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

Elevated CO2 concentrations expected in the 21st century can stim-ulate plant growth and yield, whereas tropospheric 03 suppresses plant growth and yield in many areas of the world. Recent experiments showed that elevated CO2 often protects plants from 03 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 03 stress. An O3-tolerant cultivar (Tendereffe) and an 03-sensitive selection (S156) were exposed from shortly after emergence to maturity to mixtures of CO2 and 03 in open-top field chambers. The two CO2 treatments were ambient and ambient with CO2 added for 24 h d-l resulting in seasonal 12 h d-l (0800-2000 h EST) mean concentrations of 366 and 697 FL L-l, respectively. The twvo 03 treatments were charcoal-filtered air and nonfiltered air with 03 added for 12 h d-I to achieve seasona