Linear Increase in Soybean Harvest Index during Seed‐Filling1

Abstract

Grain‐to‐stover ratio and harvest index (the ratio of seed mass to total aboveground mass) were studied during soybean [Glycine max (L.) Merr.] seed‐filling to determine the potential for using these normalized expressions of yield to improve measurements of the seed‐filling period. For example, if either grain‐to‐stover ratio or harvest index increased linearly during seed‐filling, it could be used to calculate an effective filling period. However, calculation of an effective filling period requires a linear approximation. Thus, the specific objective was to determine whether grain‐to‐stover ratio or harvest index increased as a linear function of time for various genotypes and environments. First, C and N assimilation and distribution rates within plants were simulated. Simulations predicted that harvest index increased linearly throughout seed filling, while grain‐to‐stover ratio increased curvilinearly. Next, simulations were compared with data from field‐grown plants at three northern locations. Cultivars included determinate and indeterminate types, and a range from early to late maturities. To a good approximation, harvest index increased linearly during the seed‐filling period for all but the early cultivars. The early cultivars approached maximum values of harvest index curvilinearly. Therefore, harvest index has potential for use in calculations which require linear approximations.