Grafting is a horticultural technique that joins scion (top) and rootstock plants via a graft union to create desirable above-ground and below-ground characteristics in the grafted plant (Davis et al., 2008). Grafting has been used for decades in commercial vegetable production in Asia and Europe to manage plant diseases, increase plant vigor, and improve plant production, but is still not widely used in the United States (Kubota et al., 2008; Lee, 1994; Lee et al., 2010; Peregrine and Binahmad, 1982; Sakata et al., 2007). Adoption of grafted plants among small-scale growers in the United States is limited by a shortage of research-based information regarding the production of grafted plants. While large-scale growers in the United States likely purchase grafted transplants, many small-scale growers are grafting their own (Kubota et al., 2008). Solanaceous crops (Solanaceae) are generally considered easy to graft because the splice grafting method that is most commonly used is simple and quick with a high graft survival rate (≥95% survival for tomato under controlled healing conditions). While grafting generally takes less than a minute per plant to complete, the healing process takes a week. Healing is the process of construction and establishment of the vascular connection between the scion and the rootstock plants (Davis et al., 2008; Fernandez-Garcia et al., 2004). In general, an environment that includes high relative humidity (85% to 100%), low or no light, and temperature ranging from 75 to 80 °F is believed to be a requirement for obtaining high grafting success with solanaceous crops (Bausher, 2013; Jang et al., 2011; Lee, 1994).
The scion is incapable of absorbing water from the rootstock for several days following grafting, and a high humidity environment can reduce scion transpiration and prevent plant wilting (Fernandez-Garcia et al., 2004; Turquois and Malone, 1996). The light conditions during the healing process can also affect plant transpiration since reduced light levels minimize stomatal opening and inhibit the photosynthesis process (Oda, 1999; Rivard and Louws, 2011). Overall, grafting survival is improved due to the conservation of water in the plant (Jang et al., 2011). However, photosynthesis is required for cell division, which is needed for vascular regeneration and callus formation (Hunter et al., 2004; Taiz and Zeiger, 2002). Callus proliferation in both the rootstock and the scion are needed for vascular connection at the rootstock-scion interface (Ogata et al., 2005), and poor callus formation can lead to low rates of survival of grafted plants (Johkan et al., 2008, 2009; Oda et al., 2005). Carbohydrates are required for callus formation, and are produced in the plant through photosynthesis (Hunter et al., 2004; Milthorpe and Moorby, 1979; Taiz and Zeiger, 2002). When light is limited in the healing chamber, healing of grafted plants is dependent on stored carbohydrates (Daley et al., 2014). For example, the survival of watermelon (Citrullus lanatus) grafted using the splice grafting technique (both rootstock cotyledons were removed) was 89% when rootstocks received 2% sucrose solution before grafting and the rootstock stems had 52% starch accumulation. In comparison, rootstocks that were treated with water had 6% starch accumulation in the rootstock stems and grafting success was 58% (Dabirian and Miles, 2017). There are no reports regarding increasing the carbohydrate reserves of solanaceous crops before grafting, though previous research reports that when light is provided during the healing period, callus induction, growth and graft survival are improved (Afshari et al., 2011; Moon and Stomp, 1997; Nobuoka et al., 2005).
While large-scale producers of grafted vegetable transplants use controlled environments to heal newly grafted plants, small-scale growers are using simple healing chamber structures within a greenhouse, where it can be difficult to precisely control environmental factors. There are no standard guidelines for managing small-scale healing chambers, as climate conditions and greenhouse structures vary considerably across regions (O’Connell et al., 2009). An earlier experiment carried out in a greenhouse at Mount Vernon, WA measured grafting success following healing with three different healing chamber structures: a commercial propagator’s design that was covered in clear plastic and shadecloth, and was hand misted (used by a large-scale vegetable grafting company in British Columbia, Canada); a research design that was also covered in clear plastic and shadecloth but that used a humidifier (commonly used in the southeastern United States); and a simple structure that was only covered in shadecloth and was hand misted (used by small-scale tomato growers who graft their own transplants) (Johnson and Miles, 2011). The commercial propagator healing chamber structure had the highest temperature and RH (24.9 °C and 98%, respectively), while the research structure had a slightly lower temperature and moderately lower RH (23.4 °C and 81%, respectively), and the shadecloth structure also had slightly lower temperature but substantially lower RH (23.3 °C and 52%, respectively). The authors concluded that high RH during the 7-d healing period was necessary for eggplant and watermelon graft survival (82% and 7% grafting survival overall, respectively), but not necessary for tomato graft survival (98% grafting survival overall). However, the light levels were not measured in this study.
There is a need to provide more information for small-scale growers who are healing grafted plants using simple healing structures inside a greenhouse, with a range of fluctuating light and RH levels. Thus, the objective of this study was to investigate the effect of a range of light and RH levels on the grafting success of tomato, eggplant, and pepper in a small healing chamber within a greenhouse environment.
