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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.

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(A) Light response change in the rate of ATP supplied from other flexible mechanisms rather than electron flow from water to NADP+ [vATP(Flex)] for leaves of Eucalyptus camaldulensis and tobacco; (B) Relationship between the electron flux for alternative electron sinks (Ja) and vATP(Flex) for leaves of Eucalyptus camaldulensis and tobacco, and data were obtained from light response curves (light intensities higher than 300 μmol photons m−2 s−1) measured at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4). The coefficient of correlation (r) and significance of correlation (P) are 0.98 and <0.0001 for Figure 7B, respectively.
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Figure 8: (A) Light response change in the rate of ATP supplied from other flexible mechanisms rather than electron flow from water to NADP+ [vATP(Flex)] for leaves of Eucalyptus camaldulensis and tobacco; (B) Relationship between the electron flux for alternative electron sinks (Ja) and vATP(Flex) for leaves of Eucalyptus camaldulensis and tobacco, and data were obtained from light response curves (light intensities higher than 300 μmol photons m−2 s−1) measured at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4). The coefficient of correlation (r) and significance of correlation (P) are 0.98 and <0.0001 for Figure 7B, respectively.

Mentions: Under high light, E. camaldulensis had significantly increased alternative electron flow, as indicated by the higher values of Ja and the Ja/Jg ratio (Figure 7). To examine the relationship between the alternative electron flow and photorespiration, we evaluated possible associations between Ja and Je(PCO) under light intensities higher than 300 μmol photons m−2 s−1. Interestingly, Ja was positively and linearly correlated with Je(PCO) (Figure 7A, P < 0.0001). Similarly, the Ja/Jg ratio was positively and linearly correlated with Vo/Vc ratio (Figure 7B, P < 0.0001). According to photosynthesis model, an increase in the Vo/Vc ratio requires a higher ATP/NADPH energy demand from other flexible mechanisms such as cyclic electron flow and alternative electron flow. Increased alternative electron flow promotes the formation of a proton gradient across the thylakoid membrane (ΔpH), which can be used for activating NPQ and ATP synthesis. We found that under high light the rate of ATP supplied from alternative electron sinks [vATP(Flex)] were much higher in E. camaldulensis than in N. tabacum (Figure 8A), and the rate of alternative electron flow was positively correlated to vATP(Flex) (Figure 8B, P < 0.0001). Furthermore, NPQ values under high light were significantly lower in E. camaldulensis than in N. tabacum (Figure 4B), suggesting that the main role for alternative electron flow in E. camaldulensis is not to activate NPQ but to provide extra ATP. The greater capacity of the photorespiratory pathway in E. camaldulensis plants is sustained by the enhanced alternative electron flow.


The Sclerophyllous Eucalyptus camaldulensis and Herbaceous Nicotiana tabacum Have Different Mechanisms to Maintain High Rates of Photosynthesis
(A) Light response change in the rate of ATP supplied from other flexible mechanisms rather than electron flow from water to NADP+ [vATP(Flex)] for leaves of Eucalyptus camaldulensis and tobacco; (B) Relationship between the electron flux for alternative electron sinks (Ja) and vATP(Flex) for leaves of Eucalyptus camaldulensis and tobacco, and data were obtained from light response curves (light intensities higher than 300 μmol photons m−2 s−1) measured at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4). The coefficient of correlation (r) and significance of correlation (P) are 0.98 and <0.0001 for Figure 7B, respectively.
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Figure 8: (A) Light response change in the rate of ATP supplied from other flexible mechanisms rather than electron flow from water to NADP+ [vATP(Flex)] for leaves of Eucalyptus camaldulensis and tobacco; (B) Relationship between the electron flux for alternative electron sinks (Ja) and vATP(Flex) for leaves of Eucalyptus camaldulensis and tobacco, and data were obtained from light response curves (light intensities higher than 300 μmol photons m−2 s−1) measured at 25°C and 400 μmol mol−1 CO2. Values are means ± SE (n = 4). The coefficient of correlation (r) and significance of correlation (P) are 0.98 and <0.0001 for Figure 7B, respectively.
Mentions: Under high light, E. camaldulensis had significantly increased alternative electron flow, as indicated by the higher values of Ja and the Ja/Jg ratio (Figure 7). To examine the relationship between the alternative electron flow and photorespiration, we evaluated possible associations between Ja and Je(PCO) under light intensities higher than 300 μmol photons m−2 s−1. Interestingly, Ja was positively and linearly correlated with Je(PCO) (Figure 7A, P < 0.0001). Similarly, the Ja/Jg ratio was positively and linearly correlated with Vo/Vc ratio (Figure 7B, P < 0.0001). According to photosynthesis model, an increase in the Vo/Vc ratio requires a higher ATP/NADPH energy demand from other flexible mechanisms such as cyclic electron flow and alternative electron flow. Increased alternative electron flow promotes the formation of a proton gradient across the thylakoid membrane (ΔpH), which can be used for activating NPQ and ATP synthesis. We found that under high light the rate of ATP supplied from alternative electron sinks [vATP(Flex)] were much higher in E. camaldulensis than in N. tabacum (Figure 8A), and the rate of alternative electron flow was positively correlated to vATP(Flex) (Figure 8B, P < 0.0001). Furthermore, NPQ values under high light were significantly lower in E. camaldulensis than in N. tabacum (Figure 4B), suggesting that the main role for alternative electron flow in E. camaldulensis is not to activate NPQ but to provide extra ATP. The greater capacity of the photorespiratory pathway in E. camaldulensis plants is sustained by the enhanced alternative electron flow.

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 &mu;mol mol&minus;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.