Da. Grantz et Jf. Farrar, Acute exposure to ozone inhibits rapid carbon translocation from source leaves of Pima cotton, J EXP BOT, 50(336), 1999, pp. 1253-1262
Tropospheric ozone (O-3) causes significant disruption of growth and yield
in upland and Pima cottons. Pima cotton (Gossypium barbadense L.) was expos
ed to brief pulses (0.75 h) of a range of O-3 concentrations (nominally 0.0
, 0.2, 0.5, and 0.8 mu l l(-1)) to investigate effects on phloem translocat
ion of C-14-labelled recent photoassimilate. The initial phase of rapid eff
lux from source leaves was monitored with a Geiger-Muller Tube as activity
remaining in the leaf as a function of time. Visual inspection of unprocess
ed efflux curves revealed disruption of efflux by O-3. Single exponential d
ecay functions were fitted to these efflux curves to extract first order ra
te constants for phloem loading and longitudinal transport of labelled carb
ohydrates. A single compartment model was applied, with and without an asym
ptote of non-transported carbohydrate, to calculate leaf sugar contents. Th
e effect of O-3 in retarding efflux of label, decreasing the rate constant,
and increasing calculated soluble sugar pools, was consistent regardless o
f the method of analysis. Following incorporation of the asymptote, calcula
ted rate constants and sugar pools were similar to values from the literatu
re and to preliminary measurements of sugar contents in O-3-treated cotton
leaves. Total carbohydrate transported from source leaves was reduced both
by O-3 effects on assimilation (up to 20%) and by O-3 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 plasmalemm
a or plasmodesmata of mesophyll or phloem companion cells. Evaluation of ef
fects of O-3 on tonoplast function and consequences for carbohydrate transl
ocation await a more complete compartmental efflux analysis.