| Increasing plant population densities for modern maize cultivars is one of the most efficient agronomic approaches for obtaining higher maize grain yield.However,dense plant populations may prone to lodging,as it results in increased plant height and reduced culm diameter,which has a detrimental effect on yield.Lodging is one of the major constraints,which not only reduce yield but also decrease the quality of maize grains.Besides from lodging,the limited and irregular rainfall and scarcity of irrigation water in the semiarid region is another major problem for lower grain yield of maize.The application of plant growth regulators is an effective measure for optimizing plant canopy structures,manipulating high planting densities,enhancing the crop tolerance to abiotic stresses,improving the lodging resistance,and grain yield of crops.Therefore field experiments were conducted to elucidate the efficacy of two potential plant growth regulators“paclobutrazol”and“mepiquat chloride”on plant height,the physical strength of stem,lignin content and its relationship with lodging resistant in high-density maize.While additional field experiments were conducted to study the effects of different concentrations of paclobutrazol under two different application methods（seed-soaking and seed-dressing）on root morphological and physiological characteristics,chlorophyll contents,photosynthetic characteristics,chlorophyll fluorescence,the activities of antioxidant enzymes,and its relation with grain yield of maize crop under a water deficit semiarid region.The main objective of this research was to increase the grain yield of maize crop by reducing the lodging rate under high planting density and to increase the crop tolerance under water deficit condition by application of growth regulators.For paclobutrazol,seed-soaking at the rate of 0,200,300,and 400 mg L-1,and seed-dressing at the rate of 0,1.5,2.5,and 3.5 g kg-1 were used.While for mepiquat chloride only seed-dressing at the rate 0,2.0,2.5,and 3.0 g kg-1 was used.A planting density of 82500 plant ha-1 was used to study the effects of paclobutrazol on root growth characteristics and its relation with grain yield increase（experiment 1）,while a planting density of 97500 plants ha-1was used to study the effect of paclobutrazol on lodging resistance（experiment 2）,photosynthetic characteristics and antioxidant enzymes（experiment 3）,and also to study the effects of mepiquat chloride on lodging resistance of maize（experiment 4）.The main results are as following:1)The maize crop grown under the semiarid regions are often subjected to lower and erratic rainfall,which often reduce crop growth and final grain yield.Nevertheless,application of plant growth regulators may alleviate these negative effects by improving root growth characteristics and distributions in the soil for better nutrients and water uptake.Application of paclobutrazol significantly improved the ear characteristics and grain yield of maize and was consistently higher than control plants,at low planting density(82500 plants ha-1)during 2015-2016.In seed soaking treatments,the highest average grain yield achieved was 61.1%higher,while in seed dressing it was 52.8%higher than control.Paclobutrazol-treated maize had improved root-length density（RLD）,root-surface area density（RSD）and root-weight density（RWD）at most of the soil profiles;majorly in 0-70 cm for seed-soaking,while 0-60 cm for seed-dressing,and was attributed to enhancing the grain yield.In addition,root-activity,root-bleeding sap,root dry weight,diameter and root/shoot ratio were increased by paclobutrazol,with best effects were observed with 300 mg L-11 for seed-soaking and 3.5 g kg-1 for seed-dressing treatments,across the whole growth stages in 2015-2016.These results suggest that paclobutrazol could efficiently be used to enhance root-physiological and morphological characteristics,resulting in higher grain yield.2)Dense plant cultivation is an efficient approach for increasing grain yields.However,dense plant populations may prone to lodging as it results in increased plant height and reduced culm diameter.Paclobutrazol seed-soaking and seed-dressing reduced the internode length,plant height,ear height,the center of gravity height and lodging rate.Paclobutrazol improved the culm physical strength by increasing the rind penetration strength,stalk breaking strength,culm diameter,wall thickness,and dry weight per unit length of basal third internode,compared to control plants.In addition,more lignin was accumulated in the basal internode and the activities of phenylalanine ammonia-lyase（PAL）,peroxidase（POD）,cinnamyl alcohol dehydrogenase（CAD）and 4-coumarate:CoA ligase（4CL）increased with paclobutrazol,and their maximum values were observed when paclobutrazol was used at the rates 300 mg L-1 as seed soaking and 3.5 g kg-1 as seed dressing,resulting in strong lodging resistance.Lignin content was positively and significantly correlated with the rind penetration strength,breaking strength of internode,and activities of PAL,4CL,POD,and CAD,while significantly and negatively correlated with lodging percentage.The present findings suggested that paclobutrazol may efficiently be employed to minimize the risk of lodging.Based on that all,paclobutrazol can be used to optimize the high planting densities for a better grain yield.3)The crop growth and grain yield of summer maize in a semi-arid region can be improved by regulating the photosynthetic capacity and antioxidants.Plant growth regulators are often referred as stress protectants.Therefore paclobutrazol effects on grain yield performance of summer maize,chlorophyll pigments,photosynthesis rate and antioxidant defense system was elucidated under a semiarid condition.Results showed that paclobutrazol treatments increased the chlorophyll contents,improved the photosynthetic capacity,light harvesting efficiency of photosynthesis associated with photosystem II,and enhanced the activities of antioxidants,which helped in sustaining plant growth and resulting in higher kernel number per ear-1,thousand kernel weights,and grain yield increase of maize under high planting density(97500 plants ha-1).The average maize grain yield was increased up to39%in seed-dressing and 61%in seed-soaking with paclobutrazol,compared to control plants.In addition,activities of ascorbate peroxidase（APX）,catalase（CAT）,peroxidase（POD）,superoxide dismutase（SOD）,proline and soluble protein content increased with paclobutrazol.At the same time,malonaldehyde（MDA）contents were dramatically lowered by paclobutrazol treatments in consecutive growing seasons.The concentrations of paclobutrazol which showed the most significant effects were 3.5 mg kg-1 for seed-dressing and 300 mg L-1for seed-soaking.4)Lodging is a major adversity which limits maize grain yields and quality,especially under dense population.Mepiquat chloride has been extensively used for optimizing plant canopy of cotton crop and controlling lodging under high densities.Therefore,in the present study we used mepiquat chloride to explore whether it could mitigate the risk of lodging in maize under high planting density.The results showed that mepiquat chloride application enhanced the culm physical strength,as revealed by enhanced rind puncture strength and stalk bending strength,increased dry weight per cm,stem diameter,and lignin accumulation resulting in strong lodging resistance.A significant and negative correlation was detected between stalk bending and rind puncture strength with lodging rate.Moreover,the lodging rate,plant height,internodes length and gravity center height were significantly reduced following a dose-response trend,and mepiquat chloride treatments resulted in relatively shorter plants,compared to control treatment.In addition,the grain yield,lignin contents and activities of lignin-related enzymes were enhanced by mepiquat chloride,and treatment of 2.0g kg-1mepiquat chloride showed the best effect of all treatments. |
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