Da. Grantz et Jf. Farrar, Ozone inhibits phloem loading from a transport pool: compartmental efflux analysis in Pima cotton, AUST J PLAN, 27(8-9), 2000, pp. 859-868
The rate of export of recent photoassimilate from source leaves of Pima cot
ton (Gossypium barbadense L.) is inhibited by ozone (O-3). To characterize
these effects on export, source leaves of Pima cotton were exposed to pulse
s (0.75 h) of O-3 (0.0, 0.2, 0.5 and 0.8 mu L L-1) followed by pulses of (C
O2)-C-14. Leaves were monitored by gas exchange and with a Geiger-Muller tu
be, for a sufficient period to characterize carbon assimilation (A) and a r
apid and a slower phase of export. Double exponential decay functions (two-
compartment model) were fitted and a compartmental analysis conducted. O-3
reduced by half the fast rate constant describing export from a transport p
ool, without affecting the rate constants for transport from or to a storag
e compartment. Measured soluble sugar contents increased slightly from cont
rol concentrations (1.2 g C m(-2)) by about 5-10% at all O-3 concentrations
. The calculated soluble sugar content in the transport pool increased from
about 200 to 300 mg C m(-2) with increasing exposure to O-3. The calculate
d storage pool did not respond to O-3 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 O-3 exposure. Specific inhibi
tion of rapid efflux suggests oxidant damage at the plasmalemma or plasmode
smata of mesophyll or phloem companion cells, and little effect on the tono
plast. A was affected less than export. Future research should target oxida
tion of components involved in phloem loading.