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The Sclerophyllous Eucalyptus camaldulensis and Herbaceous Nicotiana tabacum Have Different Mechanisms to Maintain High Rates of Photosynthesis

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ABSTRACT

It is believed that high levels of mesophyll conductance (gm) largely contribute to the high rates of photosynthesis in herbaceous C3 plants. However, some sclerophyllous C3 plants that display low levels of gm have high rates of photosynthesis, and the underlying mechanisms responsible for high photosynthetic rates in sclerophyllous C3 plants are unclear. In the present study, we examined photosynthetic characteristics in two high-photosynthesis plants (the sclerophyllous Eucalyptus camaldulensis and the herbaceous Nicotiana tabacum) using measurements of gas exchange and chlorophyll fluorescence. Under saturating light intensities, both species had similar rates of CO2 assimilation at 400 μmol mol−1 CO2 (A400). However, E. camaldulensis exhibited significantly lower gm and chloroplast CO2 concentration (Cc) than N. tabacum. A quantitative analysis revealed that, in E. camaldulensis, the gm limitation was the most constraining factor for photosynthesis. By comparison, in N. tabacum, the biochemical limitation was the strongest, followed by gm and gs limitations. In conjunction with a lower Cc, E. camaldulensis up-regulated the capacities of photorespiratory pathway and alternative electron flow. Furthermore, the rate of alternative electron flow was positively correlated with the rates of photorespiration and ATP supply from other flexible mechanisms, suggesting the important roles of photorespiratory pathway, and alternative electron flow in sustaining high rate of photosynthesis in E. camaldulensis. These results highlight the different mechanisms used to maintain high rates of photosynthesis in the sclerophyllous E. camaldulensis and the herbaceous N. tabacum.

No MeSH data available.


Light response changes in (A) photosynthetic electron flow through PSII (JT), and (B) the ratio of the rate of Rubisco carboxylation (Vc) to that of Rubisco oxygenation (Vo), and (C) electron flux for photorespiratory carbon oxidation [Je(PCO)] for leaves of Eucalyptus camaldulensis and N. tabacum. Measurements were conducted at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4).
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Figure 5: Light response changes in (A) photosynthetic electron flow through PSII (JT), and (B) the ratio of the rate of Rubisco carboxylation (Vc) to that of Rubisco oxygenation (Vo), and (C) electron flux for photorespiratory carbon oxidation [Je(PCO)] for leaves of Eucalyptus camaldulensis and N. tabacum. Measurements were conducted at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4).

Mentions: Under all light intensities, E. camaldulensis had significantly higher effective quantum yield of PSII (ΦPSII) compared to N. tabacum, especially under high light (Figure 4A). Concomitantly, NPQ values were lower in E. camaldulensis than N. tabacum (Figure 4B). According to the data of ΦPSII, E. camaldulensis had significantly higher values of total electron flow through PSII (JT) than N. tabacum (Figure 5A). Meanwhile, the ratios of the rate of Rubisco carboxylation (Vc) to that of Rubisco oxygenation (Vo) under saturating light intensities were much higher in E. camaldulensis (Figure 5B). On assumptions of Γ* being 40 μmol mol−1 and leaf absorbance being 0.85 in E. camaldulensis and N. tabacum, at 2000 μmol photons m−2 s−1, comparative values for JT and the Vo/Vc ratio in E. camaldulensis vs. N. tabacum were 280 vs. 178 and 0.63 vs. 0.38, respectively (Figures 5A,B). Furthermore, under light intensities above 300 μmol photons m−2 s−1, electron flux for photorespiratory carbon oxidation [Je(PCO)] were significantly higher in E. camaldulensis (Figure 5C). Values for Je(PCO) at 2000 μmol photons m−2 s−1 were 100 and 48 μmol electrons m−2 s−1 in E. camaldulensis and N. tabacum, respectively. Plotting the Vo/Vc ratio and Cc indicated that the difference in the Vo/Vc ratio between E. camaldulensis and N. tabacum was mainly determined by the difference in Cc (Figure 6), which in turn caused by the change in gm.


