Effects of Ozone and Water Stress, Separately and in Combination, on Soybean Yield

A primary concern in applying existing 0₃-effects data on crop production is the relatively unknown influence of soil moisture, which may modify plant response to 0₃. One deficiency in field experiments that have tested the influence of soil moisture on crop response to 0₃ has been lack of control of soil moisture conditions in open-top chamber plots. Most experiments have relied on the occurrence of normal drought periods during the growing season and use of irrigation to adjust soil moisture conditions. This has not allowed the control of the water stress cycles that is desirable. In 1986 a field experiment was performed with soybean [Glycine max. (L.) Merr. cv. Young] to test the influence of periodic water stress on the yield response to O₃. Open-top field chambers were used to expose plants to a range of O₃ concentrations, and rain exclusion caps were used on individual chambers to help regulate soil moisture levels. Three soil moisture treatments were used [well-watered (WW), waterstressed (WS), and well-watered with permanent rain exclusion caps that were in place from 35 d after planting until physiological maturity (WW-C)]. In the WW and WS treatments, the rain caps were put in place only during an exceptionally wet period from mid-August to mid-September. The WW and WW-C treatments had approximately the same yield and response to O₃, indicating that the presence of the caps for most of the growing season had little effect on growth or sensitivity to O₃. The WS plots yielded approximately 10% less on the average than the WW and WW-C plots, but water stress did not change the response to O₃ (i.e., no significant O₃ × water interaction). Based on a Weibull dose-response model, O₃ reduced yield of ‘Young’ soybean 13% at a concentration of 0.05 µL L⁻¹ (12 h d⁻¹ seasonal mean) compared to a hypothetical background of 0.02 µL L⁻¹. Cooperative investigations of the USDA-ARS and the North Carolina State Univ. Paper no. 11819 of the Journal Series of the North Carolina Agric. Res. Serv., Raleigh, NC 27695-7601. Research partly supported by an Interagency Agreement between the USEPA and the USDA; Interagency Agreement number DW-12931347-01. The use of trade names in this publication does not imply endorsement by the North Carolina Agric. Res. Ser. or the USDA of the products named, nor criticism of similar ones not mentioned. Although the research described in this article was partly funded by the USEPA, it has not been subjected to paper and policy review and may not reflect the views of the Agency.