*3.4. Leaf and Taproot Harvest and Biomass Partition*

In the 2016 pilot study, MQ and MQ/BK had 151% and 104% higher above-ground fresh weight (FW) than BK, and the former two did not differ significantly (Table 4). No difference was detected in above-ground dry weight (DW) between these three treatments. MQ/BK produced significantly lower below-ground FW and DW compared with BK, while similar levels of total FW and DW were observed between MQ/BK and BK.

**Table 4.** Above-ground, below-ground, and total fresh weight (FW) and dry weight (DW) of grafted, self-grafted, and non-grafted pac choi and daikon radish plants at harvest in the 2016 pilot study and the 2019 experiment.


<sup>z</sup> BK = Non-grafted 'Bora King' daikon radish; BK/BK = Self-grafted 'Bora King' daikon radish; MQ = Non-grafted 'Mei Qing Choi' pac choi; MQ/MQ = Self-grafted 'Mei Qing Choi' pac choi; MQ/BK = 'Mei Qing Choi' pac choi grafted onto 'Bora King' daikon radish. <sup>y</sup> Mean ± SE (standard error); means followed by the same letter are not significantly different at *p* ≤ 0.05 according to Fisher's LSD test.

In the 2019 study, MQ/BK showed a significant reduction in above-ground FW compared with MQ and MQ/MQ by 18% and 8%, respectively (Table 4). MQ, MQ/MQ, and MQ/BK on average had 78% higher above-ground FW than the average of BK and BK/BK. However, BK and BK/BK had significantly higher above-ground DW than other treatments and MQ/BK did not differ significantly in above-ground DW from MQ and MQ/MQ. Grafting with pac choi significantly decreased the below-ground FW and DW of daikon radish compared with non- and self-grafted daikon radish. BK had higher below-ground FW and DW than BK/BK by 33% and 23%, respectively. MQ/BK exhibited a significantly higher total FW but a reduction in total DW compared with BK and BK/BK. The water content of both radish taproots and leaves was about 93%, whereas it reached 97% in the pac choi leaves, indicating that the pac choi leafy part had a disproportional contribution to the FW of MQ/BK. The differences in total FW and DW between 2016 and 2019 studies may be due to the different growing seasons and the time between grafting and harvest as the period from grafting to harvesting was 45% longer in 2016 compared with the 2019 study. Overall, the two experiments suggested that grafting between 'Mei Qing Choi' pac choi and 'Bora King' daikon radish had a greater impact on radish taproot development than the influence on pac choi leaf growth as the taproot DW of MQ/BK was significantly lower than BK, but the leaf DW was similar to that of MQ.

The grafting procedure per se negatively affected biomass accumulation in edible part in both self-grafted BK/BK and MQ/MQ compared with their non-grafted counterparts. Grafting can be viewed as mechanical wounding that triggers redistribution of the local resources or mobilization of resources from neighboring tissues to the injured part [35], which may divert the resources that could be used for plant growth. Moreover, wounding could induce jasmonic acid production, leading to suppression of mitosis [36], and thus reduce cell number and lead to reduced plant growth.

Gibberellins (GAs) have long been known to inhibit potato tuberization possibly by their involvement in photoperiodic control of tuber formation [37,38]. Grafting potato (*Solanum tuberosum* L.) with tomato (*Solanum lycopersicum* L.) decreased stolon (underground shoot) number and length as well as tuber number, but increased the gibberellic acid (GA3) content of stolon and tuber compared with self-grafted potato [39]. The level of GAs has also been reported to play a vital role in carrot (*Daucus carota* L. var. *sativus*) elongation and expansion [40]. Leaf application of GA3 inhibited tuberous root growth but improved shoot growth in radish, while leaf application of paclobutrazol, an inhibitor of gibberellin biosynthesis, improved taproot growth [41]. Auxin may also affect hypocotyl-tuber growth in turnip as show in in-vitro studies [23]. Peres et al. [38] grafted tomato mutants, which were incapable of certain hormone biosynthesis or photomorphogenesis, onto potato plants and suggested that failure to produce certain chemicals by the tomato mutant scion may have impeded formation of the potato tuber. Hence, modifications of signal molecules produced in and transported from pac choi may have led to reduced growth of the daikon radish taproot.

In most cases, plants partition photosynthates preferentially to vegetative organs in the early to middle growth stage and to reproductive or storage organs in late growth stage [42]. However, many root vegetable plants grow the vegetative biomass and develop the storage root at the same time, leading to a balance between them [42,43]. Grafting may disturb the source–sink balance between scion and rootstock [44]. Both the leaf apical meristem of pac choi and the taproot part of daikon radish are strong sinks [45] and possibly competed for photosynthates in the pac choi–daikon radish grafts. The source-sink relationship was likely altered to support the growth of pac choi leaves at a cost of reduction in radish taproot development in the grafted plants. The dry mass of the leafy part accounted for 81% of the total DW of MQ/BK, but for non-grafted daikon radish, the leafy part only accounted for 66% of the total DW.

