Assessing the influence of exogenous ethylene on electron transport and fluorescence quenching in leaves of Glycine max

We conducted a series of modeling exercises designed to re-evaluate the light-response and CO₂-response curves of Taylor and Gunderson (Pl. Physiol. 86, 85-92, 1988) and to examine further their conclusion that ethylene-induced inhibition of electron transport may contribute to reduced CO₂ assimilation in leaves of Glycine max. By partitioning the response of CO₂ assimilation to either electron transport-limited or Rubisco-limited rates of carboxylation, we calculated that the electron transport capacity (J_max) of ethylene-treated leaves decreased by over 30% following a 4-hr exposure to 10 μl/l ethylene and noted that ethylene-induced reductions in CO₂ assimilation could be explained without a decrease in Rubisco activity (Vc_max). Measurements of in vivo Chl fluorescence supported these observations and indicated that the efficiency by which excitation energy was captured in PSII (i.e. (F_m - F_o/F_m) was reduced from 0.80 to 0.73 after a 4-hr exposure to 10 μl/l ethylene. This reduction was also accompanied by a 12% decrease in steady-state photochemical quenching (q_p), indicating that a lower proportion of open or oxidized PSII reaction centers were participating in light-dependent processes. Effects of ethylene on Chl fluorescence were amplified at increased irradiance, suggesting that photoinhibition may play a role in the ethylene-induced inhibition of CO₂ assimilation.