Performance of Chlorophyll‐Deficient and Normal Near‐Isolines of Two Soybean Cultivars

Leaves of soybean [ Glycine max (L.) Merr.] seedlings heterozygous at the y11 locus contain less chlorophyll, but have net CO 2 assimilation rates equal to rates of normal green leaves when grown at relatively low irradiance in growth chambers. The y11 allele is lethal in the homozygous condition and plants with this genotype die at the unifoliolate or first trifoliolate stage of development. The purpose of this study was to evaluate effects of the y11 allele on agronomic and physiological characteristics of isolines of two soybean cultivars when grown in the field under normal production conditions. Normal and light green near‐isolines were developed by backcrossing the y11 allele into the cultivars Century and Hobbit. Traits were evaluated in 10‐replicate tests for a 3‐yr period in the field at West Lafayette, IN. Chlorophyll concentration in leaves of the light green isolines averaged 67% of that of normal isolines at the pod‐fill stage of growth. Normal isolines averaged 20% taller at maturity and produced 22% greater yields than light green isolines. Yield reductions of the light green isolines were due primarily to fewer seeds per unit area in 2 yr with typical rainfall patterns. In a very dry year, yield reductions of the light green isolines were due to reductions in both seed number and seed size. Changes in frequency of the y11 allele were observed during four successive generations when the isolines were grown in competition with each other. The light green isolines averaged 70% of the reproductive efficiency of the normal green isolines in these competitive tests.