For Chinese cabbage grafted onto turnip, the heading of the Chinese cabbage was restricted by the thickening of the turnip taproot, resulting in a small Chinese cabbage head when the turnip taproot was harvested [8]. This imbalance was attributed to the discrepancy in crop maturity and growth cycle requirement as the heading of the Chinese cabbage requires more time than the development of the turnip taproot. In our study, 'Mei Qing Choi' pac choi and 'Bora King' daikon radish were both fast-growing cultivars with 45 and 49 d to maturity, respectively (Johnny's Selected Seeds). However, we seeded the faster maturing pac choi 6 d earlier than the slower maturing daikon radish in order to better match their stem diameters for grafting. Further examinations are needed to elucidate the scion–rootstock interactions for grafting scenarios in which accumulated biomass of both scion (shoots) and rootstock (enlarged taproot) are harvested together for economic yield. In this special scenario, competition for water and nutrients, photosynthetic capacity, and photosynthate partitioning between above- and below-ground sinks are of particular importance. Interestingly, the reciprocal grafting experiment by Sugiura et al. [42] using *Raphanus sativus* genotypes with differential hypocotyl sink activities demonstrated the genotype-dependent autonomous regulation of the hypocotyl sink activity.

#### *3.5. Mineral Nutrient Contents in Pac Choi Leaves and Daikon Radish Roots of Grafted Plants*

Dried leaves of MQ, MQ/MQ, and MQ/BK, and dried taproots of BK, BK/BK, and MQ/BK from the 2019 experiment harvest were used to examine the macronutrient and micronutrient concentrations (Table 5) and accumulation (Table 6). Leaf N concentration did not differ significantly between MQ/BK and MQ. Interestingly, MQ/MQ had a significantly higher N concentration in the leaf tissue compared with MQ/BK. However, this seems contradictory to the finding that MQ/BK had a higher leaf photosynthetic rate (Table 3) than MQ/MQ. It needs to be pointed out that the entire above-ground leaf tissue was sampled for leaf nutrient analysis, whereas the most recently mature leaves were used for photosynthesis measurements. While MQ/BK had a lower level of leaf N concentration in the above-ground biomass, its higher leaf photosynthetic rate could be due to remobilization of N compounds from the older leaves to the most recently mature leaves that were used for photosynthesis measurement [46]. Compared with non-grafted pac choi, grafting with the daikon radish rootstock significantly decreased K and S concentrations in the leaf tissue of pac choi (by 21% and 45%, respectively), but it increased Zn concentration in the leaf tissue by 37%. MQ/BK had significantly higher concentrations of N (by 14%), K (by 30%), Mg (by 47%), Ca (by 38%), B (by 36%), and Zn (by 63%) in the taproot compared with the average of BK and BK/BK, while there were no differences between BK and BK/BK. By contrast, the concentration of S in the taproot of MQ/BK was reduced by 44% compared with BK and BK/BK. Overall, there was not a clear relationship between plant nutritional status and reduction of taproot in MQ/BK. Nutrient uptake of grafted pac choi–daikon radish in relation to scion-rootstock interactions is an intriguing area to explore.

**Table 5.** Mineral nutrient concentrations in leaves of grafted, self-grafted, and non-grafted pac choi and in taproots of grafted, self-grafted, and non-grafted daikon radish plants at harvest in the 2019 experiment.


<sup>z</sup> BK = Non-grafted 'Bora King' daikon radish; BK/BK = Self-grafted 'Bora King' daikon radish; MQ = Non-grafted 'Mei Qing Choi' pac choi; MQ/MQ = Self-grafted 'Mei Qing Choi' pac choi; MQ/BK = 'Mei Qing Choi' pac choi grafted onto 'Bora King' daikon radish. <sup>y</sup> Mean ± SE (standard error); means followed by the same letter are not significantly different at *p* ≤ 0.05 according to Fisher's LSD test.

**Table 6.** Mineral nutrient contents accumulated in leaves of grafted, self-grafted, and non-grafted pac choi and in taproots of grafted, self-grafted, and non-grafted daikon radish plants at harvest in the 2019 experiment.


<sup>z</sup> BK = Non-grafted 'Bora King' daikon radish; BK/BK = Self-grafted 'Bora King' daikon radish; MQ = Non-grafted 'Mei Qing Choi' pac choi; MQ/MQ = Self-grafted 'Mei Qing Choi' pac choi; MQ/BK = 'Mei Qing Choi' pac choi grafted onto 'Bora King' daikon radish. <sup>y</sup> Mean ± SE (standard error); means followed by the same letter are not significantly different at *p* ≤ 0.05 according to Fisher's LSD test.

With respect to leaf nutrient accumulation, MQ/BK had 37% greater Zn accumulation compared with the average of MQ/MQ and MQ (Table 6). However, MQ/BK decreased N, K, and S content by 12%, 26%, and 49%, respectively, compared with MQ. In terms of nutrient accumulation in the taproot, MQ/BK showed significantly lower accumulation of all measured minerals except Fe and Cu relative to BK and BK/BK, which was likely associated with the small size of the taproot in MQ/BK. The lower accumulation of nutrients in the taproot demonstrated that pac choi–daikon radish favored growth of the pac choi leaves at the expense of the radish taproot.
