Lixia SHEN,Pu WANG,Ruofan LI

1.College of Water Conservancy and Engineering,Taiyuan University of Technology,Taiyuan 030024,China;2.College of Agronomy and Biotechnology,China Agricultural University,Beijing 100094,China

For high kernel set in maize(Zea mays L.),close synchrony pollination between pollen shed and silk emergence is required.The time gap between male(anthesis)and female (silking) flowering usually lengthens when plants are exposed to stress before anthesis,since silking is delayed more than the start of pollen shedding[1].A negative relationship exists between final kernel number(KN)and the extent of the anthesis-silking interval(ASI),so close synchrony between both events is desirable for improving kernel set.Intense selection for a shorter ASI has improved prolificacy,KN,and grain yield in maize under drought conditions[2].

Under natural conditions,pollination of ovaries at the base of the ear takes place between silking and one day after silking (DAS),while those from the tip are pollinated from 4 DAS onwards[3-4].Thus,early-fertilized ovaries at the base promote kernel abortion of late-fertilized ovaries from the tip.Delaying fertilization of early-silking ovaries allows some of the late-silking ones to compensate for their ontogenic delay and set kernels[5].Timing of pollination has a large impact on kernel set,and the disadvantage associated with an ontogetic delay in silk emergence could be partially overcome by synchronous pollination(SP)[6].SP improves kernel set in maize,but the physiological reasons behind this response are yet unclear.

In this work,we examined maize kernel growth at the tip of ear subjected to natural pollination (NP)or SP,which was obtained by hand pollination on 3 DAS and samples were taken from natural and hand-pollinated ears on 8,13,18,and 23 DAS.Fresh weight,volume and dry weight were used as outer indicators of kernel growth,and contents of soluble sugar,sucrose,starch and totalnitrogen were determined as inner ones.

Materials and Methods

Experimental details

Field experiments were conducted at the Wuqiao Experiment Station(37°47′N,116°42′E)of China Agricultural University.The soil is a salted light loam which turns clammy at deeper layer(about 130-170 cm),and with an organic matter content of 11.7 g/kg and 4.8 g/kg in the first 20 cm of depth and the second.Maize hybrid Zhengdan958 was sown on 13 June 2011,which represent normal planting dates in the region,following a randomized complete block design with three replicates.Plots were over planted and desired densities (6 plant/m2for experiment 1 and 9 plant/m2for experiment 2)were obtained by thinning at the three-leaf stage,with a uniform plant-to-plant distance within the row.Experiments were fertilized with 240 kg N hm2,100 kg P2O5hm2and 120 kg K2O hm2,to provide adequate mineral nutrition.Water stress was prevented by means of furrow irrigation,with the soil near field capacity throughout the growing season.Weeds,insects,and diseases were adequately controlled.

Treatments and measurements

Two pollination treatments were evaluated in all experiments:(i)natural pollination(NP),and(ii)synchronous pollination (SP).Each plot had 10 rows,0.6 m apart and 9 m long.On approximately 15 days before silking,at least 80 plants were tagged in the innermost six rows of each replicate,and were assigned at random to NP(at least 30 plants)or SP treatments(at least 50 plants). All visible earshoots of plants under SP were bagged before silking,and the date of silking (at least one silk visible from among the husks)was registered for all tagged plants(NP and SP).Pollination of ears assigned to the SP treatment was performed 3 DAS,which represents most silks appearing for this hybrid,by adding fresh pollen manually to all silks exposed from bagged ears.After pollination,these ears were left unbagged to allow natural pollination of late-appearing silks.For pollen collection,tassels with anthers visible only in the main branch were bagged late in the afternoon,and sampled for pollen the next morning[7-8].

For each replicate,at least 10 ears were sampled for determination of kernel growth at 8,13,18,23 DAS.Ears were placed in sealed bags to avoid desiccation and immediately brought to the laboratory.All kernels from 3rdto 13thposition according to the tip of ear were collected and mixed from each of 10 ears(total number of positions were 34-37 for experiment 1,and 27-30 for experiment 2),and from which 300 kernels were sampled and divided into three parts (100 kernels each)randomly for determination of kernel fresh weight,volume,and dry weight.For dry weight determination,kernels were put into oven for 15min at 105℃,and then dried to constant weight at 80℃.Dried kernel samples were conserved for determination of contents of soluble sugar,sucrose and starch by the method of anthronesulfuric acid colorimetry[9-10],and total nitrogen according to Kjeldahl determination[11].Contents of soluble sugar,sucrose,starch,and total nitrogen were all expressed on a dry matter basis.

