Ozone stress, carbon dioxide enrichment, and nitrogen fertility interactions in cotton

Ozone (O-3) in the troposphere can cause plant stress leading to foliar inj ury and suppressed growth and yield, whereas elevated CO2 generally enhance s growth and yield. Numerous studies have been performed to determine effec ts of O-3 and CO2 separately, but relatively few have been performed to det ermine if O-3 can affect plant response to CO2 or vice versa. Open-top fiel d chambers were used to determine if such interactions occur for cotton (Go ssypium hirsutum L.), which is relatively sensitive to O-3. Nitrogen nutrit ion is especially important in cotton production so N nutrition was include d as an experimental factor. Plants were grown in 14-L pots at low, medium, and high soil N levels and exposed to three CO2 and two or three O-3 treat ments in all combinations during two seasons. The CO2 treatments were ambie nt (370 mu L L-1) and two treatments with CO2 added for 24 h d(-1) at appro ximately 1.5 and 2.0 Limes ambient. In 1995, the O-3 treatments were charco al filtered air (CP), and nonfiltered air (NF) with O-3 added for 12 h d(-1 ) (NF+). In 1996, a NF treatment was also included to represent ambient O-3 conditions. The CF, NF, and NF+ treatments resulted in seasonal O-3 concen trations of approximately 23, 51, and 75 nL L-1. Carbon dioxide enrichment generally stimulated growth and yield whereas O-3 exposure suppressed growt h. and yield. Stimulation induced by CO2 increased as O-3 stress increased. For example, in 1995 at medium N, the percentage increase in yield caused by doubling CO2 in CF air was 0%, but was 52% in NF+ air. Comparable values for 1996 were 23% in CF air and 140% in NF+ air. These interactions occurr ed for a range of soil N levels, and were probably caused by CO2-induced pr evention of O-3 stress. The results emphasize the need to consider O-3 x CO 2 interactions to ensure correct interpretation of cause-effect relationshi ps in CO2 enrichment studies with crops that are sensitive to O-3.