Ozone inhibits phloem loading from a transport pool: compartmental efflux analysis in Pima cotton

Abstract

The rate of export of recent photoassimilate from source leaves of Pima cotton (Gossypium barbadense L.) is inhibited by ozone (O₃). To characterize these effects on export, source leaves of Pima cotton were exposed to pulses (0.75 h) of O₃ (0.0, 0.2, 0.5 and 0.8 L L^–1) followed by pulses of ¹⁴CO₂. Leaves were monitored by gas exchange and with a Geiger–Muller tube, for a sufficient period to characterize carbon assimilation (A) and a rapid and a slower phase of export. Double exponential decay functions (two-compartment model) were fitted and a compartmental analysis conducted. O₃ reduced by half the fast rate constant describing export from a transport pool, without affecting the rate constants for transport from or to a storage compartment. Measured soluble sugar contents increased slightly from control concentrations (1.2 g C m^–2) by about 5–10% at all O₃ concentrations. The calculated soluble sugar content in the transport pool increased from about 200 to 300 mg C m^–2 with increasing exposure to O₃. The calculated storage pool did not respond to O₃ but exceeded measured contents. This discrepancy is attributed to starch deposition and mobilization, which are not considered in the two-compartment model, uncertainties in slower decay parameters, and non-steady-state A induced by O₃ exposure. Specific inhibition of rapid efflux suggests oxidant damage at the plasmalemma or plasmodesmata of mesophyll or phloem companion cells, and little effect on the tonoplast. A was affected less than export. Future research should target oxidation of components involved in phloem loading.