Yunji Xu,Mengting Qiu,Yinyin Li,Xiyang Qian,Junfei Gu,Jianchang Yang*

Jiangsu Key Laboratory ofCrop Genetics and Physiology/Co-Innovation Centerfor Modern Production Technology ofGrain Crops,Yangzhou University, Yangzhou 225009,China

Polyamines mediate the effect of post-anthesis soil drying on starch granule size distribution in wheat kernels

Yunji Xu,Mengting Qiu,Yinyin Li,Xiyang Qian,Junfei Gu,Jianchang Yang*

Jiangsu Key Laboratory ofCrop Genetics and Physiology/Co-Innovation Centerfor Modern Production Technology ofGrain Crops,Yangzhou University, Yangzhou 225009,China

ARTICLEINFO

Article history:

Received 24 January 2016

Received in revised form

20 April 2016

Accepted 13 May 2016

Available online 14 June 2016

Polyamines

Polyamines(PAs)are important endogenous plant growth regulators responding to environmental stress and mediating many physiological processes including grain filling in cereals.This study investigated whether PAs mediate the effect of post-anthesis soil drying on starch granule size distribution,starch content,and weight of superior and inferior kernels of wheat(Triticum aestivum L.).Two wheat cultivars were grown in pots. Three treatments,well-watered(WW),moderate soil drying(MD)and severe soil drying (SD),were imposed from 9 days post-anthesis until maturity.PA levels in kernels and small,medium and large granules were measured.The results showed that superior kernels had much higher free spermidine(Spd)and free spermine(Spm)concentrations, larger volumes of medium starch granules,and smaller-sized large granules than did inferior kernels under all the treatments.Compared to WW,MD significantly increased the concentrations of free Spd and free Spm,activities of soluble starch synthase and granule-bound starch synthase,volume of medium granules,and starch content and kernel weight of inferior kernels,and decreased the size of large granules.SD produced the opposite effect.Application of Spd or Spm to spikes produced effects similar to those of MD, and application of an inhibitor of Spd and Spm synthesis produced effects similar to those of SD.These results suggest that PAs mediate the effect of post-anthesis soil drying on starch biosynthesis in wheat kernels by regulating key enzymes in starch synthesis and that elevated PA levels under MD increase the volume of medium granules and kernel weight of inferior kernels.

?2016 Crop Science Society of China and Institute of Crop Science,CAAS.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Starch in the endosperm of wheat(Triticum aestivum L.)is a major form of carbon reserves and accounts for 65–75%of the final dry weight of a kernel[1–3].Starch exists as discrete semi-crystalline granules with various sizes,shapes,and size distributions[4,5].Wheat starch granules can normally be divided into two types,large A-type granules(diameter＞9.9 μm)and small B-type granules(diameter＜9.9 μm)[6–9]. However,small C-type granules have also been observed[10].In our previous study[11],the thresholds for separating wheat granules into small,medium,and large sizes were＜5 μm, 5–50 μm,and＞50 μm,respectively.This separation method was new and clearer than previous methods.The starch granule size distribution of wheat is an important character that can affect starch chemical composition,and consequently, influence kernel quality[12–14].However,knowledge about the relationship between starch granule size distribution and kernel weight in wheat is very limited.

The physicochemical properties of starch and the granule size distribution in wheat are both genetically and environmentally controlled[9,15–17].Environmental factors exert a greater effect on starch components and properties than does cultivar variation[16].Dai[15]reported that the contents of A-,B-,and C-type starch granules in wheat were affected by soil water,genotype,and soil water×genotype interaction. Rainfed cultivation reduced the granules with diameters＞9.8 μm and increased the granules with diameters of 2.0–9.8 μm and＜9.8 μm compared to an irrigated treatment[18]. This finding indicates that the starch granule size distribution in a wheat kernel may vary with soil moisture.However,little is known about how post-anthesis soil drying affects starch granule size distribution.

