Feasibility of selection for high weed suppressive ability in soybean: Absence of tradeoffs between rapid initial growth and sustained later growth

In ecology, tradeoff theory has been used to understand differences among p lant species in their competitive abilities. In efforts to develop weed-sup pressive soybean, we found evidence of a tradeoff between the ability for r apid initial growth and the ability for sustained growth later in the seaso n: early maturing lines displayed more rapid initial growth but ceased grow th sooner. Such a tradeoff would increase the difficulty of obtaining a ful l-season weed-suppressive variety. To determine this tradeoff's existence a nd severity we examined two possible mechanisms that could lead to it. We t ested whether early maturing soybean lines attain higher early relative gro wth rate than late-maturing soybean lines and whether early maturing soybea n lines produce larger seeds by an environmentally-dependent or -independen t mechanism. Early maturing lines had higher relative stem elongation rates than late lines but not higher relative dry weight or leaf area increase r ates. In more northern locations and in years with shorter growing seasons, early maturity lines produced larger seeds than late maturing lines, impli cating an environmentally-dependent rather than -independent mechanism caus ing seed size differences. Relative to early lines, when late lines mature, temperature and photoperiod are in greater decline, leading to a risk of i ncomplete seed fill. Resulting seed size decreases could lead to lower init ial growth in late maturity lines and thus to an environmental rather than genetic or physiological cause of tradeoff between initial growth and susta ined growth later in the season.