Non-destructive assessment of developing hydraulic connections in the graft union of tomato

Hydraulic architecture prescribes water flow in plants and is, therefore, fundamental to many areas of plant physiology. It is usually analysed destructively, or on excised material. A method is explored here based on displacement transducers for the continuous, nondestructive assessment of functional hydraulic connections within the intact plant. The graft union was chosen as a test system. The technique involves repeated application of water at some point in the system, while simultaneously observing patterns of swelling (increase in water status) at other points. Such patterns will reflect the hydraulic resistance of the intervening pathways. It is demonstrated that the major hydraulic connections within the graft union of tomato become functional over a period of about 48 h from the fifth day after grafting. This is consistent with histological observations on the appearance of wound-xylem bridges at this time. This approach could be useful for non-destructive monitoring of changes in hydraulic connections in various other intact systems, for example, during abscission, drought-induced embolism, or attack by vascular-wilt pathogens.