EFFECT OF THE CANOPY STRUCTURE ON THE CHLOROPHYLL FLUORESCENCE YIELD AT CANOPY SCALE

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1 EFFECT OF THE CANOPY STRUCTURE ON THE CHLOROPHYLL FLUORESCENCE YIELD AT CANOPY SCALE C. Rhoul (1), Y. Goulas (1), F. Daumard (2), A. Ounis (1), I. Moya (1) (1) Laboratoire de météorologie dynamique, École polytechnique, Palaiseau France (2) Institut national de la recherche agronomique, Thiverval-Grignon France ABSTRACT The measurements of a fluorescence yield at canopy level with passive method is complex due to the interaction of the incident solar radiation and the leaves emitted fluorescence with the cover. Practically, the canopy fluorescence yield is approached with yield indexes (as apparent spectral fluorescence yield (ASFY) or fluorescence fraction (FF)) calculated with the top of canopy measured fluxes. By means of canopy fluorescence simulation, this theoretical study purpose to compare the canopy fluorescence yield with these yield indexes calculated from the simulated fluxes for different canopy structures (varying LAI and leaf angle distribution). Results show that, depending on the canopy structure, the indexes can be more or less reliable. Especially, the indexes well represent the canopy fluorescence yield when the canopy is dense with quite horizontal leaves. Keywords: fluorescence diurnal cycle, fluorescence yield, canopy structure, fluorescence index, passive measurement, fluorescence model, FluorSAIL. 1 INTRODUCTION At leaf scale, the fluorescence quantum yield is well known and measured. It is defined as the ratio between the leaves emitted fluorescence and the absorbed incoming light. Fluorescence quantum yield is the parameter linked to the plant photosynthetical activity ([1]). Monitoring the dynamics of the fluorescence yield can give information on plant physiological state especially by monitoring the diurnal cycle dynamic ([2] [5]). Active measurements permit to measure this yield at leaf scale. In this case, the fluorescence is induced by a known source of light and we measure the fluorescence generated by this source to go back up to the fluorescence yield. One would like to make use of the relationship between fluorescence yield and plant physiological state at canopy scale in the aim of monitor it at global scale. However, measuring the yield from space is impossible with active measurement. Indeed, it would require a huge powerful excitation source and pose a problem of security. To measure on larger scales, one uses passive technics where the measured fluorescence is induced by the Sun. Fluorescence fluxes are hidden in the canopy reflected light and the issue is to extract this weak signal (< 2 %). Towards this goal, one method is the Fraunhofer lines infilling that consists in measuring the fluorescence in a spectral range where the solar flux is weak as in Fraunhofer lines (for instance in O 2-A and O 2-B band on which we will focus on). The fluorescence reduces the depth of these absorption bands. Comparing the band depth between the vegetation and a non-fluorescence reference permits to retrieve the fluorescence fluxes. In the case of active measurements, the excitation source is well characterized, which permit to calculate the fluorescence yield. In the case of passive measurements, we do not know exactly the total emitted fluorescence and the absorbed solar flux to calculate a yield. These fluxes are not easily measurable, which forces to use canopy fluorescence yield indexes calculated from the available flux measurements. A commonly used proxy is the apparent spectral fluorescence yield (ASFY) where the top of canopy (TOC) fluorescence flux is normalized by the PAR. But other authors prefer to normalize the fluorescence flux by a wavelength that is well absorbed by the canopy leaves as the fluorescence fraction (FF). These indexes approach the canopy fluorescence yield but the variations of illumination conditions (ratio between direct and diffuse incoming radiation, angle of incidence), because of the complexity of the canopy structure, result in an alteration of the diurnal cycles of these indexes which is not representative of physiological changes in plants. The aim of this study is to estimate the magnitude of variations in the diurnal cycle of yield indexes that are not due to changes in plant physiological state. To this end, we used the canopy radiative transfer model FluorSAIL ([6]) to simulate for various canopy structure, the diurnal cycles of the canopy fluorescence yield and the yield indexes. We used also the simulation to output some physical quantities as PAR, APAR to understand how the structure can influence the diurnal shape of the indexes. 