Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 29, 2000
Publication Date: N/A
Interpretive Summary: Insufficient rain fall (drought) is the main limiting factor in soybean grain yield because it prevents the movement of water and nutrients to the developing seed. The xylem and phloem are the specialized vascular cells for moving water and nutrients in the plant. However, the anatomy of the xylem and phloem of the pod of the soybean fruiting structure has never been investigated. The xylem and phloem of the pod was investigated as a preliminary to the understanding of water and nutrient movement in the deposition and mobilization of storage compounds destined to be utilized by the developing seed. The pod was found to contain a well defined xylem and phloem. In addition, a layer of phloem was found internal to the xylem. Phloem is generally external to xylem and the presence of internal phloem is a rare occurrence in plants. The presence of the internal phloem indicates that it has a special function in the nourishment of the developing seed.
Low temperature scanning and transmission electron microscopy were used to observe the vascular tissue of the placental and nonplacental sutures of the carpel (pod) of soybean [Glycine max (L.) Merr.]. Pods were harvested when they reached their greatest length and then weekly until physiological maturity. The vascular tissue lies beneath a layer of thick-walled sclerenchyma tissue that is not connected to the sclerenchyma tissue of th pod pericarp. The walls of the sclerenchyma tissue become thicker with maturation. Directly below the sclerenchyma is the vascular tissue with the phloem external to the xylem. The phloem consists of sieve elements and adjoining companion cells and the xylem consists of tracheary elements. An unusual feature of the placental vasculature was the presence of an inner layer of phloem cells. These cells were similar to and joined with the outer phloem when they entered the funiculus (seed stalk). The presence of the inner phloem suggests that it may have a special function in the nourishment of the developing seed.