EFFECT OF O-3 ON HYDRAULIC ARCHITECTURE IN PIMA COTTON

Pima cotton (Gossypium barbadense L. cv S-6) exhibits foliar injury an d yield reduction at ambient concentrations of O-3. We tested the hypo theses that O-3 reduces the allocation of biomass to the root system, and that this disrupted carbohydrate allocation impairs root hydraulic capacity relative to transpiring leaf area. Both hypotheses are suppo rted, even though leaf area development is itself reduced by O-3. Seed lings were grown in pots in greenhouse fumigation chambers and exposed from planting to sinusoidal O-3 profiles with peak concentrations of 0, 0.1, 0.2, and 0.3 mu L L(-1) (12-h averages of 0, 0.037, 0.074, and 0.111 mu L L(-1)). At 8 weeks after planting, stem basal diameter, le af area, and total plant dry weight decreased by 61, 83, and 88%, wher eas root/shoot dry weight ratio declined from 0.16 to 0.09 g/g. Hydrau lic conductance decreased per plant by 85%, and per unit leaf area by 35%. Conductance of all organs declined per plant, but only root condu ctance declined per leaf area by 41%. Root resistance increased from 6 9 to 82% of whole plant resistance, a functional consequence of reduce d carbon allocation to roots. Stomatal conductance declined with root hydraulic conductance, protecting short-term lear water status. Reduce d root hydraulic efficiency may mediate O-3 injury to whole plants by reducing shoot gas exchange and biomass productivity through the inhib ition of water and nutrient acquisition.