Selection of southern soybean genotypes resistant to iron deficiency chlorosis
Phelps, Norris D.
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Soybean [Glycine max (L.) Merr.] genotypes were evaluated using three field and four greenhouse experiments to determine the extent of iron deficiency chlorosis (IDC). Evaluation of growth media was more effective when using freshly sampled soil from the field than when using fritted clay and nutrient solutions. The genotypes A7 (Maturity Group I) and T203 (MG II) were chosen as resistant and susceptible genotype checks, respectively, based on screening done with northern soybean genotypes. Mitchell (MG IV) and Bedford (MG V) were chosen as susceptible checks while Crawford (MG IV) and RA(d)41 (MG V) were selected as resistant checks of mid- and late-maturity groups, respectively. Visual chlorosis scores and leaf measurements were used as indices of the extent of iron chlorosis and for corresponding estimates of the effect of IDC on yield. Leaf samples were taken from the first two trifoliolate leaves at or near the third vegetative stage. One sample was used for measurements of dry weight, the other for optical density of total chlorophyll. Fresh weight and surface area were used from leaf disc samples while leaf area and leaf fresh weight readings were taken from whole leaf samples. Chlorophyll a, b, and total chlorophyll values were determined using measurements taken with a spectrophotometer at 6450, 6630, and 6520 angstroms, respectively, using leaf tissue extracted with 80% acetone. The experimental analyses employed a randomized complete block design. The analyses showed genotype effects (P<0.01) in both greenhouse and field screening for visual chlorosis score and total chlorophyll per leaf dry weight. Chlorosis scores, yield, and total chlorophyll per dry weight were reliable as estimates of the effects of IDC for plants stressed by field conditions. Refinements in the greenhouse screening procedures are necessary.