## 2 Materials and Methods
### Measurements and Analysis  
After 63 days, the experiment was completed and the plants were harvested, split into scion's leaves and stems and rootstock's stem and roots, respectively. All the collected samples were weighed (fresh weight; f.w.), rinsed once with tap water and twice with deionized water, then dried in an oven at 75 \({}^{\circ}\)C to a consistent dry weight, before being crushed into a fine powder that could pass through a 30-mesh screen. Afterwards, for the nutrients' analyses, 0.5 g of the dried sample was dry asked for 6 h at 550 \({}^{\circ}\)C, then dissolved in 3 mL of 6 N HCI, and, finally, diluted with deionized water. The concentrations of P, K, Ca, Mg, Fe, Mn, and Zn were determined using atomic absorption spectroscopy (Perkin-Elmer model 2380; Perkin-Elmer, Salem, MA, USA). Moreover, the concentration of P was determined colorimetrically (470 nm) by using the ammonium phosphovanandomolybdate yellow method [37]. Macronutrient concentrations were expressed in % d.w., while those of micronutrients, i.e., Fe, Mn, and Zn, were expressed in mg kg\({}^{-1}\) d.w. The content (absolute quantity) of each nutrient existing in each plant part, and the total in the entire plant, was calculated by multiplying the concentration of each nutrient found in the leaves, scion's stems, rootstock's stem, and root by the respective dry weight of each plant part. The nutrient use efficiency (NUE), which is defined as the amount of plant biomass produced per unit of a mineral nutrient, was also assessed for each nutrient [36].  
Chlorophyll fluorescence parameters (F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{m}}\), F\({}_{\mathrm{v}}\)/F\({}_{\mathrm{0}}\); F\({}_{\mathrm{0}}\): initial fluorescence, F\({}_{\mathrm{m}}\): maximum fluorescence, F\({}_{\mathrm{v}}\) = F\({}_{\mathrm{m}}\) - F\({}_{\mathrm{0}}\): variable fluorescence) were measured on the 62 nd day with a Plant Efficiency Analyzer (Hansatech 2.02), in order to determine possible impairments to photosystem II (PSII). For this reason, one fully-grown leaf per plant, located at the 3 rd or 4 th node from the top to the base of a stem, was measured after pre-conditioning in the dark for 20 min. In the following, each of these leaves was detached and used for the determination of the concentrations of chlorophyll, total soluble carbohydrates, and free proline.  
For the determination of chlorophyll concentration, 7 leaf discs, 10 mm in diameter, were sampled from each leaf. The remaining leaf tissue was put in the fridge (\(-20\)\({}^{\circ}\)C) until the determination of proline and carbohydrates. Leaf chlorophyll was extracted with ethanol (96%) in a water bath (78 \({}^{\circ}\)C). Total chlorophyll concentration (Chl\({}_{\mathrm{a+b}}\)) was calculated from the equations given by [PERSON] and [PERSON] [38]. Furthermore, 0.1 g of each leaf was cut into small pieces, placed in a glass vial containing 10 mL of 80% (\(v/v\)) ethanol, and heated at 60 \({}^{\circ}\)C for 30 min. The extract was then filtered and diluted with 80% (\(v/v\)) ethanol up to 20 mL [38]. Free proline and total soluble carbohydrates were both assayed in the aforementioned extract, following the acid ninhydrin reagent and the anthrone methods, respectively [39]. The same procedure was followed for the determination of carbohydrates and proline in root samples that were collected during the harvest of plants at the end of the experiment.