LIGHT-INTENSITY, ROW SPACING, AND PHOTOPERIOD EFFECTS ON EXPRESSION OF BRACHYTIC STEM IN SOYBEAN

Brachytic stem trait in soybean [Glycine max (L.) Merr] is characterized by a geniculate internode arrangement and markedly shortened internodes; brachytic plants are much shorter than their nonbrachytic isolines. When the brachytic trait expression is suppressed, plants have heights similar to those of their nonbrachytic isolines. A series of greenhouse and field experiments involving light intensity and photoperiod treatments were conducted to determine their effects on plant height and other stem traits in 'Tracy-M' and 'Wright' soybean and in their brachytic near-isolines, B-Tracy-M and B-Wright. The experiments demonstrated that full expression of the brachytic stem trait is a light-regulated process. Under unshaded conditions in the field, B-Wright and B-Tracy-M had a plant height 48.7 and 78.9% of that of their respective nonbrachytic isolines. The most effective shading treatment (shade applied between 5 and 7 wk after emergence) produced a 46 and a 22% increase in plant height for B-Tracy-M and B-Wright, respectively, by increasing internode length. The shade treatment also suppressed the geniculate expression, reducing the internodal angles by 5 to 20-degrees for B-Tracy-M and 6 to 26-degrees for B-Wright. After shade removal, brachytic stem development resumed in subsequently formed internodes. The wide-row (76 cm) spacing (more plants per meter of row) decreased the percentage of brachytic plants at early growth stages by 47.5% and 22.7% for B-Tracy-M and B-Wright, respectively, compared with the narrow-row (25 cm) spacing. Increasing photoperiod from 14 to 16 h in the greenhouse decreased plant height and enhanced the development of brachytic stem trait. In general, B-Tracy-M appeared to be more sensitive than B-Wright to the treatments imposed in these studies.