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 CO 2 generally enhances growth and yield. Numerous studies have been performed to determine effects of O 3 and CO 2 separately, but relatively few have been performed to determine if 03 can affect plant response to CO 2 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 CO 2 and two or three 03 treatments in all combinations during two seasons. The CO 2 treatments were ambient (370 μL L -1 ) and two treatments with CO 2 added for 24 h d -1 at approximately 1.5 and 2.0 times ambient. In 1995, the O 3 treatments were charcoal filtered air (CF), and nonfiltered air (NF) with 03 added for 12 h d -1 (NF+). In 1996, a NF treatment was also included to represent ambient 03 conditions. The CF, NF, and NF+ treatments resulted in seasonal 03 concentrations of approximately 23, 51, and 75 nL L -1 . Carbon dioxide enrichment generally stimulated growth and yield whereas 03 exposure suppressed growth and yield. Stimulation induced by CO 2 increased as O 3 stress increased. For example, in 1995 at medium N, the percentage increase in yield caused by doubling CO 2 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 CO 2 -induced prevention of 03 stress. The results emphasize the need to consider O 3 X CO 2 interactions to ensure correct interpretation of cause-effect relationships in CO 2 enrichment studies with crops that are sensitive to O 3 .