**4. Conclusions**

Successful grafts were produced between *B. rapa* var. *chinensis* and *R. sativus* var. *longipinnatus* in this study, resulting in a novel 'pac choi–daikon radish' product that may help save growing space and have added-value as perceived by farmers and consumers. More research is needed to optimize the seeding time and management of seedling production to help further improve the graft survival rate. Grafting pac choi with daikon radish did not severely impair the growth of the above-ground parts as grafted pac choi had similar SPAD value, canopy size, leaf number, and above-ground DW compared with non-grafted pac choi. Interestingly, grafting with radish increased the photosynthetic ability of the pac choi. However, grafting the daikon radish with pac choi decreased the taproot formation as reflected by the reduced length, diameter, FW and DW of the taproot. Future studies could explore different approaches such as cultivar selection and nutrient management to better balance the sizes of the above- and below-ground parts of this new pac choi–daikon radish product. Given the wide range of *B. rapa* var. *chinensis* and *R. sativus* var. *longipinnatus* cultivars, it would be interesting to explore different grafting combinations to characterize the range of graft performance. We only tested the graft performance under greenhouse conditions, and the grafted plants need to be further evaluated in field growing systems where biotic and abiotic stressors can be intensified. Generally, grafting between pac choi and daikon radish showed more negative impacts on mineral nutrient levels in radish taproots than in pac choi leaves. Sensory properties of the 'pac choi–daikon radish' product are unknown, and this aspect deserves further assessment. The inter-generic grafting between *B. rapa* var. *chinensis* and *R. sativus* var. *longipinnatus* could also provide a unique model system to further our understanding of scion-rootstock synergy and above- and below-ground sink competition in horticultural crops toward improving the use of grafting technology in sustainable vegetable production.

**Author Contributions:** Conceptualization, X.Z.; Data curation, T.G., Z.T.R. and K.E.B.; Formal analysis, T.G.; Funding acquisition, X.Z.; Investigation, T.G., Z.T.R. and K.E.B.; Methodology, T.G., Z.T.R., Z.E.B. and X.Z.; Project administration, X.Z.; Resources, X.Z. and J.K.B.; Supervision, X.Z. and J.K.B.; Writing—original draft, T.G. and X.Z.; Writing—review & editing, T.G., Z.T.R., K.E.B., Z.E.B., X.Z. and J.K.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is partly supported by the Specialty Crop Research Initiative grant No. 2016-51181-25404 from the USDA National Institute of Food and Agriculture.

**Conflicts of Interest:** The authors declare no conflict of interest.
