3.3 Overexpression of MdASMT9 improved photosynthetic
capacity in apple plants
The significant difference in plant biomass between WT andMdASMT9 -OE plants under N-deficiency conditions indicates
potential variations in the photosynthetic capacity of different
genotypes. There was no conspicuous difference in Pnbetween the WT and MdASMT9 -OE plants under control conditions
(Figure 2a). However, when exposed to N-deficiency stress, the decrease
in Pn was much less pronounced in the MdASMT9 -OE
plants. Under control and low-N conditions, OE-4 lines exhibited higher
GS than WT plants (Figure 2b). We also assessed the
total chlorophyll and carotenoid contents (Figure 2c,d) to find that
after low-N treatment, OE-3 and OE-4 lines showed higher total
chlorophyll and carotenoid contents than WT.
Chlorophyll fluorescence (ChlF) parameters are closely related to
photosynthesis under environmental stress (Gao et al., 2020). Hence, we
further examined the ChlF characteristics of WT and MdASMT9 -OE
lines with disparate tolerance to low N levels (Figure 2e,f). After
low-N treatment, F0 and F mvalues were lower whereas the FV/Fm ratio was
higher in MdASMT9 transgenic plants compared to WT plants.MdASMT9 -OE lines exhibited higher YⅡ than WT as well. Under
normal N supply and low-N conditions, OE lines showed lower Y(NO) and
higher NPQ/4. After low-N treatment, OE-4 lines showed higher Y(NPQ) and
qN than WT. These results altogether suggest that the photosynthetic
capacity in MdASMT9 -OE plants was less compromised under
N-deficiency stress.