Vasculature of Soybean Racemes with Altered Intraraceme Competition

Abscission probability differs among floral positions within soybean [ Glycine max (L.) Merr.] racemes, with proximal flowers much less likely to abscise than distal flowers. The objectives of this research were to measure phloem and xylem areas within the rachises of developing soybean racemes, to determine if the sizes of the vascular tissue areas were associated with flower abscission and pod development, and to determine if removal of proximal pods affects vascular tissue areas in distal regions of the racemes. ‘Williams 82’ soybean plants were grown in the field in 1985 and 1986 and tagged when the flower at raceme position 1 (most proximal) opened. In 1985, racemes were harvested 8 and 16 d after tagging (DAT). Reproductive structures were removed, dried, and weighed. The remaining rachises were fixed, dehydrated, embedded in paraffin, serially sectioned, and stained. In 1986, proximal pods (positions 1 and 2) were removed from one‐half the racemes 7 DAT. Fourteen and 21 DAT, treated and control racemes were harvested and treated as described for 1985. Distal pods were smaller than proximal pods at both harvests in both years. Distal internodes contained smaller phloem and xylem areas than proximal internodes, but phloem and xylem tissues were present in the most distal region of the rachis. Removal of proximal pods at 7 DAT increased distal pod weight, phloem and xylem areas, and ratio of phloem to xylem area 21 DAT. Vascular tissue expansion between 8 and 16 DAT was faster in rachis internodes 1 through 3 than in internode 4. Abscission probability was lower for proximal flowers than distal flowers. High abscission probabilities and slow pod growth were associated with a region of the raceme with small vascular tissue areas and slow vascular area expansion rates. The mechanism that inhibits growth and development of tissues within the distal region of the raceme may also influence abscission.