to (7) we can cal-culate the radiative fluxes which cannot be measured in a direct way. On the rightof Fig. 2 the measured and the calculated values are displayed for the leaf with hairs.The absorptivity A^ calculated with the aid of equation (6) is the same as might bederived from measurements and using equation (8). The calculated reflectivity R?is equal to the measured reflectivity. These coinciding values give the proof that thedifferences for the reflectivity of leaves with hairs present or absent from the lowersurface arise from the reflection of a part of the radiation by the hair layer. Further-more, they are affected by a more or less strong absorption of this radiation on itsreturn through the leaf. If the absorptivity is high, as it is at 550 nm wavelength, thenonly a small fraction of that reflected radiation contributes to an increase of theradiation coming from the upper surface as 'reflected' radiation. At 1000 nm wave-length the absorptivity is low and therefore a much greater increase of the 'reflec-tivity' of the upper surface is the result. The same relations might be shown forevery wavelength.I t must be emphasized that hairs on the lower surface do not affect the absorp-tivity to an extent that could be significant for the determination of the amount ofabsorbed energy of global radiation.Moreover, this work shows the extent to which the measured values of the re-flectivity might be miscalculated if insufficient care is taken to avoid any directincident radiation or reflected radiation falling on the reversed surface of the leafwhen the reflected radiation of the upper surface is measured. Perhaps some of thevariable results on the reflectivities of leaves have been caused by such stray light.ACKNOWLEDGEMENTSThis research work was supported by the Swiss National Foundation grant3.745.72.1 thank Dr. R. Parish for his help in the preparation of the manuscript.EUer—Spectral Properties of Hairy Leaves 1059LITERATURE CITEDBILLINGS, W. D., and MORRIS, R. J., 1951. Am. J. Bot. 38, 327-31.BISKEBAK, ROLAND, and BULEEBAK, RICHARD, 1964. Ecology, 45, 646-9.EHLEBINGER, J., BJOEBKSIAN, O., and MOONEY, H. D., 1976. Science, N.T. 192, 376-7.ELLEE, B. M., 1972. Verh. Schweiz. Naif. Gee. 152, 142-5.and WILLI, P., 1977. Oecologia 25, 179-87.GATES, D. M., KEEGAN, H. J., SOHLETEB, J. C. and WETDNEB, V. R., 1965. Appl. Opt. 4,11-20.JOHNSON, H. B., 1975. Bot. Rev. 41, 233-58.SHULL, CH. A., 1929. Bot. Gaz. 87, 583-607.SmoLAiB, R., and THOMAS, D. A., 1970. Aust. J. Bot. 18, 261-73.

Leaf Pubescence: The Significance of Lower Surface Hairs for the Spectral Properties of the Upper Surface

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