CARBON ALLOCATION AND PARTITIONING IN ASPEN CLONES VARYING IN SENSITIVITY TO TROPOSPHERIC OZONE

Clones of aspen (Populus tremuloides Michx.) were identified that diff er in biomass production in response to O-3 exposure, (14)Carbon trace r studies were used to determine if the differences in biomass respons e were linked to shifts in carbon allocation and carbon partitioning p atterns. Rooted cuttings from three aspen Clones (216, O-3 tolerant; 2 71, intermediate; and 259, O-3 sensitive) were exposed to either charc oal-filtered air (CF) or an episodic, two-times-ambient O-3 profile (2 x) in open-top chambers. Either recently mature or mature leaves were exposed to a 30-min C-14 pulse and returned to the treatment chambers for a 48-h chase period before harvest. Allocation of C-14 to differen t plant parts, partitioning of C-14 into various chemical fractions, a nd the concentration of various chemical fractions in plant tissue wer e determined. The percent of C-14 retained in recently mature source l eaves was not affected by O-3 treatment, but that retained in mature s ource leaves was greater in O-3-treated plants than in CF-treated plan ts. Carbon allocation from source leaves was affected by leaf position , season, clone and O-3 exposure. Recently mature source leaves of CF- treated plants translocated about equal percentages of C-14 acropetall y to growing shoots and basipetally to stem and roots early in the sea son. When shoot growth ceased (August 16), most C-14 from all source l eaves was translocated basipetally to stem and roots. At no time did m ature source leaves allocate more than 6% of C-14 translocated within the plant to the shoot above. Ozone effects were most apparent late in the season. Ozone decreased the percent C-14 translocated from mature source leaves to roots and increased the percent C-14 translocated to the lower stem. In contrast, allocation from recently mature leaves t o roots increased. Partitioning of C-14 among chemical fractions was a ffected by O-3 more in source leaves than in sink tissue. In source le aves, more C-14 was incorporated into the sugar, organic acid and lipi ds + pigments fractions, and less C-14 was incorporated into starch an d protein fractions in O-3-treated plants than in CF-treated plants. I n addition, there were O-3 treatment interactions between leaf positio n and clones for C-14 incorporation into different chemical fractions. When photosynthetic data were used to convert percent C-14 transporte d to the total amount of carbon transported on a mass basis, it was fo und that carbon transport was controlled more by photosynthesis in the source leaves than proportional changes in allocation to the sinks. O zone decreased the total amount of carbon translocated to all sink tis sue in the O-3-sensitive Clone 259 because of decreases in photosynthe sis in both recently mature and mature source leaves. In contrast O-3 had no effect on carbon transport from recently mature leaves to lower shoots of either Clone 216 or 271, had no significant effect on trans port to roots of Clone 216, and increased transport to roots of Clone 271. The O-3-induced increase in transport to roots of Clone 271 was t he result of a compensatory increase in upper leaf photosynthesis and a relatively greater shift in the percent of carbon allocated to roots . In contrast to those of Clone 271, recently mature leaves of Clone 2 16 maintained similar photosynthetic rates and allocation patterns in both the CF and O-3 treatments. We conclude that Clone 271 was more to lerant to O-3 exposure than Clone 216 or 259. Tolerance to chronic O-3 exposure was directly related to maintenance of high photosynthetic r ates in recently mature leaves and retention of lower leaves.