2 MATERIALS AND METHODS 2.1 Canopy Fluorescence yield At leaf scale, the fluorescence quantum yield is easily definable. By analogy, at canopy level, the fluorescence yield can be defined as the total fluorescence emitted by the leaves (before reabsorption within the canopy) 1

2 divided by the incoming light absorbed by the leaves. It is equal to φ c (λ) = F e(λ) APAR, (1) with φ c (λ) the canopy fluorescence yield at the wavelength λ, F e (λ) the total emitted fluorescence at λ and APAR the absorbed PAR (photosynthetically active radiation). The PAR is the top of canopy solar irradiance integrated between 400 nm and 700 nm. 2.2 Canopy Fluorescence Yield Proxies Practically, passive measurements are done on TOC. In this case, one accesses to the TOC fluorescence fluxes in the direction of measurement (F TOC (λ)) that is different from the total fluorescence emitted by leaves (F e (λ)) because of the reabsorption of the fluorescence within the canopy and the canopy anisotropy. To approach the canopy fluorescence yield, some authors introduce an apparent spectral fluorescence yield (ASFY) ([7]) by dividing the measured fluorescence fluxes in the direction of observation by the photosynthetically active radiation (PAR) measured TOC. ASFY(λ) = π F TOC(λ). PAR (2) We will focus only on ASFY(687 nm) and ASFY(760 nm) that correspond to the O 2-A and O 2-B absorption bands. We will call them ASFY687 and ASFY760 respectively. It is interesting to study these two wavelengths because they match roughly with two pics of the fluorescence spectrum and their radiative transfer within the canopy are totally different. 687 nm is well absorbed by leaves while 760 nm is not. This permits to study two different radiative transfer processes within the canopy. Other authors normalize the fluorescence fluxes by a physical quantity accounting for the absorption of solar incident light, that is to say a wavelength well absorbed by the canopy that is a priori more representative of the APAR than the PAR. This kind of index of canopy fluorescence yield is the fluorescence fraction, FF ([8]), defined as follows: FF(λ) = F TOC(λ) L(λ 0 ), (3) with L(λ 0 ) the vegetation radiance at λ 0, a wavelength that is well absorbed by leaves. For the same reason explaining the choice of 687 nm and 760 nm for ASFY, we choose to focus our study on FF687 and FF Comparison between Canopy Fluorescence Yield and Indexes As is shown by the definitions of φ c (λ) and canopy fluorescence yield indexes (ASFY and FF) (equations (1), (2) and (3)), the actual fluorescence yield and the indexes are not exactly equals. Indeed, the PAR does not represent accurately the APAR neither its diurnal variations. Also F TOC (λ) does not represents accurately F e (λ) neither its variations. The APAR is linked to the PAR by this relation: fapar = APAR PAR, (4) with fapar (fraction of absorbed photosynthetically active radiation) the fraction of absorbed light, between 0 and 1. The total emitted fluorescence flux and the measured TOC fluorescence flux are linked by this relation: τ c (λ) = F TOC(λ) F e (λ), (5) where τ c (λ) is the ratio of fluorescence that exits the canopy in the direction of observation. Combining the canopy fluorescence yield equation ((1)) and the τ c (λ) definition ((5)), we obtain the following development of the TOC measured fluorescence: F TOC (λ) = PAR fapar φ c (λ) τ c (λ). (6) The measured fluorescence flux is linked to the canopy fluorescence yield by three factors that depend on the canopy structure (fapar and τ c (λ)) and the illumination conditions (fapar, PAR and τ c (λ)). Note that unlike fapar, τ c (λ) depends also on the direction of observation. With the previous equation ((6)), ASFY ((2)) becomes: ASFY(λ) = fapar τ c (λ) φ c (λ) π. (7) With equation (6), the FF equation ((3)) can be rewritten similarly: FF(λ) = ASFY(λ) PAR π L(λ 0 ) = ASFY(λ) π FF(λ) = fapar τ c (λ) 1 1 ρ λ0 ρ λ0 φ c (λ), with ρ λ0 defined as the ratio between L(λ 0 ) and PAR that we will call the pseudo-reflectance of the canopy in the direction of observation at λ 0. FF is linked to φ c (λ) with the same two factors than ASFY but also with another factor dependent on the canopy structure and direction of observation: 1 ρ λ0. The interest of dividing (8) 2

