Báo cáo lâm nghiệp: The effect of nitrogen content on the of Scots pine needles and shoots photosynthesis

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Báo cáo lâm nghiệp: "The effect of nitrogen content on the of Scots pine needles and shoots photosynthesis"The effect of nitrogen content on the photosynthesisof Scots pine needles and shootsH. Smolander 1 P. Oker-Blom 2The Finnish Forest Research Institute, Suonenjoki Research Station, SF-77600 Suonenjoki, and2 University of Helsinki, Department of Silviculture, Unioninkatu 40 B, SF-00170 Helsinki, FinlandIntroduction Materials and Methods One yr old experimental shoots were collectedA close dependency between photosyn- from 5 young Scots pine (Pinus sylvestris L.)thetic capacity and nitrogen content of stands in Suonenjoki (62°39’ N, 27°05’ E) be-leaves has been shown (e.g., DeJong, tween 5 and 30 June 1988. The sites of the1982). For coniferous trees, however, a stands varied from poor sandy soil with Callu- na vulgaris vegetation to a fertile site withlarge variation in the relationship has been grass-herb vegetation. A randomly selectedfound. It has been proposed that the rea- shoot from the uppermost whorl of each treeson for this could be that the nitrogen was used in measurements of photosynthesis.content, by affecting needle growth, Morphological characteristics and silhouette to total needle area ratios were measured for achanges the pattern of mutual shading shoot from the uppermost whorl and for 1 stwithin a shoot, which is the basic element order and 2nd order shoots just below. Theused in studying photosynthesis of conif- photosynthetic capacity of 10 shoots (2erous trees (Linder and Rook, 1984). The trees/stand) and silhouette to total needle areaaim of this study was to analyze separate- ratio of 93 shoots (3 trees/stand) were mea- sured.ly the effect of nitrogen content on the The photosynthesis of excised shoots wasphotosynthetic capacity of individual measured in a diffuse radiation field using anneedles and on within-shoot shading. open flow IRGA-system. In order to eliminateNeedle photosynthesis was approximated the effect of within-shoot shading, about two- thirds of the needles on a shoot were removedby measuring the photosynthesis of the night before the day of measurement. The’thinned’ shoots in a diffuse radiation field. air temperature was 20°C, external C0 2Within-shoot shading was quantified in concentration was 340 ppm and vapor pressureterms of the mean silhouette area to total deficit 5-10 mbar in the chamber. The horizon- tal photon irradiance (measured by LI-190SR)needle area ratio of a shoot, which deter- was increased in an integrating sphere from 0mines the relative interception rate per to 1 000, in 7 or 8 steps and a stea- 1 S 2 M - UMOIunit of needle area on the shoot (Oker- dy state photosynthetic rate was obtained atBlom and Smolander, 1988). each irradiance level. A non-rectangular hyper-bola was fitted to the measured photosyntheticrates using the method of least squares and thevalue at 2000 pmol was used as an 1 s 2 m- ’estimate of the photosynthetic capacity. The silhouette area of each shoot was mea-sured photographically at 6 different inclinationsof the shoot axis to the plane of projection. Theeffect of asymmetry of the shoot was eliminatedby rotating the shoot 4 times in increments of60° for each inclination angle. The mean sil-houette area with respect to a spherical shootorientation was calculated as a weighted aver-age of the 24 silhouette areas measured. Themean silhouette to total needle area ratio(STAR) was then obtained by dividing mean sil-houette area by the total needle area of a shoot,which was approximated as x times the project-ed needle area (measured by a LiCOR leaf areameter) Oker-Blom and Smolander, 1988). Finally, needles were dried for 48 h at 80°Cand their nitrogen content determined by theCentral Laboratory of the Finnish ForestResearch Institute using a LECO CHN-600analyzer.Results and DiscussionDespite the large variation in site fertility,the range of nitrogen concentration variedonly from 0.49 to 1.5% of dry weight ofneedles. The low values of nitrogen content are due to the fact that measure- ments were made during the growing phase of new shoots. ...