Acute exposure to ozone inhibits rapid carbon translocation from source leaves of Pima cotton

Tropospheric ozone (O3) causes significant disruption of growth and yield in upland and Pima cottons. Pima cotton (Gossypium barbadense L.) was exposed to brief pulses (0.75 h) of a range of O3 concentrations (nominally 0.0, 0.2, 0.5, and 0.8 µl l−1) to investigate effects on phloem translocation of 14C-labelled recent photo-assimilate. The initial phase of rapid efflux from source leaves was monitored with a Geiger-Müller Tube as activity remaining in the leaf as a function of time. Visual inspection of unprocessed efflux curves revealed disruption of efflux by O3. Single exponential decay functions were fitted to these efflux curves to extract first order rate constants for phloem loading and longitudinal transport of labelled carbohydrates. A single compartment model was applied, with and without an asymptote of non-transported carbohydrate, to calculate leaf sugar contents. The effect of O3 in retarding efflux of label, decreasing the rate constant, and increasing calculated soluble sugar pools, was consistent regardless of the method of analysis. Following incorporation of the asymptote, calculated rate constants and sugar pools were similar to values from the literature and to preliminary measurements of sugar contents in O3-treated cotton leaves. Total carbohydrate transported from source leaves was reduced both by O3 effects on assimilation (up to 20%) and by O3 effects on efflux (up to 70%), but was clearly dominated by the impact on phloem translocation. These rapid efflux kinetics likely reflect oxidant damage at the plasmalemma or plasmodesmata of mesophyll or phloem companion cells. Evaluation of effects of O3 on tonoplast function and consequences for carbohydrate translocation await a more complete compartmental efflux analysis.