Combined effects of chronic ozone and elevated CO2 on Rubisco activity and leaf components in soybean Glycine max

Content and activity of Rubisco and concentrations of leaf nitrogen,chlorophyll and total non-structural carbohydrates(TNC) were determined at regular intervals during the1993 and 1994 growing seasons to understand the effects and interactions of[O3] and elevated [CO2] on biochemical limitations to photosynthesis duringontogeny. Soybean (Glycine max var. Essex) was grownin open-top field chambers in either charcoal-filtered air (CF, 20 nmolmol^-1) or non-filtered air supplemented with 1.5 xambient [O3] (c. 80 nmol mol^-1) at ambient (AA, 360mol mol^-1) or elevated[CO2] (700 mol mol^-1).Sampling period significantly affected all the variables examined. Changesincluded a decrease in the activity and content of Rubisco during seedmaturation, and increased nitrogen (N), leaf mass perunit area (LMA) and total non-structural carbohydrates(TNC, including starch and sucrose) through thereproductive phases. Ontogenetic changes were most rapid in O2-treatedplants. At ambient [CO2], O3 decreased initial activity (14-64% per unitleaf area and 14-29% per unit Rubisco) and content of Rubisco (9-53%), andN content per unit leaf area. Ozone decreasedLMA by 17-28% of plants in CF-AA at the end of thegrowing season because of a 24-41% decrease in starch and a 59-80% decreasein sucrose. In general, elevated CO2], in CF or O3-fumigated air, reducedthe initial activity of Rubisco and activation state while having littleeffect on Rubisco content, N and the chlorophyllcontent, per unit leaf area. Elevated CO2 decreased Rubisco activity by14-34% per unit leaf area and 15-25% per unit Rubisco content of plants ingrown CF-AA, nd increases LMA by 27-74% of the leafmass per unit area in CF-AA because of a 23-148% increase in starch.However, the data suggest that, at elevated [CO2], increases in starch andsucrose are not directly responsible for the deactivation of Rubisco. Also,there was little evidence of an adjustment of Rubisco activity in responseto starch and sucrose metabolism. Significant interactions between elevated[CO2] and [O3] on all variables examined generally resulted in alleviationor amelioration of the O3 effects at elevated CO2. These data providefurther support to the idea that elevated atmospheric CO2 will reduce orprevent damage from pollutant O3.