Stability of Soybean Harvest Index1

Abstract

AbstractPrevious work had indicated that the harvest index within cultivars of soybeans [Glycine max(L.) Merr.] might be fairly stable. Such stability of harvest index would greatly simplify the conceptualization of the influence of seed growth processes on yield. In the studies reported here harvest index of field‐grown soybean plants was measured in response to four types of environmental influence: interplant competition, extended photoperiod, timing of drought stress, and degrees of drought stress (line source irrigation). The experiments included both indeterminate and determinate cultivars, and included both a northern (Ithaca, N.Y.) and southern location (Gainesville, Fla). These experimental conditions resulted in considerable variability in plant sizes within a cultivar but the harvest index (ratio of seed mass to mass of mature plant without fallen leaves and petioles) was stable. Total masses of individual mature ‘Chippewa 64’ and ‘Bragg’ plants varied substantially in response to interplant competition, but harvest index was essentially constant (0.54 and 0.50 for Chippewa 64 and Bragg, respectively) across most of the range of plant sizes. Harvest index of smaller plants was less than for larger plants, but these smaller plants have little influence on total crop yield and harvest index. Extending the photoperiod to 14.5 h throughout the life cycle of six cultivars increasingly delayed ontogenic events as the distance from the photoperiod lamps decreased. However, no influence on harvest index was observed except at the position nearest the lamps where the standard errors of the harvest indices were increased. Harvest index did not respond to differences in the timing of severe drought stress. When rain was withheld from ‘Wilkin’ plants during vegetative growth, flowering, or seed setting, harvest index of individual plants was essentially 0.60. Only those plants stressed late in pod filling had lower harvest index and increased variability in harvest index. Seed yields (Chippewa 64, Wilkin, and ‘Evans’ for three planting dates) decreased substantially as degree of drought stress, induced by increasing distance from a line source irrigation system, increased yet, harvest index changed negligibly with distance from the line source. Thus, we conclude that harvest index is a conservative characteristic within each soybean cultivar.