Fig. 4. Effect of ozone exposure on (A) leaf chlorophyll concentration (SPAD units) of cotton sampled at different insertion levels: upper, mid-, and lower canopy (open symbols), or on nutsedge grown alone (solid squares), on (B) net carbon assimilation, and (C) stomatal conductance to water vapor of young exposed leaves of nutsedge (open squares) and cotton (open circles), grown alone. Mean separation as in Fig. 1. ** indicates P 0.01, and indicates P 0.10.