13.2.6 Carbon partitioning

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A shift in resource deployment can be expressed as a change in carbon and other element partitioning among plant parts. The notion of ‘functional equilibrium’ suggests that increasing availability of an atmospheric resource, like CO2, should increase relative partitioning of resources into roots so that a balance between assimilation of carbon and nutrients (especially nitrogen) is maintained. Some plant growth models use this concept to drive dry matter partitioning. However, examination of CO2-enrichment effects on partitioning in terms of allometric relationships (Section 6.3.1) does not support this view. Rather, changes in carbon distribution among plant parts is more a function of plant size, whether that be varied by time of harvest (i.e. age of plant) or by CO2 concentration for a given time of harvest (i.e. size of plant), rather than CO2 concentration per se (Figure 13.4).


Figure 13.4 Allometric relationships between root carbon and total carbon of isolated Danthonia richardsonii plants in response to CO2 enrichment and nitrogen supply. At any given rate of nitrogen supply, CO2 enrichment had no impact on the relationship between root carbon and total plant carbon, despite CO2 enrichment increasing plant size at any given time. As nitrogen supply rate declined, the proportion of carbon allocated to roots increased, an example of a homeostatic response of a plant depleted in nitrogen exploring a larger soil colume in order to obtain nitrogen.

= ambient CO2, low nitrogen;
= ambient CO2, mid nitrogen;
= ambient CO2, high nitrogen;
= enriched CO2, low nitrogen;
enriched CO2, mid nitrogen;
= enriched CO2, high nitrogen

(Based on J.L Lutze and RM Gifford, pers. Comm. 1996)

Figure 13.4 shows an allometric relationship between root carbon and total plant carbon for Danthonia richardsonii seedlings grown in two atmospheric CO2 concentrations and three nitrogen treatments. Increasing nitrogen supply reduced carbon allocation to roots. This is apparent from the different intercepts of the lines. Growth at elevated CO2 did not change allocation patterns because at each nitrogen level, ambient CO2 control plants (circles) and CO2-enriched treatments (triangles) fall on the same line.

In this section the general response of assimilation and growth to CO2 enrichment has been shown in isolation. However, in reality atmospheric CO2 enrichment is occurring in an environment where water availability may be restricted at some point in a plant’s lifetime, or temperatures may be suboptimal or supraoptimal. For any prediction of plant response to CO2 enrichment the interaction of CO2 enrichment with other environmental inputs must be appreciated (Section 13.3).