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

Ozone (O-3) in the troposphere can cause plant stress leading to foliar injury and suppressed growth and yield, whereas elevated CO2 generally enhances growth and yield. Numerous studies have been performed to determine effects of O-3 and CO2 separately, but relatively few have been performed to determine if O-3 can affect plant response to CO2 or vice versa. Open-top field chambers were used to determine if such interactions occur for cotton (Gossypium hirsutum L.), which is relatively sensitive to O-3. Nitrogen nutrition is especially important in cotton production so N nutrition was included 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 treatments in all combinations during two seasons. The CO2 treatments were ambient (370 mu L L-1) and two treatments with CO2 added for 24 h d(-1) at approximately 1.5 and 2.0 Limes ambient. In 1995, the O-3 treatments were charcoal 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 concentrations of approximately 23, 51, and 75 nL L-1. Carbon dioxide enrichment generally stimulated growth and yield whereas O-3 exposure suppressed growth. 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 occurred for a range of soil N levels, and were probably caused by CO2-induced prevention of O-3 stress. The results emphasize the need to consider O-3 x CO2 interactions to ensure correct interpretation of cause-effect relationships in CO2 enrichment studies with crops that are sensitive to O-3.