Selection of Peas for Photosynthetic CO 2 Exchange Rate under Field Conditions 1

The variability and heritability of photosynthetic CO 2 exchange rate (CER) in peas ( Pisum sativum L.) were examined. Twenty‐five genotypes were measured in the field during 1976 and 1977, and genotype mean CER differed significantly ranging between 12.1 and 19.5 mg CO 2 dm −2 hour −2 (1976) and between 18.9 and 35.5 mg CO 2 dm− 2 hour− 1 (1977). Three genotypes with high (‘Trapper’ ‘Alaska,’ PI356998) and three with low (JM9, PI269812, PI180700) CER were selected and retested in the field and in a controlled environment. Despite a significant genotype ✕ environment interaction, these high and low groups were consistently separable in the field. Under more equable environmental conditions in the growth cabinet however, CER differences were not apparent. Diallel crosses were made within the high and low groups and, although parental genotypes selected for high CER did not differ from each other, the F 4 lines showed significant segregation, and ft lines reselected for high CER and retested in the F 4 and F 5 generations, had higher values than the parents. F 4 progeny from high CER crosses showed some evidence of reciprocal effects, but F 1 and F 2 progeny from a high ✕ low CER cross gave no indication of non‐additivity. Heritability of CER estimated by several methods averaged 0.7. The results demonstrate the theoretical feasibility of using CER as a selection criterion, the practical limitations of methodology, and the likelihood of genotype ✕ environment interactions in a broad range of environments.