Monitoring water pathways through mungbean pods using backscattered electron microscopy

Weather damage by rainfall is a major constraint to production of high quality mungbean seed. Breeding mungbeans for weathering resistance is assisted by an understanding of the pod wall structure and how this structure can affect water absorption. This paper describes a simple microscopic technique for determining the transport route of salt solutions from the external pod surface through the pod wall tissues and into the pod lumen. Different mungbean cultivars were selected based on differences in field performance. Mungbean pods were immersed in concentrated salt solutions (lanthanum nitrate, caesium chloride and sodium chloride) for different time periods, embedded in resin blocks and polished prior to backscatter imaging in a field emission scanning electron microscope. The salts precipitated between the pod wall tissues and through timed experiments clearly demonstrated the passage of salt solution through the pod wall tissues and into the pod lumen. The salt molecules penetrated the outer epidermis and parenchyma but were unable to penetrate the dense sclerenchymatous layer beneath. The salt solution entered the lumen of the pod via the small gap between the suture cap and the dense tissue sheath. Although this technique may not provide a true determination of fresh water absorption through mungbean pods it does demonstrate a simple means of identifying more resistant pod structures suitable for use in achieving genetic improvement.