Effects Of Exogenous ABA On Physiological Mechanism And Yield Of Maize Under Drought Stress At Grain Filling Stage

Drought is a major abiotic stress that reduces the world’s food crop production and will become a serious obstacle to maintaining population growth.Improving crop drought resistance has become a new research hotspot for sustainable agricultural development.Plant hormones affect plant growth,development,maturity and senescence through signal transduction,metabolic regulation and integration of physiological responses.Abscisic acid（ABA）is usually involved in regulating stomatal opening and closing,assimilate transport and promoting dry matter accumulation in sink organs.However,there are few studies on exogenous application,especially the physiological mechanism of exogenous regulation under drought stress.Maize（Zea mays L.）plants need more water during reproductive growth than during vegetative growth stage.Maize filling stage is particularly sensitive to water stress and affects grain growth and yield.In this experiment,the barrel containers with unified specifications were used,and the corn varieties Zhengdan 958（ZD 958）and Xianyu 335（XY335）,which occupy a significant planting area in China,were used as research materials.Under normal water conditions and different degrees of drought stress,the effects of ABA on crop growth,photosynthetic function,antioxidant function,osmotic regulation function,nitrogen metabolism function,sucrose transport,starch synthesis and yield components of maize during early grain filling stage（ES）（1-10 days after pollination）and middle grain filling stage（MS）（20-30 days after pollination）were studied.The correlation between yield factors and physiological indexes was analyzed.The physiological regulation of exogenous ABA on drought stress in maize at grain filling stage had been explored.The main results are as follows:（1）Drought stress at the early stage of grain filling reduced the dry weight of leaves,stems,ears and leaf area per plant.The relative water content of maize leaves,photosynthetic pigment content,net photosynthetic rate（Pn）,maximum photochemical efficiency of PSⅡ（Fv/Fm）,photochemical quenching coefficient（q P）,stomatal conductance（Gs）,transpiration rate（Tr）and actual photochemical quantum efficiency（ФPSⅡ）decreased in varying degrees and the non-photochemical quenching coefficient（NPQ）increased due to insufficient water.The photosynthetic enzyme activity decreased the most on the 10^th day of drought stress.The decrease of Pn was not determined by stomatal or non stomatal factors alone.The effect of drought stress on the photosynthetic index of maize leaves in the early stage of grain filling was greater than that in the middle stage,and the effect on XY335 was greater than that on ZD958,indicating that XY 335 was more sensitive to water stress in the early stage of grain filling.The photosynthetic indexes of plants treated with exogenous ABA recovered in varying degrees,improved photosynthetic performance and drought resistance,alleviated the inhibition of plant growth,and some indexes in the middle filling stage could reach the control level.Chlorophyll content,gas exchange parameters,chlorophyll fluorescence and photosynthetic enzyme activity were significantly correlated with yield factors.（2）The accumulation of hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）increased under drought stress,and the phenomenon of relative electrolyte leakage（EL）was obvious.The application of exogenous ABA could alleviate the rising trend of H₂O₂and MDA content,significantly increase the activities of superoxide dismutase（SOD）and peroxidase（POD）,and alleviate the decrease of catalase（CAT）activity.Exogenous ABA up-regulated the transcription of related enzyme genes in ascorbic acid glutathione（AsA-GSH）cycle,and increased the activities of monodehydroascorbate reductase（MDHAR）,dehydroascorbic acid reductase（DHAR）,ascorbate peroxidase（APX）and glutathione reductase（GR）.Therefore,exogenous ABA plays an important role in improving the antioxidant capacity and drought resistance physiology of maize by improving the activity of antioxidant enzymes,increasing the levels of reduced ascorbic acid（AsA）and reduced glutathione（GSH）,stabilizing the redox state of AsA and GSH,and reducing the lipid peroxidation damage of cell membrane.The indexes of antioxidant system were significantly different among different treatments in the early stage of grouting,and XY335 was more sensitive.H₂O₂ content,MDA content,EL,antioxidant enzyme activity and antioxidant enzyme gene transcription level were significantly correlated with yield factors.