Responses of citrus leaf photosynthesis, chlorophyll fluorescence, macronutrient and carbohydrate contents to elevated CO2

Ninety days exposure to elevated atmospheric CO2 concentrations at and above 600 ppm resulted in ‘down-regulation’ of leaf net photosynthesis rate (A) in completely expanded and old leaves of Citrus madurensis Loureiro (Calamondin). The decrease of A at high CO2 levels could be explained by a lower activity and/or concentration of Calvin cycle enzymes, which was accompanied by a reduced leaf N content. Nevertheless, C. madurensis did not suffer from nutrient deficiency. Instead, N was redistributed in the plant according to the sink capacity for this macronutrient, which changed at elevated CO2 levels. Mutual shading of older leaves, which occurred especially at elevated atmospheric CO2 concentrations due to the accelerated tree growth, and as a consequence, adaptation to a slightly darker environment may also have played a role for the ‘down-regulation’ of A. A decrease of the chlorophyll fluorescence parameter Fv/Fm in completely expanded and old leaves of C. madurensis at and above 750 ppm CO2 is interpreted to indicate photoinhibition at high CO2 levels. Especially in young leaves of C. madurensis, but also in older ones, the increase of photosynthetic water use efficiency was an important effect of elevated atmospheric CO2 concentrations.