Development of genetic resistance to iron‐deficiency chlorosis in soybean

Abstract

One of the most thoroughly researched and agriculturally applicable examples of genetic resistance to iron‐deficiency chlorosis is in soybean [Glycine max (L.) Merr.]. Iron‐deficiency chlorosis is a serious economic problem in soybean‐producing areas with calcareous soils. The only practical solution to overcome iron‐deficiency chlorosis is utilization of genetic resistance. The objective of this paper is to review research on genetic resistance to iron‐deficiency in chlorosis and the development of resistant varieties. Genetic studies have revealed both simple inheritance patterns based on a single gene in some crosses and quantitative inheritance patterns probably based on several to many different genes in other crosses. A major obstacle to breeding for chlorosis resistance has been the difficulties associated with screening for chlorosis resistance using calcareous soils under field, greenhouse, and growth chamber conditions where chlorosis symptoms cannot be reliably replicated among experiments. This lack of replication is probably due to the complex chemical and physical criteria in both the plant and soil that must be met in order for chlorosis to occur. The high variation in chlorosis symptoms due to non‐genetic factors has reduced the effectiveness of selection for chlorosis resistance. Nutrient solution systems that mimic soil‐plant interactions responsible for chlorosis have been used successfully for inducing chlorosis symptoms similar to those observed in breeding material grown in calcareous soils in the USA, overcoming many of the limitations associated with screening in calcareous soil. Chlorosis resistance in soybeans grown in field nurseries in the USA has also been associated with iron reduction at the roots. Quantitative determination of iron reduction is a reliable indicator of the genetic potential for chlorosis resistance. Soybean iron reductases have been identified electrophoretically and some of the electrophoretic variation is associated with chlorosis resistance, particularly in chlorosis‐resistant and chlorosis‐susceptible isolines. The research conducted to date collectively suggests that resistance to iron‐deficiency chlorosis in soybeans is due to both the timing and level of iron reductase activity and that genetic variation for iron‐reductase expression in breeding material is predominantly quantitative, although some crosses apparently involve a single major allele. A substantial research effort in the past 50 years on genetic resistance to iron‐deficiency chlorosis in soybeans has produced a large body of information and several highly resistant cultivars. Further improvements in breeding programs may cause losses due to iron‐deficiency chlorosis to become negligible in the foreseeable future.