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 plant species in their competitive abilities. In efforts to develop weed-suppressive soybean, we found evidence of a tradeoff between the ability for rapid initial growth and the ability for sustained growth later in the season: early maturing lines displayed more rapid initial growth but ceased growth sooner. Such a tradeoff would increase the difficulty of obtaining a full-season weed-suppressive variety. To determine this tradeoff's existence and severity we examined two possible mechanisms that could lead to it. We tested whether early maturing soybean lines attain higher early relative growth rate than late-maturing soybean lines and whether early maturing soybean lines produce larger seeds by an environmentally-dependent or -independent mechanism. Early maturing lines had higher relative stem elongation rates than late lines but not higher relative dry weight or leaf area increase rates. In more northern locations and in years with shorter growing seasons, early maturity lines produced larger seeds than late maturing lines, implicating an environmentally-dependent rather than -independent mechanism causing seed size differences. Relative to early lines, when late lines mature, temperature and photoperiod are in greater decline, leading to a risk of incomplete seed fill. Resulting seed size decreases could lead to lower initial growth in late maturity lines and thus to an environmental rather than genetic or physiological cause of tradeoff between initial growth and sustained growth later in the season.