Sulfur dioxide-enhanced phytotoxicity of ozone to watermelon

Three watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] cultivars with different ozone (O-3) sensitivities were grown in a charcoal-filtered greenhouse and exposed in continuous-stirred tank reactor chambers to five levels (0, 100, 200, 300, or 400 nL . L(-1)) of sulfur dioxide (SO2) in the presence (80 nL . L(-1)) or absence (0 nL . L(-1)) of ozone (O-3) for 4 hours/day, 5 days/week for 22 days. In the presence of O-3, SO2 increased foliar injury in all three cultivars, but the impact was greatest for the most O-3-sensitive cultivar, 'Sugar Baby,' moderate for 'Crimson Sweet,' and least for the least O-3-sensitive cultivar, 'Charleston Gray.' For all cultivars, SO2 intensified O-3 suppression of leaf area for the first seven mainstem leaves and of dry weights for aboveground and total plant tissues. Root dry weight was independently suppressed by both pollutants, and the root : top ratio was linearly suppressed by SO2 alone. Sulfur dioxide combined with O-3 can be detrimental to crop species such as watermelon. Thus, the potential for SO2 phytotoxicity should not be summarily dismissed, especially in the vicinity of SO2 point sources where O-3 co-occurs.