3 by L(λ 0 ) in the FF definition is to compensate the fapar diurnal variation. 2.4 Modelling of Fluorescence Diurnal Cycle FluorSAIL, based on the turbid model SAIL ([9]), describes the radiative transfer within the canopy taking into account the fluorescence. It takes as input parameter the soil reflectance, the leaves reflectance and transmittance, the sun position, the fraction of direct and diffuse radiation from the sun and two canopy structure parameters. These canopy structure parameters are the LAI and the eccentricity parameter of an elliptic representation of the leaf angular distribution within the canopy (χ). In order to study the influence of the canopy structure on the diurnal variations of fluorescence yield indexes, these two parameter were varied as follows: 20 values between 0.5 and 10 for the LAI and 19 logarithmically spaced values between 0.1 and 10 for χ. For a total amount of 380 simulated canopies, very dense ( LAI = 10), very sparse ( LAI = 0.5 ), planophile ( χ = 10), erectophile (χ = 0.1), with intermediate values and all the possible combinations of these two parameters were used. The incoming solar radiance was calculated with MODTRAN4 ([10]), an atmospheric radiative transfer model. To simulate the canopy reflectance in the direction of observation, we used the canopy radiative transfer model SAIL with a hotspot function implemented. The latter taking as input the leaves reflectance and transmittance that we simulated with PROSPECT model ([11]). The fluorescence excitation spectra of the leaves were calculated with FluorMODleaf ([12]). Gathering all these models generated the canopy fluorescence yield, the indexes and physical quantities as fapar and φ c. To calculate the leaves transmittance, reflectance and quantum yield, we took common values of following parameter: Symbol σ II Value N 1.5 C ab C w C m C cx 0.5 τ I τ II σ I 33 μg cm cm g cm 2 8 μg cm ns 0.5 ns Table 2. PROSPECT and FluorMODleaf configuration used for modelling. Our simulation involve three direction of observation: with a zenithal angle of 30 directed to the south, to the north and at nadir. We show only the nadir direction of observation which is the commonest used in remote sensing. 2.5 Studying Diurnal Variability of Indexes and Other Physical Quantities The chlorophyll fluorescence yield at leaf scale was kept constant during the simulated diurnal cycles. The excitation spectrum of chlorophyll fluorescence depending on wavelength, variations of solar spectrum shape during the day (due to diffuse and direct ratio variations) involve diurnal variations of the canopy fluorescence yield. These variations are small (within 3 ) during the day, thus, to look if indexes diurnal cycle shape was close to the canopy fluorescence yield diurnal cycle shape, we only looked at their diurnal constancy. In order to explore the influence of LAI and χ on the diurnal cycle shape of indexes and other physical quantities, we plotted versus LAI and χ the relative diurnal variation of the quantity Q (ΔQ) calculated as follows: ΔQ = Q(12: 00) Q(8: 00) Q(8: 00) + Q(12: 00) (9) where Q can be ASFY687, ASFY760, FF687, FF760, fapar, τ c 687, τ c 760 and ρ 685, calculated at noon and at 8:00. 3 RESULTS 3.1 Diurnal Cycles Diurnal Cycle of Fluorescence Figure 1 shows the PAR and the fluorescence diurnal cycles at 687 nm for three different canopies. The fluorescence fluxes increase before noon and decrease after, following roughly the PAR variations. The slopes of the increase and the decrease depend on canopy structure: they decline with erectophilia. Around noon there can be different shapes: concave, flat or convex, depending on canopy structure. Indeed, the erectophilia makes the shape around noon diminished. In addition, the shape of the fluorescence diurnal cycle changes with the direction of observation (not shown). 3

