Response of soybean to low concentrations of ozone: II. Effects on growth, biomass allocation, and flowering

Soybean plants were subjected to ozone (O₃) and water stress to determine whether the two stresses interact to alter growth. Potted soybean (Glycine max [L.] Merril cv. Hodgon) plants were exposed to O₃ for 6.8 h daily in four controlled environment chambers. The air supply to the chambers was filtered, and constant amounts of O₃ were added: 0.01, 0.05, 0.09, or 0.13 μL L^-1. Within each O₃ treatment, half of the plants were watered daily and half had water withheld 2 or 3 d per week. At 2-week intervals plants were harvested and divided into leaves, stems, and roots. Exposure to increasing concentrations of O₃ resulted in a linear decline in growth, but did not influence the allocation of biomass to roots, stems, and leaves. Ozone also delayed the onset of flowering. Weekly water stress periods of 2 or 3 d duration significantly inhibited the growth of soybean but did not alter the allocation of biomass or delay the onset of flowering. Water stress acted to close stomates and reduce the effective O₃ dose which resulted in a reduced percent reduction in growth that could be attributed to O₃ stress.