Abscission, Total Soluble Sugars, and Starch Profiles within a Soybean Canopy

Reproductive abscission is greatest in the lower one-third of soybean (Glycine max L. Merr.) canopies. It has been hypothesized that low carbohydrate availability is responsible for abscission of soybean flowers and pods, The cv. Williams was used to determine canopy distribution of flower and pod number, reproductive abscission, and the concentration of total soluble sugars (TSS) and starch in soybean leaflet, petiole, stem, and reproductive tissues. The plants were field grown (Beltsville silt loam; a fine-loamy, mixed, mesic Typic Fragiudult) and harvested four times during reproductive development (R1.5, R3.0, R5, R6). Two treatments, no shade and 75% shade, were used. Shades were in place 11 days for each physiological stage. Flower number, pod number, and reproductive abscission percentages were determined during these 11 day periods for both treatments. All characters were determined for three strata within the canopy. Total abscission throughout the growing season was 68 and 61% in 1978 and 1979. Abscission within the upper, middle, and lower strata, averaged over 2 years, was 38, 59, and 98%, respectively. Partial shade increased percent abscission at R3 and R6 of 1978 and not at all in 1979. In 1978, TSS concentration within leaflets, petioles, and stems reached a maximum value at R5, R3, and R6, respectively. In 1979, TSS of petioles and stems increased throughout reproductive development, but TSS of leaflets fluctuated between stages. Stage of development had an effect only on the starch concentration of petioles. In this instance maximum starch concentration occurred at R5. Significant canopy strata effects on TSS and starch concentration of stems and petioles were found at most stages of development. In these instances, concentrations of the lower one-third canopy were smaller than concentrations of the middle and/or upper canopy region. Large abscission percentages were associated with small concentrations of stored carbohydrates.