Growth under elevated atmospheric CO2 concentration accelerates leaf senescence in sunflower (Helianthus annuus L.) plants

Abstract

Some morphogenetic and metabolic processes were sensitive to a high atmospheric CO2 concentration during sunflower primary leaf ontogeny. Young leaves of sunflower plants growing under elevated CO2 concentration exhibited increased growth, as reflected by the high specific leaf mass referred to as dry weight in young leaves (16days). The content of photosynthetic pigments decreased with leaf development, especially in plants grown under elevated CO2 concentrations, suggesting that high CO2 accelerates chlorophyll degradation, and also possibly leaf senescence. Elevated CO2 concentration increased the oxidative stress in sunflower plants by increasing H2O2 levels and decreasing activity of antioxidant enzymes such as catalase and ascorbate peroxidase. The loss of plant defenses probably increases the concentration of reactive oxygen species in the chloroplast, decreasing the photosynthetic pigment content as a result. Elevated CO2 concentration was found to boost photosynthetic CO2 fixation, especially in young leaves. High CO2 also increased the starch and soluble sugar contents (glucose and fructose) and the C/N ratio during sunflower primary leaf development. At the beginning of senescence, we observed a strong increase in the hexoses to sucrose ratio that was especially marked at high CO2 concentration. These results indicate that elevated CO2 concentration could promote leaf senescence in sunflower plants by affecting the soluble sugar levels, the C/N ratio and the oxidative status during leaf ontogeny. It is likely that systemic signals produced in plants grown with elevated CO2, lead to early senescence and a higher oxidation state of the cells of these plant leaves.