Temperature Control of Node Appearance and Node Initiation in Soybean.

Soybean production is expanding rapidly into cooler and warmer environments. However, influence of temperature on node appearance rate (NAR) and node initiation rate (NIR), two critical parameters that determine potential leaf area size (source) and seed number (sink) in soybean, has been poorly documented. The objective of this study is to establish the relationship between NAR and NIR and temperature in indeterminate soybean and assess the degree to which these two processes are co-ordinated. To accomplish this objective, we used a combination of greenhouse and field experiments that explored a wide range of mean seasonal temperatures (15-26ºC) and soybean varieties with contrasting maturity group. Number of appeared and initiated nodes was periodically measured during the growing season through visual inspect of the plants and dissections of stem apex . We found that, while node appearance ceased at the beginning of seed filling, node initiation ended at an earlier stage (beginning of pod setting). There was a strong relationship between NIR and NAR and temperature, from which it was possible to derive a constant plastochron of 36ºCd and a phyllochron that decreased from 83ºCd during early vegetative stages to 58ºCd during appearance of trifoliolate nodes, with a base temperature of 10ºC for both NIR and NAR. Despite variation in weather, soil type, varieties, and management across experiments, we found a robust two-phase relationship between node initiation and node appearance that suggests a strong co-ordination between these two processes. In all treatments, we found that the final number of initiated nodes were in excess relative to the final number of appeared nodes, with the magnitude of the difference depending upon the thermal time between beginning of pod setting and start of seed filling. Findings from this study can be used for a more mechanistic simulation of node appearance and initiation in soybean, which, in turn, can help improve predictions of leaf area dynamics, growth, and final seed yield.

Keywords: soybean, Glycine max (L.) Merr., node appearance, node initiation, phyllochron, plastochron, temperature.