The Sclerophyllous Eucalyptus camaldulensis and Herbaceous Nicotiana tabacum Have Different Mechanisms to Maintain High Rates of Photosynthesis
Light response changes in (A) photosynthetic electron flow through PSII (JT), and (B) the ratio of the rate of Rubisco carboxylation (Vc) to that of Rubisco oxygenation (Vo), and (C) electron flux for photorespiratory carbon oxidation [Je(PCO)] for leaves of Eucalyptus camaldulensis and N. tabacum. Measurements were conducted at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4).
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Figure 5: Light response changes in (A) photosynthetic electron flow through PSII (JT), and (B) the ratio of the rate of Rubisco carboxylation (Vc) to that of Rubisco oxygenation (Vo), and (C) electron flux for photorespiratory carbon oxidation [Je(PCO)] for leaves of Eucalyptus camaldulensis and N. tabacum. Measurements were conducted at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4).
Mentions: Under all light intensities, E. camaldulensis had significantly higher effective quantum yield of PSII (ΦPSII) compared to N. tabacum, especially under high light (Figure 4A). Concomitantly, NPQ values were lower in E. camaldulensis than N. tabacum (Figure 4B). According to the data of ΦPSII, E. camaldulensis had significantly higher values of total electron flow through PSII (JT) than N. tabacum (Figure 5A). Meanwhile, the ratios of the rate of Rubisco carboxylation (Vc) to that of Rubisco oxygenation (Vo) under saturating light intensities were much higher in E. camaldulensis (Figure 5B). On assumptions of Γ* being 40 μmol mol−1 and leaf absorbance being 0.85 in E. camaldulensis and N. tabacum, at 2000 μmol photons m−2 s−1, comparative values for JT and the Vo/Vc ratio in E. camaldulensis vs. N. tabacum were 280 vs. 178 and 0.63 vs. 0.38, respectively (Figures 5A,B). Furthermore, under light intensities above 300 μmol photons m−2 s−1, electron flux for photorespiratory carbon oxidation [Je(PCO)] were significantly higher in E. camaldulensis (Figure 5C). Values for Je(PCO) at 2000 μmol photons m−2 s−1 were 100 and 48 μmol electrons m−2 s−1 in E. camaldulensis and N. tabacum, respectively. Plotting the Vo/Vc ratio and Cc indicated that the difference in the Vo/Vc ratio between E. camaldulensis and N. tabacum was mainly determined by the difference in Cc (Figure 6), which in turn caused by the change in gm.

View Article: PubMed Central - PubMed

ABSTRACT

It is believed that high levels of mesophyll conductance (gm) largely contribute to the high rates of photosynthesis in herbaceous C3 plants. However, some sclerophyllous C3 plants that display low levels of gm have high rates of photosynthesis, and the underlying mechanisms responsible for high photosynthetic rates in sclerophyllous C3 plants are unclear. In the present study, we examined photosynthetic characteristics in two high-photosynthesis plants (the sclerophyllous Eucalyptus camaldulensis and the herbaceous Nicotiana tabacum) using measurements of gas exchange and chlorophyll fluorescence. Under saturating light intensities, both species had similar rates of CO2 assimilation at 400 μmol mol−1 CO2 (A400). However, E. camaldulensis exhibited significantly lower gm and chloroplast CO2 concentration (Cc) than N. tabacum. A quantitative analysis revealed that, in E. camaldulensis, the gm limitation was the most constraining factor for photosynthesis. By comparison, in N. tabacum, the biochemical limitation was the strongest, followed by gm and gs limitations. In conjunction with a lower Cc, E. camaldulensis up-regulated the capacities of photorespiratory pathway and alternative electron flow. Furthermore, the rate of alternative electron flow was positively correlated with the rates of photorespiration and ATP supply from other flexible mechanisms, suggesting the important roles of photorespiratory pathway, and alternative electron flow in sustaining high rate of photosynthesis in E. camaldulensis. These results highlight the different mechanisms used to maintain high rates of photosynthesis in the sclerophyllous E. camaldulensis and the herbaceous N. tabacum.

No MeSH data available.