The Regulation Effect Of Paclobutrazol On Low Light Tolerance In Tall Fescue

Turf is a neat and uniform grassland that has been artificially planted and maintained.It plays an important role in environmental protection,landscaping and recreation.It can be used as a carrier of urban ecosystems and one of the symbols of ecological civilization.However,tall buildings and trees in cities cause varying degrees of shade on turf,slight shade leads to the decline of turf quality,and heavy shade leads to turf degradation and even death.Therefore,the establishment and maintenance management of turf under the shade is one of the bottleneck problems to be solved urgently at present.Plant growth regulators represented by paclobutrazol（PBZ）are widely used in regulating the growth and yield of crops and ornamental plants,but there are few studies on turfgrass,especially those related to the regulation of low-light stress resistance in turfgrass.Therefore,this study explored the physiological mechanism of PBZ regulating the low-light（LL）tolerance of tall fescue（Festuca arundinacea Schreb.）by indoor control of light intensity and foliar spraying of PBZ.The results as follows:（1）Different concentrations(50-500 mg L^-1)of PBZ were sprayed on tall fescue under shading stress,and then it was found that the regulation effect of PBZ on the LL tolerance of tall fescue was inhibited by high concentration and promoted by low concentration.The most suitable PBZ concentration is 200 mg L^-1.Therefore,we used this concentration as the optimal concentration of PBZ in regulating LL tolerance in subsequent studies.（2）Under LL stress,the tiller numbers,leaf width,and biomass of tall fescue decreased significantly（P<0.05）.Low light also limits the synthesis of chlorophyll,reducing the content of chlorophyll precursors and photosynthetic pigments.However,PBZ treatment improved the morphological performance and photosynthetic pigment levels of tall fescue under LL stress.In addition,PBZ also alleviated the inhibition of LL stress on photosynthetic parameters such as net photosynthetic rate and Fv/Fm.The above results indicated that PBZ could increase the net photosynthetic rate and chlorophyll fluorescence parameters,thereby alleviating the damage of LL stress to photosynthesis.（3）Under LL stress,the electrolyte leakage and ROS level in tall fescue significantly increased,while the membrane lipid peroxidation level decreased significantly after spraying PBZ under LL（P<0.05）.LL stress also led to a significant increase in the activities of antioxidant enzymes.After PBZ treatment,the activities and gene expression levels of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,ascorbate peroxidase（APX）,and glutathione reductase（GR）were further enhanced,the activity of GR was the most significant（P<0.05）.In addition,PBZ enhanced the As A-GSH cycle,increased As A,GSH content and ratios of As A/DHA,GSH/GSSG.Therefore,PBZ could improve the LL tolerance of tall fescue by increasing the levels of antioxidant enzymes and non-enzymatic antioxidants.（4）LL stress had a significant effect on carbon and nitrogen metabolism in aboveground and underground parts.First,the activities of the nitrate reductase（NR）and glutamine synthetase（GS）in the aboveground and underground parts under low light were significantly reduced,and the activity of ribulose-1,5-bisphosphate carboxylase（Rubisco）of the aboveground parts was also significantly decreased（P<0.05）.However,spraying PBZ significantly increased the content of starch and soluble sugar in the aboveground and underground parts,while the content of reducing sugar in the aboveground part decreased significantly.In nitrogen metabolism,simultaneous treatment of LL and PBZ decreased the content of ammonium nitrogen in the aboveground and underground parts,increased the content of organic nitrogen（free amino acids and soluble protein）in the aboveground part,and decreased the content of free amino acids in the underground part.This indicated that PBZ enhanced the ability of nitrogen and carbohydrate synthesis in the aboveground part of tall fescue,transported more carbohydrates to the ground for energy,and finally enhanced the resistance to LL stress.（5）LL stress also affected the metabolic levels of gibberellin（GA）and abscisic acid（ABA）in tall fescue.Under LL stress,the contents of GA and ABA in tall fescue significantly increased,and the treatment of PBZ significantly decreased the content of GA and increased the content of ABA（P<0.05）.In addition,the expression levels of genes encoding GA synthases,such as Fa KS,Fa KO,Fa KAO,and Fa GA20ox,were significantly increased under LL stress,while these genes were significantly down-regulated after spraying PBZ.The expression level of the gene Fa GA2ox encoding GA degrading enzyme also showed a similar trend.Meanwhile,the expression levels of Fa NCED1,Fa PYR1 and Fa PYL1 were significantly increased under the simultaneous treatment of LL and PBZ.Therefore,PBZ may also regulate the balance of GA and ABA,thereby enhancing the LL tolerance of tall fescue.