Báo cáo lâm nghiệp: " A comparison of the photosynthetic radiation response of Scots pine shoots in direct and diffuse radiation"

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Nội dung Text: Báo cáo lâm nghiệp: " A comparison of the photosynthetic radiation response of Scots pine shoots in direct and diffuse radiation"

A comparison of the photosynthetic radiation response of Scots pine shoots in direct and diffuse radiation T. Lahti 3 P. Oker-Blom 1 H. Smolander 2 ! University of Helsinki, Department of Silviculture, Unioninkatu 40 B, 00170 Helsinki, 2 Finnish Forest Research Institute, Suonenjoki Research Station, SF-77600 Suonenjoki, and 3 University of Helsinki, Botanical Museum, Unioninkatu 44, SF-00 170 Helsinki, Finland Introduction Materials and Methods The material consisted of 9, 1 yr old shoots col- The directional distribution of radiation lected from a young Scots pine stand. The net incident on a coniferous shoot has been rate of photosynthesis of the excised shoots shown to have a large effect on the rate of was measured in a direct and a diffuse (spheri- shoot photosynthesis (e.g., Zelawski et cal) radiation field, using an open flow IRGA- al., 1973). In a multidirectional radiation system (URAS 3G). The temperature in the assimilation chamber was 20°C, ambient C0 2 field, the irradiance on the needle surface concentration was 340 ppm and the air water area of a shoot becomes more evenly dis- vapor pressure deficit was 9 ± 1 mbar. tributed than in the case of a highly The distribution of radiation within each shoot directional field, and the rate of photosyn- simulated using a Monte Carlo method (cf. was thesis per unit of intercepted radiation Smolander et al., 1987) and using a model de- should logically be higher (cf. Oker-Blom, scribing shoot geometry based on certain mor- 1985). The aim of this study was to com- phological characteristics of the shoot (cf. Oker-Blom et al., 1983). Using the simulated pare the rates of photosynthesis of Scots distributions and assuming the photosynthetic pine (Pinus sylvestris L.) shoots in diffuse light curve for the photosynthetic unit to be a and direct radiation and to test a shoot Blackman type curve (c£ Oker-Blom, 1985), photosynthesis model based on the hypo- shoot photosynthesis was calculated as the integrated response of the photosynthetic units. thesis that shoot photosynthesis can be Parameters of the Blackman curve were esti- expressed as the integrated response of mated iteratively using the method of least the photosynthetic units of the shoot which squares to give the best fit between measured are assumed to have an invariant photo- and calculated photosynthesis for the shoot in synthetic light-response curve. direct radiation. * Present address: University of Georgia. School of Forest Resources, Athens, GA 30602, U.S.A. .eorqia ores,t esources.
In simulating the irradiance distributions, 2 length of the photon pathway within the needle different approaches were used. In the 1st before reaching the point under consideration. case, the photosynthetic units of the shoot were represented by needle surface area elements, i.e., the distribution of irradiance on the needle surface area was simulated. In the 2nd case, the photosynthetic units were represented by Results points within the needles and the irradiance (the photon field strength) at these points was simulated. The first approach is consistent with Measured rates of photosynthesis of a the assumption that the photosynthetic units are shoot subjected to direct and diffuse radia- evenly distributed on the needle surface and that needles are optically black, i.e., there is no tion, respectively are shown in Fig. 1. a needle. In the transmission of radiation within When the radiation is expressed in terms 2nd approach, the photosynthetic units are of the (simulated) mean irradiance on the assumed to be uniformly distributed within the needle surface area (Fig. 1 B), the rate of needle and the transmission of radiation was assumed to be an exponential function of the photosynthesis represents the photosyn-