4 passive measurements, L(λ 0 ) is chosen among available channels. When measuring in an absorption band (in our case at 687 nm), a channel gives the off-band radiance of the vegetation (in our case at 685 nm). This wavelength can be used as λ 0 if it is well absorbed by leaves. The graphic of Figure 3 shows the diurnal cycle of FF687 and FF760 for three canopies with various χ. Figure 1. Diurnal cycle of PAR (dark thick line) and fluorescence flux at 687 nm for planophile (light green; χ = 3.6), spheroidal (blue; χ = 1) and erectophile (dark brown; χ = 0.3) canopies with LAI = Diurnal Cycle of ASFY Figure 2 shows the ASFY diurnal cycle calculated at 687 nm and 760 nm for three kinds of canopy. When calculating the ASFY, the greater value of fluorescence around noon is diminished because of dividing by the PAR that presents also a greater value around noon. So, the bell shape of the diurnal fluorescence curve is roughly cancelled when calculating the ASFY. But depending on the canopy structure, the ASFY diurnal curve can even vary. Figure 3. Diurnal cycle of FF687 (solid line) and FF760 (dashed line) for planophile (light green; χ = 3.6 ), spheroidal (blue; χ = 1) and erectophile (dark brown; χ = 0.3) canopies with LAI = 3. We can see that the FF has a concave shape around noon that growth with erectophilia and a plateau for planophile canopy. In addition, as for ASFY, the mean value of FF decrease with erectophilia and the mean value at 760 nm is greater than at 687 nm Diurnal Cycle of fapar, τ c and ρ 685 During the day, for LAI = 3, the fapar is concave for erectophile canopies and nearly constant for planophile canopies (Figure 4). It can reach roughly 0.96 for a planophile canopy with large LAI (χ = 10; LAI = 10) and can come down to 0.2 at noon for an erectophile canopy with very low LAI (χ = 0.1; LAI = 0.5). Figure 2. Diurnal cycle of ASFY687 (solid line) and ASFY760 (dashed line) for planophile (light green; χ = 3.6 ), spheroidal (blue; χ = 1 ) and erectophile (dark brown; χ = 0.3) canopies with LAI = 3. Compared with fluxes, ASFY diurnal cycle shows less variations, but the remaining variations depend on the canopy structure. We can also see that the mean value of the diurnal cycle depends on the canopy structure. The more the canopy is erectophile, the less this mean value is. Furthermore, ASFY760 is greater than ASFY Diurnal Cycle of FF We choose to use 685 nm for the value of λ 0 in the definition of FF687 and FF760 ((3)). Practically, in Figure 4. Diurnal cycle of fapar for planophile (light green; χ = 3.6 ), spheroidal (blue; χ = 1 ) and 4

5 erectophile (dark brown; χ = 0.3) canopies with LAI = 3. If we have a look at τ c variations (Figure 5)., we can see that for a fixed LAI, the shape of the diurnal curve is convex for both erectophile and planophile canopies. The mean value of τ c decreases with erectophilia and is greater at 760 nm than 687 nm. The diurnal variation of the pseudo canopy reflectance at 685 nm, ρ 685, presents a particular shape (Figure 6). The shape is convex for a planophile canopy, for a spheroidal canopy the curve is lower and more convex and for an erectophile canopy the curve presents a bell shape centred around noon that goes above the planophile and spheroidal curves. In addition, the pseudo canopy reflectance diminishes with erectophilia in the morning. Figure 5. Diurnal cycle of τ c 687 (dashed line) and τ c 760 (solid line) for planophile (light green; χ = 3.6 ), spheroidal (blue; χ = 1) and erectophile (dark brown; χ = 0.3) canopies with LAI = 3. Figure 6. Diurnal cycle of ρ 685 for planophile (light green; χ = 3.6 ), spheroidal (blue; χ = 1 ) and erectophile (dark brown; χ = 0.3) canopies with LAI = Maps of Diurnal Variations Figure 7 to Figure 14 show ΔASFY, ΔFF, ΔfAPAR, Δτ c and Δ1/ρ 685 versus LAI and χ. The colour scale shows the relative diurnal variation. These maps show how much the indexes or