Ozone impacts on allometry and root hydraulic conductance are not mediated by source limitation nor developmental age

O-3 could reduce growth and carbohydrate allocation to roots by direct inhibition of photosynthesis and source strength. Alternatively, O-3 could reduce growth indirectly by inhibition of root hydraulic development through a primary lesion in carbohydrate translocation. Another alternative is that O-3 could slow the rate of plant development, only apparently altering carbohydrate allocation at a given plant age. Pima cotton (Gossypium barbadense L.) is used to address these possibilities, and four hypotheses were tested: (1) O-3 exposure reduces leaf pools of soluble sugars; (2) pruning leaf area and reducing source strength to match that of O-3-treated plants reproduces O-3-effects; (3) pruning lower leaf area more closely reproduces O-3 effects than pruning upper leaf area; and (4) manipulating plant age and thereby plant size to match O-3- treated plants reproduces O-3-effects. All were falsified. Soluble sugars did not decline. Pruning upper and lower leaves and manipulating plant age all reduced biomass and leaf area similarly to O-3-exposure, but neither reproduced O-3 effects on biomass allocation nor root function. It is concluded that O-3 induces an allometric shift in carbohydrate allocation that is not mediated by photosynthetic inhibition nor by alteration of developmental age. Effects of O-3 could be mediated by direct effects on phloem loading, with consequent inhibition of translocation to roots and root system development.