thetic response per unit of intercepted representative value which corresponds to transmission of 5% per mm of path radiation and the difference between the a length within the needle (cf. Gates et al., respective rates of photosynthesis result from differences in the distribution of 1965). In Fig. 3B, the model is applied to radiation over the shoot. diffuse radiation. The root mean square error of predicted rates by this 2nd method In Fig. 2A, the photosynthetic rate of a varied between 0.31 and 1.58 and aver- shoot in direct radiation is calculated aged 0.89 pmol (C0 . 1 s m- ’ ) 2 based on the simulated irradiance distribu- tion on the needle surface area and a pho- tosynthetic light curve with parameters a (initial slope) 0.040 and P (maximum m = Discussion rate) 10.92 pmol (C0 estimat- , 1 s m- ’ ) 2 = ed by the method of feast squares to give the best fit to measured values. Using the clear difference be- Our results showed a same parameters and the simulated ir- tween the rates of shoot photosynthesis in radiance distribution in diffuse radiation, direct and diffuse radiation. When the the rate of photosynthesis in the diffuse radiation is expressed in terms of horizon- radiation field was predicted (Fig. 2B). The tal photon irradiance (Fig. 1A), the dif- root mean square error of predicted rates ference is exaggerated because, at an in diffuse radiation varied between 1.45 equal horizontal irradiance, the amount of and 3.65 and averaged 2.41 umol intercepted radiation is many times great- -s-’ )-m- 2 (C0 for the 9 shoots. er in the spherical radiation field. In a In Fig. 3A, the photosynthetic rate of a direct radiation field, the amount of inter- shoot in direct radiation is calculated using cepted radiation, which is determined by the distribution of radiation within the the projected shoot area, has been shown needles and a Blackman curve giving the to be the major component causing varia- best fit to measured values. The extinction tion in the photosynthetic response (Smo- coefficient along the path within the needle lander et al., 1987). Thus, much of the was taken as 3 mm- an arbitrary but , 1 variation in photosynthesis caused by
shoot structure and direction is eliminated irradiance on the needle surface area, when the rate of photosynthesis is ex- which partly eliminates the effect of shoot pressed as a function of mean irradiance structure. The effect of radiation field geo- metry is, however, not completely offset by or, alternatively, on a projected shoot area basis. In the diffuse radiation field, how- this method, which means that the rela- ever, the rate of photosynthesis per unit of tionship between intercepted radiation and intercepted radiation was still clearly photosynthesis depends upon, e.g., the shares of diffuse and direct radiation, higher (Fig. 1 A), indicating that the more even distribution of radiation in the case of respectively. For analyzing the effect of diffuse radiation is an important compo- radiation field geometry, the method pre- sented here was found to be promising. nent, too. In the direct radiation field, the fit of the measured rates to the estimated curve was rather good (Fig. 2A). When applied to shoot photosynthesis in a diffuse radia- References tion field, however, the model gave clearly higher rates of photosynthesis than the measured ones (Fig. 2B). This deviation Gates D.M., Keegan H.J., Schleter J.C. & Weid- may be due to the assumption of optically ner V.R. (1965) Spectral properties of plants. Appl. Opt. 4, 11-20 black needles resulting in an overesti- mated difference between the irradiance Oker-Blom P. (1985) Photosynthesis of a Scots pine shoot: simulation of the irradiance distribu- distributions of photosynthetic units for tion and photosynthesis of a shoot in different direct and diffuse radiation. Therefore, an radiation fields. Agric. For. Meteorol. 34, 31-40 alternative model was developed which Oker-Blom P., Kellomaki S. & Smolander H. calculates the distribution of irradiance (1983) Photosynthesis of a Scots pine shoot: within the needles, assuming that the the effect of shoot inclination on the photosyn- attenuation of radiation within needle a thetic response of a shoot subjected to direct decreases This model exponentially. radiation. !gnc..Mefeoro/. 29, 191-206 con- siderably improved the agreement be- Smolander H., Oker-Biom P., Ross J., KellomA- tween measured and calculated rates of ki S. & Lahti T. (1987) Photosynthesis of a Scots pine shoot: test of a shoot photosynthesis shoot photosynthesis in diffuse radiation model in a direct radiation field. Agric. For. (Fig. 3). Meteorol. 39, 67-80 In conclusion, it is proposed that a more Zelawski W., Szaniawski R., Dybczynski W. & invariant response of shoot photosynthe- Piechurowski A. (1973) Photosynthetic capacity sis to radiation may be obtained by of conifers in diffuse light of high illuminance. expressing the radiation in terms of mean Photosynthetica 7, 351-357