Carbon isotope composition of bulk leaf tissue

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Figure 4.11.:δ¹³C values of bulk leaf tissue of the four birch clones during the summer of 1999. Open circles and dashed lines show the values of the droughted plants (D99), filled squares and solid lines those of the well watered controls (W99). Means and standard deviations of 10 third and fourth leaves (counted from the terminal bud of a twig) from different plants are given. Asterisks indicate significant differences (P ≤0.05) between droughted and well watered plants. After the day indicated by the black arrow the volumetric soil water content of the droughted pots was below 3 %.

Figures 4.11 and 4.12 show δ¹³C values of bulk leaf tissue of third and fourth leaves, as measured on 5 or 6 dates during the summer of 1999 and on 4 dates in 2000. In 1999, the curves started with higher (less negative) values in mid-June, which then decreased in the course of mid-summer before rising again towards the end of August (Fig. 4.11). The δ¹³C

4. Results

Figure 4.12.:δ¹³C values of bulk leaf tissue of the four birch clones during the summer of 2000. Open circles and dashed lines show the values of the droughted plants (D99/D00), filled squares and solid lines those of the well watered controls (W99/W00) and crosses and dotted lines those of the drought-and-recovery treatment (D99/W00). Means and standard deviations of 10 third and fourth leaves (counted from the terminal bud of a twig) from different plants are given. Asterisks indicate significant differences (P ≤0.05) between droughted and well watered plants. After the day indicated by the black arrow the volumetric soil water content of the droughted pots was below 3 %.

values of the well watered trees averaged in 1999 to -28.85 ‰ in Aust.1, -29.35 ‰ in Bp-Aust.2, -29.32 ‰ in Bp-Swed. and -30.92 ‰ in Bp-Germ.. Values of δ¹³C of the German clone were significantly (P ≤ 0.05) lower than those of all other genotypes throughout the whole season, while the values of Bp-Aust.1 were continuously least negative, the differences in comparison with Bp-Aust.2 and the Swedish clone being significant at two and three dates of sampling, respectively. The δ¹³C values of the droughted trees were getting progressively less negative than those of the well watered ones from the early drought phase in late June to early July on. Average values of the droughted trees ranged from -27.38 ‰ in Bp-Aust.1 to -29.57 ‰ in Bp-Germ., exhibiting the same order of the genotypes as in the well watered trees:

values of Bp-Aust.1 were significantly less negative than those of Bp-Swed. and Bp-Germ. at all measured dates and less negative than those of Bp-Aust.2 on four dates, while Bp-Aust.2 and Bp-Swed. reached less negative values than the German trees on three and two occasions, respectively. At the end of August 1999 differences between the W99/W00-treated and the droughted trees of one clone averaged to 2.12 ‰.

In the summer of 2000, theδ¹³C values of the well watered plants showed a less pronounced annual variation than in 1999, being more or less constant from early June to late August

4. Results

June July August June July August

ci [ppm]ci [ppm]ci [ppm]ci [ppm]

Figure 4.13.: Leaf-internal CO2 concentration (c_i) as calculated from foliar δ¹³C values of the four birch clones during the growing seasons of 1999 and 2000. Open circles and dashed lines represent the droughted plants (D99 and D99/D00), filled squares and solid lines the well watered controls (W99 and W99/W00), and crosses and dotted lines the drought-and-recovery treatment in 2000 (D99/W00).

Means and standard deviations of 10 third and fourth leaves (counted from the terminal bud of a twig) from different plants are given. Asterisks indicate significant differences (P ≤0.05) between droughted (D99 and D99/D00) and well watered plants (W99/W00). After the day indicated by the black arrow the volumetric soil water content of the droughted pots was below 3 %.

(Fig. 4.12). They were more negative than in 1999 in all genotypes, with the values of the German trees being significantly more negative than those of the other clones at all sampled

4. Results

dates. Values ofδ¹³C of the droughted trees were slightly less negative than those of the well watered ones from early June on, the difference was significant only in the German clone. They increased rapidly to late July in all clones and remained constantly higher than those of the well watered plants from then on. Annual averages ranged from -27.11 ‰ in Bp-Aust.1, which exhibited significantly less negative values than both Bp-Swed. and Bp-Germ., to -28.62 ‰ in the German genotype. Values reached by the droughted trees of Bp-Aust.2 were usually less negative than those of the Swedish and the German genotype as well. At the end of August 2000,δ¹³C values of the droughted Bp-Aust.1 and Bp-Germ. were 3.00 ‰ and 3.24 ‰ less negative than those of the well watered ones, while the differences between these two treatments were only 1.89 ‰ in Bp-Aust.2 and 2.05 ‰ in the Swedish genotype. The drought-and-recovery treatments of all clones exhibited the sameδ¹³C values as the well watered trees throughout the whole season. Analyses of variance (Tab. B.5) confirmed the seasonal course of theδ¹³C values, as well as the marked influence of the genotype and the treatment.

Plasticity with regard to theδ¹³C values (Tab. A.5) devided the clones into two pairs: while Bp-Aust.2 and the Swedish clone showed plasticities of 1.63 ‰ and 1.47 ‰ in 1999, the second Austrian and the German genotype reached plasticities of 2.24 ‰ and 2.12 ‰. All clones showed more plastic responses in 2000, when differences between them were even more pronounced: while the plasticity of Bp-Aust.2 and Bp-Swed. was about 2 ‰, plasticities of both Bp-Aust.1 and the German clone reached more than 3 ‰.

Fig. 4.13 shows intercellular CO₂ concentrations (c_i) as calculated from the δ¹³C values (HUBICK et al. 1989) with an assumed CO2 concentration of the surrounding air of 360 ppm.

They mirror theδ¹³C values exactly, though with opposite signs, but they clarify the ecophysi-ological meaning of differences between the treatments and genotypes. While the well watered trees of the Austrian clones and Bp-Swed. carried out photosynthesis in 1999 with an average c_iof 287 to 296 ppm, trees of the German genotype had 322 ppm to operate with. Thec_iof the droughted trees averaged to between 263 ppm (Bp-Aust.1) and 300 ppm (Bp-Germ.). From June to August 2000, CO₂fixation was carried out with an average 300 ppm (Austrian clones) to 328 ppm (Bp-Germ.) in the well watered trees, whilec_i ranged from 259 ppm (Bp-Aust.1) to 285 ppm (Bp-Germ.) within the droughted plants. By multiplying the difference betweenc_i of the D99/D00-treated and the W99/W00-treated plants of one genotype with the carboxyla-tion efficiency CE of the well watered plants of this genotype (both measurements from early August, Tab. 4.5), effects of a lowered CO₂ on the rate of photosynthesis are elucidated. If the reduction had occured in the linear part of the CO₂ response curve, net-assimilation of the droughted plants would have been reduced by 1.57 µmol m⁻²s⁻¹ (Bp-Swed.) to 3.51 µmol m⁻²s⁻¹ (Bp-Aust.1) in August 2000. Since all c_i values calculated from δ¹³C values were well below saturating CO₂concentrations, (CO_2sat, see Tab. 4.5), reduction of net assimilation should be lower than this estimate, but nevertheless substantial.