Source Strength Influence on Soybean Yield Formation during Early and Late Reproductive Development

Abstract

Soybean [Giycine max (L.) Merr.] yield is affected more by assimilatory capacity (source strength) during reproductive (R1-RT) than vegetative (emergence to RI) growth periods, but relative sensitivities of yield components to source strength have not been clearly demanstrated. This study was conducted to verify that source strength (R1-R7) affects yield primarily through regulation of pod number. Source strength effects (R1-R6) on pod number and effects of altered sourcesink strength during R5 to R7 on seed per pod and seed size were determined. Soybean cultivar Centennial was planted at an optimal date in 1991 and 1992 near St. Gabriel, LA. Source strength during RI to R6 and source-sink strength during R~ to R7 were altered with three treatments: control, 30% shade (R1-R7), and partial pod removal 10 d after RS. Shade vs. control reduced source strength and yield 30% through a 20% reduction in pod number. Number of branch nodes and main stem pods per reproductive node, yield components shown previously to respond to source strength at late plantings, responded similarly to altered source strength at optimal planting dates. In contrast, a 28% decrease in source-sink strength during R5 to R7 reduced seed per pod and seed size by only 7.7 and 4.5%, respectively. The results substantiate that source strength during RI to R7 influences soybean yield primarily by affecting pod number and that RI to R6 is the most responsive part of the reproductive period to altered source strength. K NOWLEDGE of soybean yield component responses to assimilatory capacity (source sensitivity), coupled with identification of the growth periods in which yield components are formed, can aid in designing genetic and cultural strategies for optimizing yield. Previous studies (Brun, 1978; Christy and Porter, 1982) have determined that yield was affected by altered source strength during the reproductive (R1-R7, stages according to Fehr and Caviness, 1977) rather than the vegetative period (emergence to R1). However, reports of source sensitivity for yield components formed during early (R1-R5) and late (RS-R7) reproductive periods have not been consistent. During R1 to R5, yield components affecting pod number (node number, reproductive node number, and pods per reproductive node) are formed. Final determination of pods per reproductive node occurs near R6, 10 to 12 d after R5 (Board and Tan, 1995). Drastic reductions in source strength (i.e., total defoliation) applied shortly after R6 have little effect on pod number (Board et al., 1994).

Relationship between Photosynthesis and Seed Number at Phloem Isolated Nodes in Soybean

“…This relationship indicates that the primary effect of the environment on pod and seed number was expressed through its effect on NCI. These results agree with other reports of a close association between canopy photosynthesis and seed number (Herbert and Litchfield, 1984; Ramseur et al, 1985; Egli and Yu, 1991; Egli, 1993; Board and Harville, 1994; Board et al, 1995). In addition, treatments designed to alter canopy photosynthesis (CO 2 enrichment, Cooper and Brun, 1967; Jones et al, 1984; light enrichment, Schou et al, 1978) and the availability of assimilate also produced corresponding changes in pod and seed number.…”

“…Treatments that alter canopy photosynthesis during this period, such as CO 2 enrichment (Cooper and Brun, 1967; Jones et al, 1984), or increased irradiance (Schou et al, 1978) caused corresponding changes in seed number. Herbert and Litchfield (1984), Ramseur et al (1985), Egli and Yu (1991), Egli (1993), Board and Harville (1994), and Board et al (1995) reported linear relationships between crop growth rate (CGR), measured during flowering and podset, and seed number in soybean. The variation in CGR was created by differences in row spacing and plant population (Herbert and Litchfield, 1984; Board and Harville, 1994), irrigation and intra‐row spacing (Ramseur et al, 1985), shade (Egli and Yu, 1991; Egli, 1993), and defoliation (Board et al, 1995) The relationship for a single cultivar was consistent across years, but there were large differences between cultivars (Egli and Yu, 1991; Egli, 1993).…”

Interference of GR® Volunteer Corn Population and Origin on Soybean Grain Yield Losses

“…The reduction in the soybean yield components in the three experiments indicates that the intense competition executed by VC over soybean interfered negatively on the number of produced pods per plant and/or caused a decrease in the grain number per pods. It is far more likely that it has reduced the number of vegetables per soybean plant, since this variable is more responsive to competition stress among competing species, whereas the number of grains per pods and the grain weight have more individual genetic control, having less competition influence (Board et al, 1995).…”

Impact of Conyza bonariensis density and establishment period on soyabean grain yield, yield components and economic threshold

“…The literature demonstrates that the number of pods per plant is the soyabean yield component most affected by weed competition (Juan et al ., ; Lamego et al ., ; Silva et al ., ). The magnitude of the seed mass is less affected by environmental fluctuations (Board et al ., ); thus, it is less sensitive to weed interference (Fig. D).…”