Growth and Yield Responses of Snap Bean to Mixtures of Carbon Dioxide and Ozone

Elevated CO 2 concentrations expected in the 21st century can stimulate plant growth and yield, whereas tropospheric O 3 suppresses plant growth and yield in many areas of the world. Recent experiments showed that elevated CO 2 often protects plants from O 3 stress, but this has not been tested for many important crop species including snap bean ( Phaseolus vulgaris L.). The objective of this study was to determine if elevated CO 2 protects snap bean from O 3 stress. An O 3 –tolerant cultivar (Tenderette) and an O 3 –sensitive selection (S156) were exposed from shortly after emergence to maturity to mixtures of CO 2 and O 3 in open‐top field chambers. The two CO 2 treatments were ambient and ambient with CO 2 added for 24 h d −1 resulting in seasonal 12 h d −1 (0800–2000 h EST) mean concentrations of 366 and 697 μL L −1 , respectively. The two O 3 treatments were charcoal‐filtered air and nonfiltered air with O 3 added for 12 h d −1 to achieve seasonal 12 h d −1 (0800–2000 h EST) mean concentrations of 23 and 72 nL L −1 , respectively. Elevated CO 2 significantly stimulated growth and pod weight of Tenderette and S156, whereas elevated O 3 significantly suppressed growth and pod weight of S156 but not of Tenderette. The suppressive effect of elevated O 3 on pod dry weight of S156 was approximately 75% at ambient CO 2 and approximately 60% at elevated CO 2 (harvests combined). This amount of protection from O 3 stress afforded by elevated CO 2 was much less than reported for other crop species. Extreme sensitivity to O 3 may be the reason elevated CO 2 failed to significantly protect S156 from O 3 stress.