Afshari, R.T., Angoshtari, R. & Kalantari, S. 2011 Effect of light and different plant growth regulators on induction of callus growth in rapeseed (Brassica napus L.) genotypes Plant Omics J. 4 60 67
Brust, G. 2008 Using nitrate-N petiole sap-testing for better nitrogen management in vegetable crops. Univ. Maryland Ext. Publ. Sept. 2008
Dabirian, S. & Miles, C. 2017 Increasing survival of splice-grafted watermelon seedlings using a sucrose application HortScience 52 579 583
Daley, S.L., Adelberg, J. & Hassell, R.L. 2014 Improvement of grafted watermelon transplant survival as a result of size and starch increased over time caused by rootstock fatty alcohol treatment: Part 1 HortTechnology 24 343 349
Davis, A.R., Perkins-Veazie, P., Sakata, Y., Lopez Galarza, S., Maroto, J.V., Lee, S.G., Huh, Y.C., Sun, Z., Miguel, A., King, S., Cohen, R. & Lee, J.M. 2008 Cucurbit grafting Crit. Rev. Plant Sci. 27 50 74
Fernandez-Garcia, N., Carvajal, M. & Olmos, E. 2004 Graft union formation in tomato plants: Peroxidase and catalase involvement Ann. Bot. 93 53 60
Hochmuth, G.J. 1994 Plant petiole sap-testing for vegetable crops. Univ. Florida. Hort. Sci. Dept. Circ. 1144
Hunter, J.J., Volschenk, C.G., le Roux, D.J. & Adams, L. 2004 Plant material quality: A compilation of research. ARC Infruitec-Nietvoorbij, Stellenbosch, South Africa
Jang, Y.A., Goto, E., Ishigami, Y., Mun, B.H. & Chun, C.H. 2011 Effects of light intensity and relative humidity on photosynthesis, growth and graft-take of grafted cucumber transplants during healing and acclimatization Hort. Environ. Biotechnol. 52 331 338
Johkan, M., Oda, M. & Mori, G. 2008 Ascorbic acid promotes graft-take in sweet pepper plants (Capsicum annuum L.) Scientia Hort. 116 343 347
Johkan, M., Mitukuri, K., Yamasaki, S., Mori, G. & Oda, M. 2009 Causes of defoliation and low survival rate of grafted sweet pepper plants Scientia Hort. 119 103 107
Johnson, S. & Miles, C. 2011 Effect of healing chamber design on survival of grafted eggplant, tomato, and watermelon HortTechnology 21 752 758
Johnson, S., Kreider, P. & Miles, C. 2011 Vegetable grafting: Eggplants and tomatoes. Washington State Univ. Ext. Publ. FS052E. 30 Nov. 2017. <http://extension.wsu.edu/publications/wp-content/uploads/sites/54/publications/fs052e.pdf>
Johnson, S., Miles, C., Kreider, P. & Roozen, J. 2016 Vegetable grafting: The healing chamber. Washington State Univ. Ext. Fact Sheet. FS051E. 15 Nov. 2017. <http://cru.cahe.wsu.edu/CEPublications/FS051E/FS051E.pdf>
Kubota, C., McClure, M., Kokalis-Burelle, N., Bausher, M.G. & Rosskopf, E.N. 2008 Vegetable grafting: History, use, and current technology status in North America HortScience 43 1664 1669
Lee, J.M., Kubota, C., Tsao, S.J., Bie, Z., Echevarria, P.H., Morra, L. & Oda, M. 2010 Current status of vegetable grafting: Diffusion, grafting techniques, automation Scientia Hort. 127 93 105
Milthorpe, F.L. & Moorby, J. 1979 An introduction to crop physiology. Cambridge Univ. Press, Cambridge, UK
Moon, H.K. & Stomp, A.M. 1997 Effects of medium components and light on callus induction, growth, and frond regeneration in Lemna gibba (Duckweed) In Vitro Cell. Dev. Biol. Plant 33 20 25
Nobuoka, T., Nishimoto, T. & Toi, K. 2005 Wind and light promote graft-take and growth of grafted tomato seedlings J. Jpn. Soc. Hort. Sci. 74 170 175
O’Connell, S., Hartmann, S., Rivard, C.L., Peet, M.M. & Louws, F.J. 2009 Grafting tomatoes on disease resistant rootstocks for small scale organic production. 15 Nov. 2017. <http://ofrf.org/funded/highlights/oconnell_07f30.html>
Oda, M., Maruyama, M. & Mori, G. 2005 Water transfer at graft union of tomato plants grafted onto Solanum rootstocks J. Jpn. Soc. Hort. Sci. 74 458 463
Oda, M. 1999 Grafting of vegetables to improve greenhouse production. Food Fert. Technol. Ctr. Ext. Bul. 480
Ogata, T., Kabashima, Y., Shiozaki, S. & Horiuchi, S. 2005 Regeneration of the vascular bundle at the graft interface in auto-and hetero grafts with juvenile nucellar seedlings of satsuma mandarin, yuzu and trifoliate orange J. Jpn. Soc. Hort. Sci. 74 214 220
Rivard, C.L. & Louws, F.J. 2011 Tomato grafting for disease resistance and increased productivity. Sustainable Agr. Res. Educ. (SARE) Factsheet GS05-046
Rivard, C.L. & Louws, F.J. 2006 Grafting for disease resistance in heirloom tomatoes. North Carolina State Univ. Coop. Ext. Serv. 675
Taiz, L. & Zeiger, E. 2002 Plant physiology. 3rd ed. Sinauer Assoc., Sunderland, MA
Turquois, N. & Malone, M. 1996 Nondestructive assessment of developing hydraulic connections in the graft union of tomato J. Expt. Bot. 47 701 708
Vu, N.T., Kim, Y.S., Kang, H.M. & Kim, I.S. 2014 Effect of red LEDs during healing and acclimatization process on the survival rate and quality of grafted tomato seedlings Protected Hort. Plant Factory 23 43 49