Effects of exogenous Z-3-HAC on seedling growth, plant biomass and root morphology under drought stress
As shown in Fig. 1a, wheat seedlings primed with Z-3-HAC grew significantly better than those un-primed in both normal and drought stress conditions. In detail, shoot fresh weight and dry weight of seedlings primed with Z-3-HAC were respectively 33.64% and 28.70% higher than those un-primed under normal growth conditions (Fig. 1b and c). Drought stunted wheat seedling growth as indicated by the significant decreases in shoot fresh weight and dry weight by 45.50% and 23.15%, respectively (Fig. 1b and c). However, priming of Z-3-HAC significantly mitigated the adverse effects of drought stress as indicated by no significant difference between the control and “Z-3-HAC + PEG” treatments (Fig. 1b and c). Similar results were obtained in root fresh weight and dry weight analysis. Root fresh weight and dry weight of seedlings primed with Z-3-HAC were respectively 50.33% and 47.83% higher than those un-primed under normal growth conditions (Fig. 1d and e). Drought stress decreased root fresh weight and dry weight by 42.14% and 23.91%, respectively (Fig. 1d and e). And priming of Z-3-HAC made no significant difference of root fresh weight and dry weight exist between the control and “Z-3-HAC + PEG” treatments (Fig. 1d and e).
As root plays an important role in crop drought resistance, root morphology was investigated detailly. Intuitively, roots of seedlings with Z-3-HAC priming showed a clear advantage regardless of normal growth or drought stress conditions (Fig. 2a). Quantitative data of root morphological parameters showed that exogenous application of Z-3-HAC significantly increased total root length, total root surface area and total root volume compared with the non-drought stressed control (Fig. 2c-e). The total root length, total root surface area, and total root volume were decreased by 53.88%, 48.51%, and 43.61%, respectively, under drought stress (Fig. 2c-e). Whereas, the application of Z-3-HAC before drought stress increased them by 67.60%, 41.77%, and 41.33%, respectively, compared to the drought stressed control (Fig. 2c-e). However, no significant difference was observed between treatments in root average diameter (Fig. 2b).
Effects of exogenous Z-3-HAC on RWC and REC under drought stress
It was showed that exogenous application of Z-3-HAC had no effect on the RWC and REC of wheat seedlings under normal growth conditions (Fig. 3). Compared with the control, drought stress significantly decreased RWC by 27.62% (Fig. 3a), while significantly increased REC by 196.35% (Fig. 3b). Whereas, compared with the drought stressed control, the application of Z-3-HAC increased the RWC by 14.47% (Fig. 3a), while significantly decreased the REC by 32.36% (Fig. 3b).
Effects of exogenous Z-3-HAC on gas exchange parameters, chlorophyll fluorescence parameters and total chlorophyll content under drought stress
Firstly, there is no obvious difference of photosynthetic indexes between seedlings primed with and without Z-3-HAC under normal growth conditions (Fig. 4). Seedlings treated with only drought stress displayed significant decreases of 72.45% in Pn (Fig. 4a), 23.29% in Ci (Fig. 4b), 94.34% in Gs (Fig. 4c), and 85.98% in Tr (Fig. 4d), respectively. Exogenous Z-3-HAC significantly reduced the adverse effects of drought stress on seedlings. It was specifically indicated by significant increases of Pn by 101.60%, Gs by 366.67% and Tr by 210.14%, respectively, while a significant decrease of Ci by 17.28%.
Drought stress significantly decreased Fv/Fm by 29.11%. Again, Fv/Fm was significantly increased by 30.36% when seedlings were previously primed with Z-3-HAC (Fig. 5a). Fv/Fm status in different treatments was also indicated by pseudo color images of the leaves (Fig. 5a). Similarly, drought stress significantly decreased the other chlorophyll fluorescence parameters, such as Fv′/Fm′, ΦPSII, and NPQ, by 35.71%, 24.44%, and 57.14%, respectively (Fig. 5b-d). However, only Fv′/Fm′ was significantly increased by 33.33% (Fig. 5b), while ΦPSII (Fig. 5c) and NPQ (Fig. 5d) showed no change, when seedlings were previously primed with Z-3-HAC. Besides, drought stress significantly decreased chlorophyll content by 72.81%, while exogenous Z-3-HAC increased it by 140.86% (Fig. 5e).
Effects of exogenous Z-3-HAC on ROS accumulation and lipid peroxidation under drought stress
Firstly, the accumulation of H2O2 and O2-, two representative ROS, was detected by histochemical method. H2O2 and O2- accumulated slightly following the application of Z-3-HAC under normal growth conditions, and seriously following the treatment of only drought stress (Fig. 6a and b). Notably, the H2O2 and O2- accumulation of seedlings under drought stress was largely reduced by the exogenous Z-3-HAC (Fig. 6a and b). All these were basically confirmed by the quantitative data. Drought stress significantly increased H2O2 and O2- by 58.33% and 79.39%, respectively, while exogenous Z-3-HAC decreased them by 15.79% and 9.79%, respectively, compared with only drought stress (Fig. 6c and e). The lipid peroxidation of wheat seedlings was detected according to the accumulation of MDA. Drought stress significantly increased MDA content by 24.00%. Consistent with the effect of Z-3-HAC on ROS accumulation, exogenous Z-3-HAC significantly reduced the MDA content by 26.30% under drought stress conditions (Fig. 6d).
Effects of exogenous Z-3-HAC on antioxidant metabolism and osmolytes accumulation under drought stress
Under normal growth conditions, exogenous application of Z-3-HAC showed almost no effect on the activities of SOD, POD, CAT and APX (Fig. 7). Under drought stress, the activities of all these four antioxidant enzymes increased significantly, SOD by 22.29%, POD by 50.33%, CAT by 228.08%, and APX by 200.07%, respectively. And exogenous Z-3-HAC further significantly increased activities of SOD by 16.01%, POD by 24.14%, CAT by 37.78%, and APX by 24.44% (Fig. 7).
Low molecular weight organic compounds, such as TSS, Fru, FAA and Pro, are the main components of osmotic substances in plants. Under normal growth conditions, exogenous application of Z-3-HAC significantly increased the concentrations of TSS by 20.62%, Fru by 18.29%, FAA by 113.12% and Pro by 57.11%, respectively (Fig. 8). Under drought stress conditions, concentrations of TSS were significantly increased by 26.80%, FAA by 216.87% and Pro by 151.38%, respectively. While concentration of Fru was slightly increased by 10.49% (Fig. 8). As shown in Fig. 8, exogenous Z-3-HAC significantly increased TSS by 17.48%, and Pro by 13.92%, respectively, compared with only drought stress. However, concentration of Fru was just slightly increased by 11.70%, while concentration of FAA was significantly decreased by 12.49% instead.
Principal component analysis
A principal component analysis (PCA) integrating all the information of four treatments was performed. The two components of PCA collectively explained 95.54% of data variability (Fig. 9). The first PC (PC1) accounted for 73.58% of the total qualitative variation and had REC, APX, CAT, Pro, POD, SOD and FAA with high negative loadings (Fig. 9a). The second PC (PC2) accounted for 21.96% of the total qualitative variation and had Fru, RDW, SDW and TSS with high positive loadings (Fig. 9b). SOD, POD, CAT, APX, TSS, FAA, Pro and Fru were located toward the negative end of PC1 axis and the positive end of PC2 axis in the second quadrant (Fig. 9b). It corresponded to the result of “Z-3-HAC + PEG” (Fig. 9a). In conclusion, the antioxidant enzymes and osmoregulation substances were the most important factors in response to Z-3-HAC under drought stress.