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.