physical quantities vary during the day, with a sign that shows if this variation is positive or negative between 8:00 and 12:00 ((9)) Maps of ΔASFY Figure 7 and Figure 8 show the diurnal variations of ASFY687 and ASFY760 when looking at nadir. If ΔASFY is low, for example 1%, the ASFY does not vary more than 1 % during the day and so its diurnal variation is close to the canopy fluorescence yield variation (3 ). On Figure 7, for erectophile canopies (χ < 1) with LAI < 4, the diurnal variations of ASFY687 are negative and we can see that the more the canopy is erectophile or the less the LAI is, the more the absolute value of ΔASFY687 is large in this area. For planophile canopies (χ > 1) with LAI > 4, ΔASFY687 is positive up to 6 %. Between these two area, there is one where ΔASFY687 is near 0 (the zone between 1 % and 1 % is delimited by thick line on the maps). On Figure 8 we can see roughly the same trends. For erectophile canopies with LAI < 6, the diurnal variations of ASFY760 are negative, and ΔASFY760 is positive up to 4 % for planophile canopies with LAI > 6. A similar zone with variations between 1 % and 1 % separates these two zones (highlighted with thick lines) Maps of ΔFF The maps of Figure 9 and Figure 10 show ΔFF687 and ΔFF760 respectively. They present a greater range of diurnal variation than for ASFY. At 687 nm, we can see a huge diurnal variation: ΔFF687 goes down to 70 % for erectophile canopies. For planophile canopies with LAI > 3, ΔFF687 is between 1 % and 1 %. At 760 nm, the same trend is observable but with an offset. For erectophile canopies, ΔFF760 goes down to 72 % and for planophile canopies stay between 4 % and 1 % Maps of ΔfAPAR, Δτ c and Δ1/ρ 685 Figure 11 shows ΔfAPAR versus LAI and χ. fapar is rather constant during the diurnal cycle for canopies with large LAI and χ. fapar does not vary neither when LAI is low and χ large or LAI large and χ low. fapar diurnal cycle presents a depression around noon as the LAI and χ decrease. This is represented by negative values on the map that decrease with LAI and χ decrease. Figure 12 shows Δ 1 ρ 685 versus LAI and χ. Its values are only negative. It means that the reflectance is always greater at noon than at 8:00. We can see that for 5

6 erectophile canopies, the reciprocal of the pseudo canopy reflectance diurnal variations become more negative with erectophilia: down to 78 %. For planophile canopies, Δ 1 ρ 685 goes up to 4 %. For erectophile canopies, when the LAI increases, Δ 1 ρ 685 decreases (until LAI = 5 when χ = 0.1 or until LAI = 2 when χ = 1). When the LAI keeps increasing, Δ 1 ρ 685 decreases. Figure 13 and Figure 14 show Δτ c versus LAI and χ for all the simulated canopies at 687 nm and 760 nm. Δτ c 687 is positive between 1 % and 9 %. Δτ c 760 is positive between 0.5 % and 7 %. Figure 7. ΔASFY687 versus LAI and χ. Figure 8. ΔASFY760 versus LAI and χ. Figure 9. ΔFF687 versus LAI and χ. Figure 10. ΔFF760 versus LAI and χ. 6

7 Figure 11. ΔfAPAR versus LAI and χ. Figure 12. Δ 1 ρ 685 versus LAI and χ. Figure 13. Δτ c 687 versus LAI and χ. Figure 14. Δτ c 760 versus LAI and χ. 4 DISCUSSION Maps sum up the results and show that with LAI > 6 or χ > 1, ASFY687 and ASFY760 represent the canopy fluorescence yield diurnal variation within 6 %. On the contrary, for LAI < 6 and χ < 1 ASFY687 and ASFY760 present diurnal variations between 0 and 18 %. φ c diurnal variation are represented by FF687 within 1 % for planophile canopies with LAI > 3. For the same canopies, FF760 represents φ c variations with 2 % to 4 %. On the contrary, for erectophile canopies or canopies with LAI < 3, FF687 and FF760 show diurnal variations between 0 and 70 %. These diurnal variations can be explained by looking at diurnal variation of fapar, τ c and 1/ρ fapar fapar is rather constant during the diurnal cycle for canopies with large LAI and χ. This means that the canopy absorbs the same fraction of PAR (fapar) regardless of the solar zenithal angle. Indeed, at noon, leaves present a maximal area to the Sun because of the planophilia of the canopy so the fapar is maximal. When the Sun is low, the canopy has a maximal fapar too, although the leaves present a lower projected area to the Sun, the projected canopy height compensate the lower leaves projected area. fapar does not vary neither when LAI is low and χ large nor LAI large and χ low. This is because in a case the reduction of LAI is compensate by the large χ and in the other case the reduction of χ is compensate by the 7