Results

Kernel fresh weight,volume and dry weight

Under different plant densities of 6 plants/m2for experiment1 and 9 plant/m2for experiment 2,kernel fresh weight,volume and dry weight on 8,13,18,23 DAS as outer indicators of kernel growth at the tip of ear,were all higher for maize grown under SP than NP,though changed according to a similar trend.The growth rates of ker-nelfreshweight,volumeand dry weight were lower on 8-13 DAS,then increased quickly on 13-18 DAS.There were significant differences for kernel fresh weight,volume and dry weight on 18 DAS between SP and NP under different plant densities of 6 plant/m2(P＜0.05)and 9 plant/m2(P＜0.01),which indicated that SP could significantly improve kernel growth,and the effects were more significant under higher plant density than lower(Fig.1).

Contents of soluble sugar,sucrose and starch

In kernel at the tip of ear with different plant densities,the contents of soluble sugar,sucrose and starch on 8,13,18,23 DAS,were all higher for maize grown under SP than NP.Under different plant densities and pollination patterns,the contents of soluble sugar,sucrose and starch changed similarly on 8 to 23 DAS,of which,the contents ofsoluble sugarand sucrose increased on 8 to 18 DAS and decreased on 18 to 23 DAS,while the content of starch increased slowly on 8 to 13 DAS with little gap between SP and NP,and increased quickly on 13 to 23 DAS with big gap.From 8 DAS to 13 DAS,the contents of soluble sugar and sucrose were increased under SP,while changed a little under NP.Under both SP and NP,the contents of starch on 8,13,18,23 DAS were all higher for maize grown under low plant density of 6 plant/m2than high plant density of 9 plant/m2(Fig.2).

Contents of nitrogen and ratios of C/N

Under different plant densities,the contents of nitrogen in kernel at the tip of ear were all higher for maize grown under SP than NP,which increased slowly on 8 to 13 DAS,quickly on 13 to 18 DAS,and decreased on 18 to 23 DAS.Under both plant densities,the ratios of soluble sugar to nitrogen(C/N)were all higher for maize grown under SP than NP,while the gap between SP and NP was higher under high plant density of 9 plants/m2than low plant density of 6 plant/m2(Fig.3).

Discussion

SPsignificantlyimproveskernelset inmaize,bothbetweenearsatlowplant population and within the apical ear at high stand densities[6].Larger pollination gaps along florets determine larger differences in growth rate between growing kernels from the base of the ear(dominant sinks)and silking ovaries from the tip(dominated sinks),giving the latter a reduced opportunity for setting a kernel.Cárcova and Otegui characterized ovary and kernel growth from different floret positions along the ear(base,middle and tip)of plants subjected to NP or SP,and reported that florets along the ear differed in ovary weight,but differences were largerforNP than forSP plants[12].The range in ovary growth rate along the ear was drastically reduced under SP on 5 DAS.The larger the tip-to-base ratio in ovary growth rate,the larger the number of kernels set per plant.In agree with them,our results showed thatkernelfresh weight,volume and dry weight on 8,13,18,23 DAS as outer indicators of kernel growth at the tip of ear,were all higher for maize grown under SP than NP.However,the mechanism behind this response remains unclear,and the inner indicators of kernel growth,such as the contents of soluble sugar,sucrose,starch,and nitrogen,and carbon-nitrogen metabolism,the main physiological activity in kernel growth,maybe give some explanations.Our results showed that the contents of soluble sugar,sucrose,starch,nitrogen and C/N in kernel at the tip of ear were all higher for SP than NP.Being the main ingredient of kernel,starch is composed of sucrose,the major product of photosynthesis,which is transported to kernel through phloem system of ear.Higher contents of soluble sugar,sucrose and starch in kernel mean higher ability of starch syntheses with more photosynthesis product transported under SP than NP.Being another important ingredient of kernel,protein is indicated by the content of nitrogen,which is higher in kernel under SP than NP too.Higher contents of soluble sugar,nitrogen and C/N in kernel mean not only higher ability of carbon syntheses and accumulation,but also higher ability of carbon-nitrogen metabolism,which greatly promote the kernel growth under SP than NP.

In conclusion,SP improves kernel growth at the tip of ear,and not only increase kernel fresh weight,volume and dry weight,but also promote inner matter syntheses and carbon-nitrogen metabolism,whichstrengthen their competition for assimilation with kernel at the base of ear.

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