A New Protocol May Help Reveal Pre- and Post-grafting Environment Effects on Early-phase Healing in Grafted Tomato Seedlings

Grafting combines two or more varieties to create tomato plants that often outperform standard ungrafted ones, especially under stress. Grafting requires severing the root-shoot axis and, therefore, immediate and presumably total interruptions of photosynthesis and communication and transport throughout the plant axis. How quickly do xylem flow above the graft union and scion growth return? Are these rates influenced by pre- and post-grafting environments (e.g., light levels)? The literature appears to offer neither answers to these questions nor a protocol to develop them. Typically, grafted plant survivorship is scored subjectively using a binary scale; however, structural and other events required to return the new plant axis to full function are rarely documented. We set out to develop a quantitative protocol for assessing the resumption of xylem flow above the graft union and scion growth. Hypothesizing that gross carbon budgets influence wound healing, we tested this protocol using groups of plants exposed to different light levels after grafting. Tomato (Solanum lycopersicum; Maxifort – rootstock and Celebrity – scion) seedlings were grafted using the cleft method and placed into one of two climate-controlled chambers (0 PPF – dark, 135 PPF 14 h – light) for 9 days at 24/20 C (day/night). Healing responses including graft union development and whole plant re-growth were monitored 3, 5, 7, 9 days after grafting. Early-phase vasculature reconnection was assessed using a translocateable dye (Erythrosine). Grafted plants were excised at the soil line and placed in 0.7% (m/v) dye solution for 15 min; distances that the dye traveled from the point of excision and graft union were recorded, along with measures of stem diameter and rootstock and scion length. Digital images of the developing canopy and transmittance readings of dyed stems (spectroradiometer) were also taken. Survivorship equaled 100% in both groups of plants but dye traveled farther and stem diameter and length were larger in plants exposed to 135 versus 0 PPF. The data illustrate that graft healing is a continual process, benchmarks for which can change with time and light levels. Dye distance and other measures detected differences between groups of experimental plants that external observation (i.e., survivorship ratings) did not. Additional use of this method may lead to a greater understanding of the process of graft healing.