Tomato Grafted on Potato Altered its Root Traits and Enhanced its Mineral Allocation in Response to Saline Irrigation

Poor irrigation water quality is considered to be a key limiting factor for tomato cultivation in arid environments. Grafting is a promising tool to address this, and different rootstock combinations may provide stronger tolerance to saline irrigation. Grafted (tomato scion on potato rootstock) plants were tested under saline irrigation in a controlled greenhouse to study the root responses that confer tolerance to the shoot. The root traits (total length, surface area, diameter, volume and density) of tomato (cv. Ikhram), potato (cv. Charlotte) and grafted (cv. Ikram/Charlotte) plants were examined after 135 days of saline irrigation (EC = 1 and 5 dS/m). Tomato root growth was significantly affected by saline irrigation. However, the grafted plants showed significantly higher total root length, root diameter, root surface area, and root volume under non-saline irrigation, and they remained unaffected by saline irrigation. The root density (length and mass) of the grafted plants was enhanced under saline irrigation. The grafted plants compensated the total length of coarse roots (> 2.0 mm, diameter) by increasing their fine roots (< 0.5–1.0 mm, diameter) under saline irrigation. The potato rootstock modulated its leaf Na⁺ content by reducing Na⁺ accumulation in leaves and fruits, thus enabling higher K⁺/Na⁺, Ca²⁺/Na⁺ and Mg²⁺/Na⁺ ratios. More carbon and nitrogen were allocated to the roots and leaves of the grafted plants under saline irrigation. Below and aboveground carbon indicated the root and shoot growth of graft under saline irrigation. The presented results suggest that the enhanced tolerance of the grafted plants to saline irrigation, reflected in the maintenance of tomato growth, is due to the effective mineral allocation by the graft-induced fine roots.