6.2.6  CO2 x nutrients

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CO2 is a further prerequisite for growth, and also shows a positive interaction with nutrient supply on plant growth indices. CO2 effects on NAR which translate to faster RGR have been documented (Section 6.2.3). Initially strong responses that diminished over time were attributed to a shift in allocation of photoassimilate under elevated CO2 which resulted in reduced LAR, due in part to decreased SLA plus increased root mass relative to shoot mass in some cases. Photosynthetic acclimation to elevated CO2 was an additional factor restricting NAR (hence lower RGR), especially on low-nutrient supply. A positive interaction between CO2 and nutrient supply on NAR would be expected and if nutrient input drives leaf expansion to the extent demonstrated earlier (Section 6.2.4) then the combined effects of LAR × NAR on RGR will be compounded.

Using CO2 and N supply as driving variables, Wong et al. (1992) tested these ideas on seedlings of four species of eucalypt which represented ecologically distinctive groups, namely Eucalytus camaldulensis and E. cypellocarpa (both fast growing, widely distributed and reaching immense size) versus E. pulverulenta and E. pauciflora (more limited distribution, smaller final size and restricted to poor sites). In addition, two subgenera were represented: E. camaldulensis, E. cypellocarpa and E. pulverulenta belong to the subgenus Symphyomyrtus, whereas E. pauciflora belongs to Monocalyptus. Systematic differences between subgenera in physiological attributes have been noted (Noble 1989). According to that scheme, E. pauciflora would show more muted response to CO2 × nutrient inputs compared with the other three species.

In Wong et al. (1992) early exponential growth showed a strong response to CO2 × N treatments where CO2-dependent increase in RGR (DRGR) was clearly influenced by N supply. All three Symphyomyrtus species returned a greater DRGR on high N. By contrast, the Monocalyptus species E. pauciflora showed no such CO2 × N interaction.

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Figure 6.16 Growth of Eucalyptus camalduleusis (cultured 90 d in unshaded greenhouses) shows a positive and interactive response to factorial combination of N supply and CO2 (1.2 or 6.0 mM nitrate with 330 or 660 ppm CO2).Treatments left to right are: low N + low CO2, high N + low CO2, low N + high CO2 and high N + high CO2. Scale bar = 50 mm. (Further details in Wong etal. 1992) (Photograph courtesy R E. Kriedemann)

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Figure 6.17 Overall growth of Eucalyptus camaldulensis showed a strong positive response to CO2 × N (see Figure 6.16). However, mean area per leaf at each node depended mainly on N supply, showing no consistent interaction with CO2. In contrast, the number of nodes was increased by CO2, so that plants were taller and carried more leaves. N and CO2 were supplied in a 2 × 2 factorial combination of 1.2 or 6.0 mM nitrate with 330 or 660 ppm CO2) for 90 d in unshaded greenhouses (Based on Wong et el. 1992)

Given the scale of CO2 × N effects on canopy growth (Figure 6.16, and Wong et al. 1992), E. camaldulensis was taken for more detailed analysis at final harvest (Figure 6.17). CO2-enriched plants on high N were clearly tallest and carried the largest canopies (Figure 6.16) but maximum area per leaf (around node 12 in Figure 6.17) was driven by N rather than CO2. Nutrient impact on leaf expansion is well known (Section 6.2.4), and present effects are consistent with those general responses. Accordingly, CO2 × N interaction on canopy area of E. camaldulensis can be attributed to stem extension and generation of leaf number (CO2 effect at high N), as well as greater size per leaf (nutrient effect and independent of CO2).

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Table 6.7

Leaf function is also reflected in leaf-N productivity (whole-plant dry mass formed per unit leaf N per day; Table 6.7). Species differences are again evident where E. camaldulensis and E. cypellocarpa were decidedly higher while E. pulverulenta and E. pauciflora were somewhat lower. In addition, elevated CO2 increased leaf-N productivity for both E. camaldulensis and E. cypellocarpa on either high N or low N, whereas the other two species, E. pulverulenta and E. pauciflora, varied in scale and direction. Indeed, high N may have proved supraoptimal for those two species, and especially in combination with high CO2.

Leaf N is ultimately responsible for carbon gain, so that NAR and leaf-N productivity are functionally related. In those species adapted to fast capture of nutrient-rich sites such as E. camaldulensis and E. cypellocarpa a capacity for high NAR based upon efficient use of leaf N (i.e. high leaf-N productivity) would confer a selective advantage. By contrast, E. pulverulenta and E. pauciflora were collected from resource-poor sites where fast growth would have been selectively neutral.

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