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.