Effects Of Exogenous 6-Benzyladenine And Salicylic Acid On Tolerance Of Pinellia Ternate To Combined Heat And Drought Stress

With the changing environment,heat and drought have become serious natural disasters disturb agricultural production,and often occur together. Pinellia ternata is one of important traditional Chinese herbal medicine. In its cultivation period, environmental disasters such as heat, drought and the combined stress initiate P. ternata " sprout tumble " phenomenon, leading to a dramatic decline in yield and quality, and severely restricted the scientific production of P. ternata tuber. In order to explore the influence of heat and drought combination on physiology and quality of P. ternata and improve the tolerance of P. ternata to the combined stress, in this study the responses of antioxidant system, photosynthetic system and metabolites of P. ternata to heat and drought combination were examined with application of the different concentrations of phytohormones of 6-benzyladenine (6-BA) and salicylic acid (SA).the main results are shown as follows:1. Heat and drought combination resulted in higher levels of leaf superoxide anion (O2) and malondialdehyde (MDA) content compared with its control. With prolonged heat and drought combination, O2- and MDA content continued to increase and the disparity enlarged. However, O2-and MDA content of P. ternate significantly decreased.and was less than the combined heat and drought alone when exogenous 6-BA at appropriate concentration was foliar applied. Similarly, the less content of O2- and MDA in P. ternate under heat and drought combination was obtained with exogenous SA treatment. It could be interfered that high temperature and drought combination could cause oxidative damage to P. ternate plants. Applications respectively of 6-BA and SA at appropriate concentration were able to alleviate the oxidative damage caused by the combined stress.2. The activities of superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) of P. ternate under combined heat and drought stress exhibited a rise at the beginning and then decline. With extended periods of the combined stress, SOD and POD activities were lower, but APX activity was slightly higher than the plant under normal condition. Only catalase (CAT) activity was less than its control plants in each period. Treatment with exogenous suitable concentration of 6-BA could enhance the SOD, POD and APX activities, and suppress the loss of CAT activity. Plants treated with 6-BA at 20μmol·L-1 showed the highest activities of SOD, POD, CAT and APX enzymes in each 6-BA treatment. Treatment with exogenous SA at suitable concentration were associated with increasing activities of SOD, CAT, POD and APX enzyme. In the 12d, the highest POD and APX activities were observed at 0.05 mmol·L-1 SA treatment, and the highest SOD and CAT activities at 0.1 mmol·L-1 SA treatment. The results suggested that treatment respectively with 6-BA and SA at appropriate concentration could improve antioxidant enzyme activities of P. ternate plants under the combined stress, and enhance the capability of scavenging active oxygen.3. Heat and drought combination led to a loss of Chla, Chlb and total Chi content in P. ternate leaf,but carotenoid content was slightly elevated in the early then dropped continuely. The extent of decline varied with the extended periods of the combined stress. Photosynthetic pigments of plants under the combined stress were significantly lower than normal plants at 12d. Exogenous foliar-applied 6-BA inhibited the reduction of Chla, Chlb, total Chi and Car content significantly. Even plants treated with 20μmol·L-1 6-BA still had a higher Car content than nomal plant at 12d. Plants applied with SA presented a tendency of rising early and then decline for Chla, Chlb, total Chi content. Especially, plants treated with 0.1mmol·L-1 SA had Chla, Chlb, total Chi content only slightly lower but Car content higher than normal plants. Conclusion could be made that combined heat and drought stress led to lower photosynthetic pigment content, but exogenous application of 6-BA and SA could improve content of photosynthetic pigments and prevent the photosynthetic pigments degradation caused by heat and drought combination.4. Heat and drought combination also resulted in a decline in photochemical efficiency (Fv/Fm), effective photochemical efficiency (Fv’/Fm’), photochemical quenching (qP) and actual photochemical efficiency (ΦPSⅡ), but non-photochemical quenching (NPQ) initially increased and then decreased. However, the declines in Fv/Fm, Fv’/Fm’, qP and ΦPSⅡ were inhibited significantly, and NPQ was less in the early but higher later than the combined stress alone when plants were treated with 6-BA at suitable concentration. When exogenous application of SA at appropriate concentration (0.05mmol·L-1, O.lmmol·L-1) to P. ternate plants was carried out, the decline in Fv/Fm, Fv’/Fm’, qP and ΦPSⅡ was also mitigated significantly,but NPQ maintained a continuous increase and still remained at a high level at 12d. Application of SA may protect the vitality of PSⅡ photosynthetic apparatus and alleviate heat and drought combination damage to the photosynthetic apparatus by enhancing the efficiency of non-photochemical.5. Contents of soluble sugar, reducing sugar, guanosine and adenosine of P. ternate tuber decreased significantly under combined heat and drought stress. Treatment with exogenous suitable concentration of 6-BA could improve the soluble sugar, reducing sugar, guanosine and adenosine content. Especially application of 6-BA at 20μmol·L-1 showed the highest soluble sugar, reducing sugar, guanosine and adenosine content in each 6-BA treatment. Applications of exogenous SA at appropriate concentration could also increase contents of soluble sugar, reducing sugar, guanosine and adenosine of P ternate tuber under the combined stress, among SA treatments, application of SA at 0.1mmol·L-1 presented the best effective,but at higher SA concentrations contents of soluble sugar, reducing sugar, guanosine and adenosine decreased.In summary, heat and drought stress increased leaf superoxide anion and malondialdehyde content, reducing the photosynthetic pigments (Chla, Chlb, Car, total chlorophyll) content, disorganizing the normal light system Ⅱ (a decline in Fv/Fm, Fv ’/ Fm’, qP, ΦPSⅡ), and resulted in loss of metabolites of soluble sugar, reducing sugar, adenosine and guanosine in P. ternate tuber. However, exogenous application of 6-BA and SA at appropriate concentration could increase leaf antioxidant enzymes (SOD, POD, CAT and APX) activities of P. ternate under combined heat and drought stress, reducing the accumulation of superoxide anion and malondialdehyde, restraining photosynthetic pigments content, alleviating the decline in Fv/Fm, Fv ’/ Fm’, qP and ΦPSⅡ, and increase soluble sugar, reducing sugar, guanosine and adenosine content of P.ternate tuber. Finally, the adverse impact of the combined stress on P. ternate plant was mitigated, and the resistance of P. ternate plant to combined heat and drought stress was reinforced.