Effects of fruit load on photosynthesis and ribulose 1,5-bisphosphate carboxylase activity in satsuma mandarin (Citrus unshiu Marc.) Trees under long-term elevated carbon dioxide

The effects of long-term carbon-dioxide (CO₂) enrichment on photosynthetic rate (Pn), ribulose 1,5-bisphosphate carboxylase (RuBPCO) activity, chlorophyll content, and carbon accumulation in leaves were investigated in potted satsuma mandarin (Citrus unshiu Marc.) trees with and without fruit load. The Pn of whole satsuma mandarin trees at 1,000 μl CO₂·liter^-1 for the first 13 days increased significantly, up to about 200%, compared to that at a normal atmospheric CO₂ (360 μl CO₂·liter^-1). After that, Pn began to decrease gradually until it reached 170% on the 35th day. The Pn of the bearing trees remained higher than those of non-bearing trees. Although there was no difference in the chlorophyll content, the bearing trees had smaller specific leaf weight (SLW) and carbon content in the leaves. It seemed that the decrease in Pn during the long-term exposure to the elevated CO₂ concentration was partly due to a decrease in RuBPCO activity and/or an accumulation of photoassimilates in the leaves. The sink capacity of fruit may be important for the maintenance of Pn in long-term exposure to an elevated CO₂ concentration because the non-bearing trees had a particularly rapid decrease in Pn.