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 µL L -1 ) followed by pulses of 14 CO2. 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 com- partmental 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 sug- gests 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.