AUTHOR=Huang Wei , Zhang Shi-Bao , Hu Hong 

TITLE=Sun leaves up-regulate the photorespiratory pathway to maintain a high rate of CO2 assimilation in tobacco

JOURNAL=Frontiers in Plant Science

VOLUME=5

YEAR=2014

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2014.00688

DOI=10.3389/fpls.2014.00688

ISSN=1664-462X

ABSTRACT=<p>The greater rate of CO<sub>2</sub> assimilation (<italic>A</italic><sub>n</sub>) in sun-grown tobacco leaves leads to lower intercellular and chloroplast CO<sub>2</sub> concentrations and, thus, a higher rate of oxygenation of ribulose-1,5-bisphosphate (RuBP) than in shade-grown leaves. Impairment of the photorespiratory pathway suppresses photosynthetic CO<sub>2</sub> assimilation. Here, we hypothesized that sun leaves can up-regulate photorespiratory pathway to enhance the <italic>A</italic><sub>n</sub> in tobacco. To test this hypothesis, we examined the responses of photosynthetic electron flow (<italic>J</italic><sub>T</sub>) and CO<sub>2</sub> assimilation to incident light intensity and intercellular CO<sub>2</sub> concentration (<italic>C</italic><sub>i</sub>) in leaves of ‘k326’ tobacco plants grown at 95% sunlight (sun plants) or 28% sunlight (shade plants). The sun leaves had higher photosynthetic capacity and electron flow devoted to RuBP carboxylation (<italic>J</italic><sub>C</sub>) than the shade leaves. When exposed to high light, the higher Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) content and lower <italic>C</italic><sub>i</sub> in the sun leaves led to greater electron flow devoted to RuBP oxygenation (<italic>J</italic><sub>O</sub>). The <italic>J</italic><sub>O</sub>/<italic>J</italic><sub>C</sub> ratio was significantly higher in the sun leaves than in the shade leaves under strong illumination. As estimated from CO<sub>2</sub>-response curves, the maximum <italic>J</italic><sub>O</sub> was linearly correlated with the estimated Rubisco content. Based on light-response curves, the light-saturated <italic>J</italic><sub>O</sub> was linearly correlated with light-saturated <italic>J</italic><sub>T</sub> and light-saturated photosynthesis. These findings indicate that enhancement of the photorespiratory pathway is an important strategy by which sun plants maintain a high <italic>A</italic><sub>n</sub>.</p>