8 large LAI. Thus, the interception of sunlight by the canopy does not reduces when the zenithal angle of the sun is high. For erectophile canopies, when the zenith angle of the Sun decreases, the canopy intercepted light decreases because more soil intercepts this light. This explains the depression around noon as χ decreases and the negative values of fapar variation on Figure 11. The diminution of LAI gives similar trend to fapar variation because the more the LAI decreases, the less the interception of sunlight is important around noon. 4.2 τ c Figure 5 shows that the diurnal mean value of τ c 760 is higher than the one of τ c 687. This is due to the lower absorption of light at 760 nm than at 687 nm. Indeed, the higher leaves transmittance and reflectance at 760 nm makes the fluorescence going out of the canopy easier. We can also see that the erectophilia diminishes this mean value. Indeed, the fluorescence emission being considered lambertian in the model, when the sunlight goes deeply in straight line within the canopy it does not go out of the canopy easily. The fact that Δτ c is positive for both 687 nm and 760 nm (Figure 13 and Figure 14) means that around noon, the emitted fluorescence goes out of the canopy more easily than in the morning. at 760 nm It does not depend significantly on leaf angle neither LAI. This can be explained by the fact that at this wavelength, being less reabsorbed, the fluorescence travels more than at 687 nm, thus the canopy seems more homogenous. 4.3 Δ1 ρ 685 Figure 6 shows that in the morning ρ 685 diminishes with erectophilia. This is due to the nadir direction of observation. Indeed, the observer does not see the illuminated part of leaves if the canopy is erectophile. For planophile canopies, even if the leaves are horizontal and the light oblique, the observer sees the illuminated part of leaves which make ρ 685 higher. ρ 685 increases with erectophilia at 12:00, because around noon more soil is illuminated and viewed at nadir than for planophile canopies. The soil reflectance is about 20.3 % at 685 nm which is higher than leaves reflectance at this wavelength. This increase of ρ 685 for erectophile canopies is also shown by Figure 12 by the increase of the absolute value of Δ1 ρ 685 with erectophilia. The diminution of Δ1 ρ 685 and the reversion of this diminution when LAI increases can be explained by the fact that the diminution of the morning value of ρ 685 with LAI is faster than the one at noon, but this diminution slows down for lower LAI than for noon reflectance inducing a reversion of the trend of the relative difference Δ1 ρ ASFY and FF Diurnal variations of ASFY and FF can be explained by the observed variation of fapar, τ c and 1 ρ 685. As a reminder, equation (7) and (8) say So we have: and ASFY = fapar τ c φ c, FF = fapar τ c ρ 685 φ c. ΔASFY ΔfAPAR + Δτ c + Δφ c (10) ΔFF ΔfAPAR + Δτ c + Δ1/ρ Δφ c (11) Thus, for example equation (10) says that if we add Figure 11 (ΔfAPAR) and Figure 13 (Δτ c 687), we obtain a graph very close to Figure 7 (ΔASFY687). This means, in this example, that the thin band where ΔASFY687 is between 1 % and 1 % (Figure 7) is due to the compensation of Δτ c 687 positive values and ΔfAPAR negative values that are opposite. As another example, if we look at Figure 9 for ΔFF687, we can see the same effect. The large negative values of Δ 1 ρ 685 for erectophile canopies induce large negative values of ΔFF687 for these canopies. For planophile canopies with large LAI, the 6 % of Δ 1 ρ 685 is compensate by +6 % of Δτ c 687 that result in close to null value for ΔFF687 for these canopies. 5 CONCLUSION The effects of structure are very important on the radiative transfer of fluorescence and incoming sunlight especially for erectophile canopies. Increasing the LAI can reduce these effects. In the case of ASFY, erectophile canopies with LAI < 6, make the diurnal cycle shape of this index differ from the one of the canopy fluorescence yield. For other sort of canopy, ASFY diurnal variations are close to the canopy fluorescence yield variations (within 7 %). For canopies with large LAI and χ, the diurnal variations of FF687 are the same as the canopy fluorescence yield (within 1 %). For FF760, the canopy fluorescence yield is well represented too (within 4 %) for planophile canopies with large LAI. However, for erectophile canopies, both FF687 and FF760 present diurnal 8

