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 m easured in wheat plants (Triticum aestivum L.) grown in field chambers at either 350 or 700 mu L L-1 CO2. Rates of NCE decreased with leaf a ge in both CO2 treatments during 1997. A stimulation of NCE initially occurred in response to CO2 enrichment, but this was not observed on t he final eight of 33 experimental days. Net photosynthetic rates in re sponse to growth at elevated CO2 were decreased ca. 22% on average for all measurement dates in 1997 and when all NCE rates measured at elev ated CO2 for 1995-1997 were averaged (P less than or equal to 0.0001). Soluble protein, alpha-amino nitrogen, and Chi a + b concentrations w ere significantly lower (P less than or equal to 0.0001) in elevated c ompared with ambient CO2-grown wheat flag leaves in 1997. The treatmen t by date interactions for these Rag leaf constituents were nonsignifi cant (P greater than or equal to 0.05). Flag leaf storage carbohydrate s 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 greater than or equal to 0.05) and only occurred during late stages of senescence. These findings supported the suggestion tha t premature senescence contributed to the photosynthetic decline obser ved in wheat flag leaves during growth at elevated CO2. Changes of alp ha-amino nitrogen were correlated with photosynthetic decline, but aci d proteinase activity probably was under endogenous control.