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Stem Cutoff Enhances Selection for Improved Iron Efficiency of Soybean¹

Five cycles of recurrent selection for iron (Fe) efficiency resulted in a soybean [Glycine mar (L.) Merr.] population with a high percentage of segregates that showed little or no yellowing when grown on calcareous soil in the field. Further improvement for Fe efficiency in the population is not possible unless a method is available to enhance the expression of genetic variability. This study was conducted to evaluate stem cutoff as a method for enhancing the expression of genetic variability for Fe efficiency in a population with a high proportion of Fe-efficient segregates. The parents of Cycles 0 through 5 and 100 S₀-derived lines from the Cycle 5 population of AP9 were grown on calcareous soil. At stage V3, when the second trifoliolate leaf was fully developed, the entries were scored for chlorosis (1 = no yellowing to 5 = severe yellowing); then the main stem was cut off above the unifoliolate node. The new leaves produced were evaluated for chlorosis 2 weeks after treatment. Stem cutoff significantly increased the chlorosis of the parents and S₀-derived lines by an average score of 1.4. The rate of genetic improvement from Cycle 0 to Cycle 5 was the same before and after treatment, with an average decrease in chlorosis score of –0.16 per cycle before treatment and –0.15 per cycle after treatment. Genetic variability among the 100 random lines was significantly increased by stem cutoff. The genotypic variance component estimate was 0.02 ± 0.01 before treatment and 0.10 ± 0.02 after treatment. The results indicated that stem cutoff is an effective method of increasing chlorosis in a population with a high level of Fe efficiency, permitting selection for further improvement of the character.

Key Words: Glycine max (L.) Merrill • Plant stress • Recurrent selection • Breeding • Iron chlorosis

² Graduate research assistant, professor,and research associate, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication November 27, 1985.