Growth and yield responses of snap bean to mixtures of carbon dioxide and ozone

Elevated CO2 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 CO2 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 CO2 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 CO2 and O-3 in open-top field chambers. The two CO2 treatments were ambient and ambient with CO2 added for 24 h d(-1) resulting in seasonal 12 h d(-1) (0800-2000 h EST) mean concentrations of 366 and 697 muL 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 CO2 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 03 on pod dry weight of S156 was approximately 75% at ambient CO2 and approximately 60% at elevated CO2 (harvests combined). This amount of protection from 03 stress afforded by elevated CO2 was much less than reported I I or other crop species. Extreme sensitivity to O-3 may be the reason elevated CO2 failed to significantly protect S156 front O-3 stress.