USE OF C-13 AND N-15 ISOTOPES TO INVESTIGATE O-3 EFFECTS ON C-METABOLISM AND N-METABOLISM IN SOYBEANS .1. C-FIXATION AND TRANSLOCATION

Short- and long-term C uptake/transaction dynamics were studied using stable isotope techniques and leaf gas exchange to investigate the mec hanism(s) of O-3 action on plant yield and C partitioning. Glycine mar (L.) Merr. (soybean) plants were grown in open-top chambers and expos ed to one of three O-3, regimes: half-ambient, ambient, or 2 x ambient for nearly the entire growing season. The seasonal 7 h average O-3 co ncentrations (nl l(-1)) were 25, 43, and 76 nl l(-1), respectively. Wh ole plant C translocation was measured using pulse-labeled (CO2)-C-13 (99 atom %C-13) at two distant growth stages (R2 and R5). Translocatio n parameters were as follows: %C-13 (sink strength), % C-13/g dry weig ht (sink intensity), and % C-13/% Organ dry weight (relative specific uptake). Single leaf photosynthesis (P-n) was measured at four growth stages (V7, R2, R3, and R4). Ozone significantly affected translocatio n, but the effect was dependent upon growth stage and the time followi ng the C-13 pulse. At the stage of rapid seed fill within the pods (R5 ), and at 42 h post-labeling, all three leaf translocation parameters had a significant positive linear relationship with O-3 exposure. Conv ersely, root nodule values were all inversely related to O-3 exposure. Generally, at 0.5 h post-labeling, no significant effects were observ ed for leaves and nodule translocation patterns, with the exception of an inverse relationship between sink strength and O-3 exposure. No si gnificant differences were observed for single leaf P-n among treatmen ts. Our results indicate that the mechanism of chronic O-3 action invo lves inhibition of translocation, implying reduced phloem loading and the inhibition may be occurring without a concomitant reduction in the amount of C fixed. In addition, C-13 pulse labeling appears to be a v ery useful technique for investigating integrated long-term C transloc ation dynamics which might not otherwise be evident using instantaneou s methods such as short-term labeling or limited leaf gas exchange mea surements.