*2.3. Plant Growth Measurements*

In the 2019 follow-up experiment, leaf relative chlorophyll content and canopy size were measured at 33 and 41 DAG. A SPAD 502 Plus Chlorophyll Meter (Spectrum Technologies, Aurora, IL, USA) was used to measure leaf relative chlorophyll content on three randomly chosen plants per treatment per block by averaging four readings obtained from two distal areas of the leaf blade for each of the two most recent mature leaves per plant. The canopy size was measured on 3 plants of MQ/BK, MQ/MQ, and MQ for each block using digital photographs processed with ImageJ/Fiji (version 2.0.0) [14]. A ruler held in the frame of each photograph set the scale for pixels per linear cm and enabled digital measurement of length and width of the plant canopy. The canopy size was then determined by multiplying the canopy length and width.

#### *2.4. Gas Exchange Measurements*

Gas-exchange was measured in the 2019 follow-up experiment at 34 and 46 DAG between 10:00 am and 3:00 pm by using an open gas exchange system (Li-6800; Li-Cor Inc., Lincoln, NE, USA) on three plants per treatment per block. Leaf transpiration rate (E, mmol H2O m−<sup>2</sup> s−1), net CO2 assimilation rate (A, mmol CO2 m−<sup>2</sup> s−1), intercellular CO2 concentration (Ci, μmol CO2 mol−<sup>1</sup> air), and stomatal conductance to water (gsw, mmol H2O m−<sup>2</sup> s−1) were measured at steady-state on the third (fully expanded) leaf from the top of each plant [15]. The PPFD was set at 800 μmol m−<sup>2</sup> s−1, with CO2 concentration at 400 ppm, vapor pressure deficit at 1.2 kPa, and leaf temperature at 27–29 ◦C. Instantaneous water use efficiency (iWUE) (μmol CO2 mmol−<sup>1</sup> H2O) was calculated as A/E [16] and stomatal conductance (Gs, mol m−<sup>2</sup> s<sup>−</sup>1) was calculated as gsw/1.6 [17].

#### *2.5. Yield Components and Biomass Accumulation at Harvest*

For the 2016 pilot study, the above-ground part (above soil line) of all the plants of MQ/BK, non-grafted MQ, and non-grafted BK were harvested at 68 DAG. The number of leaves longer than 4 cm were counted for each plant. The MQ/BK and non-grafted BK were then uprooted, and the taproots were separated and rinsed with water to remove excess potting soil from the roots. Taproot length (from the stem base to the end of the radish taproot) of each harvested plant was recorded, and the diameter of the widest part of each radish taproot was measured with a digital caliper. For the 2019 follow-up experiment, harvest and destructive sampling were carried out at 47 DAG. Five out of six plants per treatment per replication were randomly sampled. The above-ground part (above soil line) of each plant was removed from the pot, and leaves longer than 4 cm were counted and scanned with a leaf area meter (LI-3100; Li-Cor Inc., Lincoln, NE, USA). Only taproots from MQ/BK, BK, and BK/BK were harvested and cleaned. Pak choi and daikon radish leaves and taproots from the 2016 and 2019

experiments were first weighed then dried at 65 ◦C for 7 d (until constant weight) to determine the above-ground and below-ground fresh and dry biomass.

### *2.6. Mineral Nutrient Contents in Leaf and Root Tissues*

In the 2019 experiment, the dried root samples from the BK, BK/BK, and MQ/BK treatments and dried leaf samples from the MQ, MQ/MQ, and MQ/BK treatments were ground using a Thomas Wiley Laboratory mill (Model 4; Arthur H. Thomas Company, Philadelphia, PA, USA) and sent to Waters Agricultural Laboratories (Camilla, GA, USA) to measure the concentrations of the macronutrients N, phosphate (P), sulfate (S), potassium (K), calcium (Ca), and magnesium (Mg) and the micronutrients boron (B), zinc (Zn), manganese (Mn), iron (Fe), and copper (Cu). Nutrient accumulation was calculated by multiplying the nutrient concentration by dry tissue biomass.
