Morphophysiological Characteristics Of Pollen Grains In Summer Maize Under Abiotic Stresses

As one of the most widely planted food crops in the world,maize plays an irreplaceable role in global agricultural production.More than 23%and more than 24%of the world’s maize were consumed and produced in China.The steady increase in maize yield is of great significance to China’s agricultural economic development and food security.However,with global warming,extreme weather events occur frequently,heat and drought stress have become important factors affecting maize yield.In addition,due to the continuous increase of population and the decrease of arable land area in China,in order to meet the food demand,high-density planting has become one of the main directions of maize production.However,the competitive effect brought by high density greatly affects the potential productivity of maize.In this study,Annong 591(AN591)and Denghai 605(DH605),AN591 and Xianyu 335(XY335),AN591 and Jinqiu 119(JQ119)were used as experimental materials under heat,drought and density stress,respectively.The high temperature experiment was carried out in a thermostatic incubator,tassels were collected and put into incubator after tasseling.25/25,28/25,31/25,34/25,37/25,40/25 ~oC were set in 2020,and 25/25,30/25,35/25,40/25 ~oC were set in 2021.The drought experiment was carried out in Hefei High-tech Agricultural Park in 2021.CK(75±5%field capacity)and drought(55±5%field capacity)treatments were set at tasseling stage.The density experiment was carried out in the Nongcuiyuan of Anhui Agricultural University in2021,with four treatments of 45000,67500,90000 and 112500 plants/hm~2.The effects of heat,drought and density stress on flowering stage,the key stage of grain number formation,were analyzed by investigating the phenotypic characteristics of tassel,ASI,pollen size,pollen viability,pollen germination and pollen physiological characteristics.The main results are as follows:(ⅰ)Under heat stress,pollen size of both AN591 and DH605 changed significantly.Pollen grain shriveled,and the length of horizontal and vertical axes decreased significantly,but the aspect ratio had no significant difference.The pollen dyeing rate and germination rate decreased continuously,and the pollen viability of DH605 decreased more than that of AN591 with the increase of temperature.At the initial stage of germination(0-2 h),the pollen tube length showed a linear increase trend,but the k value(slope)of the fitting equation decreased with the increase of temperature,that is,the pollen tube elongation rate slowed down due to heat stress.The pollen tube elongation rate of AN591 decreased significantly at 37 ~oC,while that of DJH605 decreased significantly at34 ~oC.The pollen tube elongation rate of DH605 decreased more than that of AN591,thus the effect of high temperature on filament fertilization of DH605 was more obvious.In terms of physiological characteristics of pollen,the contents of soluble sugar and sucrose increased,while the contents of glucose and fructose decreased under heat stress.The change of sucrose content was related to the activities of sucrose decomposition related enzymes i.e.cell wall invertase(CWINV),vacuolar invertase(VINV),sucrose synthase(SS,decomposition direction)and neutral invertase(NI).The activities of four enzymes decreased significantly under heat stress,resulting in the decrease of sucrose decomposition rate and sucrose accumulation in pollen grains.(ⅱ)The tassel length,spindle length and tassel dry weight decreased significantly under drought stress,while the number of branches and tassel cob diameter decreased slightly but not significantly.Phenotypic index of tassel in XY335 decreased more than that in AN591 under drought stress.Similar to the heat stress,the dyeing rate and pollen germination rate dropped significantly,a marked increase in the content of soluble sugar and sucrose,and decrease in the content of glucose and fructose under drought stress.Sucrose accumulation is also due to a decrease in the activity of invertase enzymes i.e.CWINV,VINV,SS(decomposition direction)and NI caused by drought stress.Pollen of XY335 was more obviously affected by drought stress,and the sucrose accumulation and enzyme activity in pollen decreased more than that of AN591.(ⅲ)The tassel length,spindle length,tassel branch number and dry weight in both AN591 and JQ119 decreased with the increase of density.Morphological characteristics of tassel in AN591 had a significant difference at 11,2500 plants/hm~2,while JQ119 had a significant change at 9,0000 plants/hm~2.AN591 had no significant difference in cob diameter,but it was shortened for JQ119 at 112500 plants/hm~2.Plant density had little effect on pollen size,pollen germination rate and dyeing rate,except that the pollen horizontal axis length,germination rate and dyeing rate of JQ119 and pollen germination rate of AN591 showed significant differences at 11,2500 plants/hm~2.However,with the increase of density,the soluble sugar content of the two hybrids increased,while the sucrose,glucose and fructose contents decreased.With the decrease of sucrose content,invertase(CWINV,VINV and SS)activities also decreased,and the decrease of JO119 was greater,while the NI activity of the two hybrids showed no significant change.In conclusion,this study showed that different stress environments had an impact on flowering period of summer maize,however the effect varied with specific stresses.Heat stress is reported to mainly affect pollen viability.Aborted pollen will cause the failure of fertilization,eventually leading to kernel abortion.Drought stress affected the ASI and tassel length,which may result in insufficient pollen uptake in filament stigma,pollen viability also decreased to a certain extent under drought stress.In addition,high plant density mainly affected the filament fertilization and seed setting rate by prolonging ASI,hindering tassel development and shortening tassel length.Interesting,all three conditions led to a decrease in invertase activity.The reasons for the decrease of invertase activity caused by heat and drought stress are consistent,that is,the enzyme was inactivated.The decrease of invertase activity caused by high density planting may be related to the decrease of sucrose content caused by competition for nutrient within hybrid.