Control of iron‐deficiency chlorosis in soybeans by plant breeding

Abstract

A series of research studies has been conducted at Iowa State University to determine the possibilities and problems associated with the development of soybean (Glycine max (L.) Merrill) cultivars with improved resistance to iron‐deficiency chlorosis on calcareous soils. An evaluation was made of the amount of yield reduction associated with different levels of chlorosis symptoms. Cultivars ranging from highly resistant to highly susceptible to iron chlorosis were tested on calcareous and noncalcareous soils. Chlorosis symptoms were rated visually with scores ranging from 1, no yellowing, to 5, severe yellowing with some necrosis. The average percentage yield loss increased linearly by 20% for each unit increase in chlorosis score. There was yield reduction whenever yellowing was observed, even if the symptoms were slight. To completely avoid yield loss from iron chlorosis, cultivars must be developed that have no symptoms any time during plant development.

Tests of soybean cultivars and plant introductions from foreign countries indicated that no genotype was available that did not exhibit some chlorosis symptoms on some calcareous soils in Iowa. A breeding program was initiated to develop lines with an improved level of resistance to iron chlorosis. Lines were developed with resistance superior to any genotype that has been tested for chlorosis resistance in Iowa. The selection program and related inheritance studies have indicated that resistance to iron chlorosis is controlled by multiple genes and can be considered a quantitative character for breeding purposes, even though a major gene Fe is related to iron efficiency in soybeans.

Cultivars with high yield and improved resistance to iron chlorosis have been developed and released to farmers for use in minimizing loss from iron chlorosis, but susceptible cultivars with higher yield than resistant cultivars are available. A farmer with spots of calcareous soil in a field may prefer to plant the higher yielding susceptible cultivar, but may not want to lose yield on the calcareous areas. One solution is to plant a seed mixture of a resistant and a susceptible cultivar. Our research indicates that only 30% of the resistant component in the mixture may be needed to almost completely compensate in yield for loss of the susceptible component on calcareous soil.

The efficiency of breeding could be improved if a laboratory test were developed to screen genotypes for resistance to iron chlorosis.

Key words:

• Seed yield
• agronomic performance
• genetics
• recurrent selection
• seed blends
• calcareous soil