3.12 Chloroplast-located Fld improved growth and tuber yield under chronic water restriction
Observations described in the preceding sections indicate that expression of a chloroplast-targeted Fld increased tolerance to short-term water limitation and attenuated drought transcriptional and metabolic responses in potato leaves. Thus, tuber yield was determined to evaluate possible Fld effects during a long-term water restriction regime with episodic rehydration applied to tuber-producing plants. Impact of drought on tuberization depends not only on the stress intensity but also upon timing, with the most damaging effects on yield occurring when the stress condition was applied at the stolon and tuber initiation stages (Obidiegwu, Bryan, Jones & Prashar 2015). Therefore, WT and Stpfld 252 plants were grown in soil at 100% FiC for 30 days, at the time when stolons were set. Water irrigation was interrupted until soil reached 40% FiC, rehydrated to 70% FiC and this protocol repeated for a total treatment of 90 days, when tuberization was extensive in both WT and Stpfld 252 lines (Supplementary Figure S2; see Materials and Methods). The protocol applied was similar to those employed to compare potato genotypes with different drought susceptibilities (Drapal et al. 2017).
Water restriction affected aerial growth in both lines, but significantly less in the transformant. Compared to control watered conditions, stressed WT plants accumulated only 10% aerial FW at the end of the 90-days treatment, whereas Stpfld 252 siblings reached ~25% (Figure 9a, b). Declines in water potential were significantly lower in Fld-expressing plants under drought (Figure 9c). Differential stress protection by Fld was also reflected at the level of tuber production. Water limitation reduced tuber yield by more than 80% in WT plants, but less than 70% in their Stpfld 252 counterparts (Figure 9d, e), resulting in absolute yields of 65 ± 6 vs 48 ± 2 g per plant for Stpfld 252 and WT lines, respectively (Figure 9e).