Plant growth, gas exchange and yield
To determine whether rootstock ABA overproduction can alleviate salt stress, two independent tomato transgenic lines, SP5 and SP12, in the genetic background of the wild-type (WT) cultivar Ailsa Craig (AC), as previously reported (Thompson et al. 2000), were used in this study as rootstocks of the commercial cherry variety Sugar Drop. At the end of the growing cycle (up to 200 days of irrigation with saline water), plants grafted onto NCED OE rootstocks had almost twice the leaf area, leaf and shoot biomass (shoot fresh weight; SFW), and stem diameter of plants grafted onto WT rootstocks (Figure 1a; Table 1). However, the root biomass of SP12 and SP5 rootstocks was 30% and 60% smaller than WT rootstocks, respectively (Table 1). Visually, these NCED OE grafts had less a complex root system architecture (the spatial configuration of a root system in the soil), than the WT (Figure 1c). Moreover, plants grafted onto NCED OE rootstocks had up to 20-30% increases in length and weight of the 3rd fruiting truss, fruit number, fruit weight and total fruit yield (Table 1; Figure 1b). Thus, NCED OE rootstocks promoted shoot (and fruit) growth at the expense of root system growth.
Plants grafted onto NCED OE rootstocks had higher photosynthesis rate (AN ) on certain measurement occasions, with similar gs (Figure 2a, b) and transpiration (data not shown) to plants grafted on WT rootstocks. Accordingly, NCED OE rootstocks increased WUEi (Figure 2b). Electron microscopy revealed that leaves of scions grafted on SP12 rootstocks had altered leaf and mesophyll structure, with a more disorganized palisade and spongy cell layers (Figure 2c), and smoother and more elongated epidermis and trichome cells in the adaxial surface (Figure 2e; Table 2) than those grafted on WT rootstocks. Those differences could explain the lower sub-stomatal CO2 concentration (Ci ) in the leaves grafted onto the NCED OE lines (Figure 2d). The SP12 rootstock also seems to lead to fewer epicuticular wax crystals on both adaxial and abaxial leaf surfaces, without affecting stomatal density and aperture (Figure 2e; Table 2), supporting the lack of effect on gs (Figure 2b) and transpiration. Thus, NCED OE rootstocks affected leaf structure and function.