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

Abstract

O3 could reduce growth and carbohydrate allocation to roots by direct inhibition of photosynthesis and source strength. Alternatively, O3 could reduce growth indirectly by inhibition of root hydraulic development through a primary lesion in carbohydrate translocation. Another alternative is that O3 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) O3 exposure reduces leaf pools of soluble sugars; (2) pruning leaf area and reducing source strength to match that of O3-treated plants reproduces O3-effects; (3) pruning lower leaf area more closely reproduces O3 effects than pruning upper leaf area; and (4) manipulating plant age and thereby plant size to match O3-treated plants reproduces O3-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 O3-exposure, but neither reproduced O3 effects on biomass allocation nor root function. It is concluded that O3 induces an allometric shift in carbohydrate allocation that is not mediated by photosynthetic inhibition nor by alteration of developmental age. Effects of O3 could be mediated by direct effects on phloem loading, with consequent inhibition of translocation to roots and root system development.