13.2.5  Photosynthetic acclimation

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‘Photosynthetic acclimation’ is a commonly observed phenomenon where a plant grown with CO2 enrichment has a different light-saturated short-term leaf photosynthetic CO2 response curve from that of an equivalent leaf grown in ambient air. Sage (1994) identifies six common patterns of response of assimilation resulting from the interaction of three semi-independent processes. These processes are the amount/activity of Rubisco, the rate of thylakoid-dependent RuBP regeneration capacity and the phosphate regeneration capacity. Figure 13.3 gives three examples of photosynthetic responses of soybean grown at ambient and about twice ambient CO2 concentrations. Substantial downregulation (Figure 13.3a), substantial upregulation (Figure 13.3c), or no change (Figure 13.3b) were all observed. The upregulated case was for plants (cultivar Bragg) grown to pod-fill stage in naturally illuminated cabinets. The downregulated case was for pod-filling plants (cultivar Frosty) grown under fluorescent light when the leaves measured were 21–26 d old. The unchanged ones came from the same group of plants as in Figure 13.3(c) but leaves were only 12–14 d post-emergence. The reasons for the different responses are unknown. There was little or no reduction in Rubisco activity per unit area of leaf as a result of growth with CO2 enrichment despite lower initial slopes in Figure 13.3(a) and (c). In many studies, however, down-regulation is accompanied by reduced Rubisco activity, as implied by a decline in the slope of the initial part of the A:pi response.

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Figure 13.3   Soybean, grown under ambient (solid line) or CO2 – enriched conditions (dashed line) can show (a) downregulation, (b) no change or (c) upregulation of assimilation. Downregulation was observed for old leaves of plants grown under fluorescent lights. No change was observed for younger plants grown as in (a), but upregulation was observed in old leaves grown under natural lighting. The numbers along the lines show the percentage change in assimilation rate between control and CO2-enriched plants. (Based on data from Xu et al. 1994; Campbell et al. 1988)

Mechanisms underlying acclimation

Diverse hypotheses are being explored to explain acclimation. Reports of downregulation are more common than of upregulation. While downregulation is the main focus of such investigations of acclimation, any complete explanation will have to accommodate instances of upregulation.

Early hypotheses emphasised the role of excessive starch and sugar accumulation in inhibiting photosynthesis. Inhibition resulted from a physical obstruction of chloroplasts by starch grains or by the sequestration of phosphate in sugar phosphates. A suboptimal supply of phosphate to the chloroplast has an immediate negative impact on rates of photosynthesis. Such responses are frequently observed in plants grown in nutrient-limiting conditions or in small pots, where root growth (and hence sink activity) is limited. Subsequent hypotheses stressed that downregulation is the result of the CO2-enriched plant allocating nitrogen away from the photosynthetic carbon-fixing apparatus (especially Rubisco), investing instead in other functions which become relatively more growth limiting when photosynthesis is CO2 stimulated, such as light harvesting or root growth (i.e. the maintenance of sink activity). From this perspective, downregulation is seen as an expression of suboptimal nitrogen nutrition. Indeed in many experiments where downregulation was not observed, the plants were not nutrient deficient. Later these two hypotheses were linked by suggestion of a regulatory effect of sugars on gene expression. Van Oosten et al. (1992, 1994) have shown that hexose accumulation in leaves resulting from artificial supply of hexoses or from growth in a CO2-enriched environment can cause a decline in the number of transcripts for chlorophyll-binding proteins and Rubisco activase. In addition, excision of leaves (which prevents phloem translocation from leaves) increased the sensitivity of nuclear genes (but not chloroplast genes) to CO2 enrichment. Some mitochondrial enzymes were also reduced in activity. Clearly, CO2 enrichment can influence expression of nuclear, chloroplast and mitochondrial genes differentially, and possibly through sugar accumulation and phosphate supply to chloroplasts.

No clear comprehensive hypothesis has yet emerged to explain both upregulation and downregulation. However, photo-synthetic downregulation probably represents a shift in resource deployment (especially nitrogen) so that plants exposed to CO2 enrichment are re-optimised to make better use of resources that are available.

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