Use of 13C and 15N isotopes to investigate O3 effects on C and N metabolism in soybeans. Part I. C fixation and translocation

Abstract

Short- and long-term C uptake/transaction dynamics were studied using stable isotope techniques and leaf gas exchange to investigate the mechanism(s) of O3 action on plant yield and C partitioning. Glycine max (L.) Merr. (soybean) plants were grown in open-top chambers and exposed to one of three O3 regimes: half-ambient, ambient, or 2 × ambient for nearly the entire growing season. The seasonal 7 h average O3 concentrations (nl l−1) were 25, 43, and 76 nl l−1, respectively. Whole plant C translocation was measured using pulse-labeled 13CO2 (99 atom %13C) at two distant growth stages (R2 and R5). Translocation parameters were as follows: %13C (sink strength), % 13C/g dry weight (sink intensity), and % 13C/% organ dry weight (relative specific uptake). Single leaf photosynthesis (Pn) was measured at four growth stages (V7, R2, R3, and R4). Ozone significantly affected translocation, but the effect was dependent upon growth stage and the time following the 13C 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 O3 exposure. Conversely, root nodule values were all inversely related to O3 exposure. Generally, at 0.5 h post-labeling, no significant effects were observed for leaves and nodule translocation patterns, with the exception of an inverse relationship between sink strength and O3 exposure. No significant differences were observed for single leaf Pn among treatments. Our results indicate that the mechanism of chronic O3 action involves 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, 13C pulse labeling appears to be a very useful technique for investigating integrated long-term C translocation dynamics which might not otherwise be evident using instantaneous methods such as short-term labeling or limited leaf gas exchange measurements.