Evidence that Premature Senescence Affects Photosynthetic Decline of Wheat Flag Leaves during Growth in Elevated Carbon Dioxide

Net CO2 exchange (NCE) rates and various flag leaf constituents were measured in wheat plants (Triticum aestivum L.) grown in field chambers at either 350 or 700 μL L-1 CO2. Rates of NCE decreased with leaf age in both CO2 treatments during 1977. A stimulation of NCE initially occurred in response to CO2 enrichment, but this was not observed on the final eight of 33 experimental days. Net photosynthetic rates in response to growth at elevated CO2 were decreased ca. 22% on average for all measurement dates in 1997 and when all NCE rates measured at elevated CO2 for 1995–1997 were averaged (P ≤ 0.0001). Soluble protein, α-amino nitrogen, and Chl a + b concentrations were significantly lower (P ≤ 0.0001) in elevated compared with ambient CO2-grown} wheat flag leaves in 1997. The treatment by date interactions for these flag leaf constituents were nonsignificant (P ≥ 0.05). Flag leaf storage carbohydrates were measured on 10 dates in 1997, but only starch and sucrose were affected by the elevated CO2 treatment. An increase of acid proteinase activity was observed on the last two measurement dates of this study. However, changes of acid proteinase activity were unaffected by CO2 enrichment (P ≥ 0.05) and only occurred during late stages of senescence. These findings supported the suggestion that premature senescence contributed to the photosynthetic decline observed in wheat flag leaves during growth at elevated CO2. Changes of α-amino nitrogen were correlated with photosynthetic decline, but acid proteinase activity probably was under endogenous control.