13.3.2  Sink strength

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Relative growth rate can be limited by utilisation rather than generation of photoassimilates. Such sink limitation can lead to feedback inhibition of photosynthesis (Section 13.2.5). Those species with a high intrinsic relative growth rate (RGR) in any given light environment are likely to be able to respond to CO2 enrichment better than species with a lower RGR for the same light environment. Under shade conditions, plants with a high RGR are more likely to benefit from the direct and compounding benefits of an increased photoassimilate supply. Plants with a low RGR, when subject to abundant illumination, may be so well supplied with photoassimilate that sinks are growing almost as fast as the prevailing temperature allows. In that scenario, further photosynthetic stimulation by CO2 enrichment would not lead to increased sink activity and feedback inhibition of photosynthesis may follow.

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Figure 13.6 Number of branches produced under different combinations of irradiance and atmospheric CO2 concentration in Acmena smithii and the more apically dominant Doryophora sassafras. A. smithii has a greater phenotypic plasticity and is better able to increase sink number in response to CO2 enrichment. (2 × standard errors of means are shown) (D.J. Wiggins, M.C. Ball, R.M. Gifford and G.D. Farquhar, pers. comm. 1996)

In addition to strong RGR (strong sinks) some species possess a greater phenotypic plasticity and are able to generate additional sinks according to growing conditions (i.e. indeterminate growth). Such plants show a larger and more sustained response to CO2 enrichment than determinate species (Figure 13.6). Doryophora sassafras Endl. and Acmena smithii (Poir.) Merr. and Perry are two species endemic to warm temperate rainforests of New South Wales. D. sassafras typically establishes under low light regimes and A. smithii establishes under a wide range of light environments. Both species are capable of germination under mature rainforest canopies and persist in low light environments.

A. smithii differs from D. sassafras in its response to both low and high light environments. Not only can A. smithii achieve a faster RGR than D. sassafras at all light levels, but A. smithii also has the phenotypic plasticity to increase sink number, largely through an increase in branching (Figure 13.6). Enhanced branching thus increases the capacity of A. smithii to use photoassimilates in response to higher light. In contrast, D. sassafras is more apically dominant and is unable to increase sink strength commensurate with the enhanced capacity to produce photoassimilates.

The response of A. smithii to CO2 enrichment is similar to its response to increased light. Increased availability of both resources may increase photosynthetic capacity and both have the ability to induce physiological and morphological changes in many species. Both factors influence leaf elongation and branching and thereby lead to a positive interaction on whole-plant RGR by sustaining faster expansion over a larger number of leaves.

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