（3）The contents of soluble sugar,soluble protein and proline in maize leaves increased under drought treatment.At the early stage of grain filling,the content of osmoregulation substances increased with the increase of stress degree;there was no significant difference among different stress treatments in the middle stage of grain filling.Exogenous ABA participated in the production pathway of osmoregulation substances and played a promoting role,so that the water potential pressure inside and outside the cells was in dynamic balance and alleviated the metabolic obstacles of crops.The contents of soluble protein,proline and free amino acids were significantly correlated with yield factors.（4）The activities of nitrate reductase（NR）,glutamine synthetase（GS）and glutamate synthase（GOGAT）decreased significantly under the condition of water deficit growth,but the activity of glutamate dehydrogenase（GDH）increased.The change range of nitrogen metabolic enzymes in the early stage of grain filling was greater than that in the middle stage of grain filling.Exogenous ABA alleviates the decline of nitrogen metabolism enzyme activity,was conducive to nitrogen assimilation and thesynthesis of amino acid and proteins,relieves the limitation of nitrogen transfer in vegetative organs,and maintains the balance of carbon and nitrogen metabolism.In the middle stage of grouting,the recovery effect of exogenous ABA on ZD 958 can basically reach the control level.There was a significant correlation between yield factors and nitrogen metabolism enzyme activity.（5）In the early stage of grain filling,the content of sucrose（Suc）in the control group decreased gradually with the process of grain filling,while the contents of fructose（Fru）and glucose（Glu）increased.Drought stress significantly increased the content of Suc in grains and decreased the contents of Fru and Glu.There was a significant difference between treatments.The change range was from 4^th day<7^th day<10^th day.In the middle stage of grain filling,the contents of three sugars in the grains of the control group showed an opposite trend with the early stage of grain filling.The activities of sucrose synthase（SS）,vacuolar invertase（VIN）,cell wall invertase（CWIN）and cytoplasmic invertase（CIN）in maize grains decreased after drought treatment.ABA treatment partially restored these enzyme activities and increased the expression of CWIN and VIN related genes.These results suggest that the increase of sucrose metabolizing enzyme activity induced by ABA may be an effective mechanism to improve the drought resistance of maize at grain filling stage.The activities of sucrose metabolizing enzymes changed significantly in the early stage of grain filling,but not in the middle stage of grain filling.The contents of sucrose,glucose and fructose in grains,the activity of sucrose transporter and the transcription level of carbon metabolizing enzyme gene were significantly correlated with yield factors.（6）The starch content of corn grains in the control group increased gradually with the filling process.The starch content in grains was significantly affected by drought stress in the early filling stage,but not in the middle filling stage.Exogenous ABA significantly increased the starch content,and there was no significant difference between the middle filling stage and the control.The activities of ADPG pyrophosphorylase（AGPase）,granule bound starch synthase（GBSS）and starch branching enzyme（SBE）decreased significantly under the influence of water deficit.In the middle filling stage,starch synthase（SSS）increased under drought stress.At the early stage of grain filling,SBE activity decreased the most on the 4^th day of stress,and other enzymes decreased the most on the 10^th day.ABA treatment inhibited the decline of starch synthase activity,and some enzyme activities returned to the control level in the middle filling stage.ABA can improve the rate of starch synthesis and reduce the negative impact on yield by regulating the activity of starch synthase.The activities of AGPase and SSS were significantly correlated with yield factors.（7）Water deficit led to the bald tip of maize,the grain number and 1000 grain weight decreased.The effect of drought stress on maize grain yield factors in the early filling stage was greater than that in the middle filling stage.At the early stage of grain filling,the bald tip length increased significantly with the stress time.The bald tip length of stress for 10 days was greater than 4 and 7days,and the change trend of grain number and 1000 grain weight was the same.At the middle filling stage,there was no significant difference in the change of bald tip of plants under different drought stress treatments.Exogenous ABA treatment inhibited the increase of bald tip length,increased 1000 grain weight and grain number,and the effect in the early stage of grain filling was greater than that in the middle stage.The yield index of plants with insufficient water was lower than that of normal growth treatment.