Delayed Leaf Senescence in Soybean¹

Abstract

Leaves of commercial soybeans [ Glycine max (L.) Merr.] typically senesce and abscise during seed development, but certain genetic lines produce mature seeds and show a delayed leaf senescence (DLS) phenotype in which leaves remain green until killed by frost. Field studies with such DLS lines provide information on the physiological and agronomic traits associated with such plants. Acetylene reduction and carbon exchange rates of DLS plants declined greatly after seeds matured, but positive values were measured until frost. Leaves formed before flowering which remained on DLS plants at the R8 stage had significantly (P ≤ 0.01) higher concentrations of reduced N and starch than those measured in leaves of normal plants just prior to abscission. In addition, a new population of axillary leaves containing substantial N and starch often developed during pod filling in DLS lines. The highest seed yield measured in DLS materials was 2370 kg/ha, which was 75 and 86% of the yields recorded in plots of ‘Elf’ and ‘Clark’, respectively. The highest yielding DLS lines accumulated as much N as Elf under conditions where both entries obtained about 75% of their N from N 2 . Progeny row studies of the DLS trait in two different genetic backgrounds showed significant (P ≤ 0.001) negative correlations between seed yield and the DLS phenotype. Reciprocal shoot/root grafts between Clark and DLS plants established that the DLS trait was controlled by the shoot genotype. It is concluded that expression of the DLS trait does not require severe inhibitions of reproductive development, such as is associated with male sterility. However, in the materials used in this study, the DLS phenotype was associated with a decrease in seed yield.