9 variations that do not correspond to the canopy fluorescence yield variations. To conclude, FF represents better than ASFY the diurnal variations of the fluorescence yield but in a very restrictive area of LAI and χ. If we look at the absolute mean value of ΔFF, it is much worse than ΔASFY. The difference between 687 nm and 760 nm is also noticeable. For ASFY, the area of well representation of the fluorescence yield diurnal variations is a bit larger at 687 nm. For FF, at 687 nm, the representation of the fluorescence yield variations is a bit better than at 760 nm. In the case of planophile canopies with large LAI, the radiative transfer is close to that of a big leaf, thus, there is almost no diurnal variation induced by the canopy structure for these two indexes. So, to interpret the diurnal cycles of canopy yield indexes, it is important to well characterize the canopy structure to avoid confusion between structure effects and vegetation physiological status on diurnal cycle of indexes. 6 REFERENCES [1] G. C. Papageorgiou and Govindjee, Chlorophyll a Fluorescence: A Signature of Photosynthesis. Springer, [2] Z. G. Cerovic, Y. Goulas, M. Gorbunov, J.-M. Briantais, L. Camenen, and I. Moya, Fluorosensing of water stress in plants: Diurnal changes of the mean lifetime and yield of chlorophyll fluorescence, measured simultaneously and at distance with a τ- LIDAR and a modified PAM-fluorimeter, in maize, sugar beet, and kalanchoë, Remote Sens. Environ., vol. 58, no. 3, pp , Dec [3] A. Rosema, J. F. H. Snel, H. Zahn, W. F. Buurmeijer, and L. W. A. Van Hove, The Relation between Laser-Induced Chlorophyll Fluorescence and Photosynthesis, Remote Sens. Environ., vol. 65, no. 2, pp , Aug [4] J. Flexas, J.-M. Briantais, Z. Cerovic, H. Medrano, and I. Moya, Steady-State and Maximum Chlorophyll Fluorescence Responses to Water Stress in Grapevine Leaves: A New Remote Sensing System, Remote Sens. Environ., vol. 73, no. 3, pp , Sep [5] J. Flexas, J. M. Escalona, S. Evain, J. Gulías, I. Moya, C. B. Osmond, and H. Medrano, Steadystate chlorophyll fluorescence (Fs) measurements as a tool to follow variations of net CO2 assimilation and stomatal conductance during water-stress in C3 plants, Physiol. Plant., vol. 114, no. 2, pp , [6] W. Verhoef, Extension of SAIL to model solarinduced canopy fluorescence spectra, presented at the 2nd International Workshop on Remote Sensing of Solar Induced Vegetation Fluorescence, Canada: Montreal, [7] J. Louis, Z. G. Cerovic, and I. Moya, Quantitative study of fluorescence excitation and emission spectra of bean leaves, J. Photochem. Photobiol. B, vol. 85, no. 1, pp , Oct [8] F. Daumard, S. Champagne, A. Fournier, Y. Goulas, A. Ounis, J. F. Hanocq, and I. Moya, A field platform for continuous measurement of canopy fluorescence, Geosci. Remote Sens. IEEE Trans. On, vol. 48, no. 9, pp , [9] W. Verhoef, Light scattering by leaf layers with application to canopy reflectance modeling: the SAIL model, Remote Sens. Environ., vol. 16, no. 2, pp , [10] A. Berk, G. P. Anderson, L. S. Bernstein, P. K. Acharya, H. Dothe, M. W. Matthew, S. M. Adler- Golden, J. H. Chetwynd Jr, S. C. Richtsmeier, and B. Pukall, MODTRAN4 radiative transfer modeling for atmospheric correction, in SPIE s International Symposium on Optical Science, Engineering, and Instrumentation, 1999, pp [11] S. Jacquemoud and F. Baret, PROSPECT: A model of leaf optical properties spectra, Remote Sens. Environ., vol. 34, no. 2, pp , [12] R. Pedrós, Y. Goulas, S. Jacquemoud, J. Louis, and I. Moya, FluorMODleaf: A new leaf fluorescence emission model based on the PROSPECT model, Remote Sens. Environ., vol. 114, no. 1, pp ,