Polyamines(PAs),including spermidine(Spd),spermine (Spm),and putrescine(Put),are considered endogenous plant growth regulators or intracellular messengers responding to environmental stress and mediating many physiological processes including grain filling of cereals[19–23].In maize(Zea mays L.),PAs have been observed to be presentatmuch higher levels in normal kernels than in aborting kernels,and that higher ratios of Spd or Spm to ethylene are closely associated with higher kernel set and kernel weight[24,25].Similar results have been observed in rice and wheat[26–29].Application of Spd or Spm accelerated kernel filling of wheat[29,30].Application of Spd increased the contents of amylose,amylopectin and total starch,and a higher Spd content increased the formation of B-type starch granules in wheat kernels under severe water deficit[31].It is not known, however,whether PAs could mediate the effect of post-anthesis soildrying on starch granule size distribution,starch content,and kernel weight.

Kernels on a wheat spike can be classified into superior or inferior ones based on their flowering date or locations on the spike[32,33].Usually,superior kernels show a higher kernel filling rate and a heavier weight than inferior ones[34,35].The difference in kernel weight between superior and inferior kernels may be attributed to many factors,such as differences in assimilate participation,hormonal levels,and starch biosynthesis ability[34,36,37].However,no information is available on starch granule size distribution between superior and inferior kernels or whether PAs can regulate the distribution when wheat plants are subjected to post-anthesis soil drying.

The objective of this study was to test the hypothesis that PAs mediate the effect of post-anthesis soil drying on starch granule size distribution,starch content,and kernel weight by regulating the key enzymes in starch synthesis,and that PAs are involved in the difference in kernel weight between superior and inferior kernels of wheat.PA levels,starch granule size distribution,and activities in kernels of two key enzymes in starch synthesis,soluble starch synthase(SSS)and granule bound starch synthase(GBSS),were investigated.

2.Materials and methods

2.1.Plant material and growth conditions

The experiment was conducted at a research farm of Yangzhou University,Jiangsu province,China(32°30′N, 119°25′E,21 m altitude),during the wheat(Triticum aestivum L.) growing season(November 2013–June 2014),and was repeated during the wheat growing season in 2014–2015.Two cultivars, Yangmai 16(YM16)and Ningmai 13(NM13),that are currently used in local production,were grown in porcelain pots.Each pot (30 cm in height and 25 cm in diameter,14.72 L in volume)was filled with 18 kg of sandy loam soil[Typic Fluvaquent,Entisol (U.S.taxonomy)]that contained 20.2 g kg?1organic matter, 105 mg kg?1alkali hydrolyzable N,34.2 mg kg?1Olsen phosphorus,and 68.0 mg kg?1exchangeable potassium.Twenty seeds were sown in each pot.At the three-leaf stage,plants were thinned to 10 plants per pot(equivalentto a density of204 plants m?2).The plants were watered daily by hand to maintain the soil water content close to field capacity(soil moisture content 0.189 g g?1)until 9 days post-anthesis(DPA),when soil drying treatments were initiated.YM16 and NM13 headed 153 and 154 days after sowing(DAS),respectively,and flowered during 160–166 DAS.

2.2.Soil drying treatments

The experiment was a 2×3(two cultivars and three levels of soil water status)factorial design with six treatment combinations.Each treatment had 48 pots as repetitions in a randomized complete block design.From 9 DPA until maturity, three levels of soil water potential(ψsoil)were imposed on the plants by control of water application.The well-watered(WW) treatment was maintained at?20±5 kPa(soil moisture content 0.155 g g?1),a moderately soil drying(MD)treatment was maintained at?40±5 kPa(soil moisture content 0.119 g g?1), and a severely soil drying(SD)treatment was maintained at?60±5 kPa(soil moisture content 0.091 g g?1).Soil water potential was monitored at 15-cm to 20-cm soil depth.A tension meter consisting of a 5-cm-long sensor(Soil Science Research Institute,China Academy of Sciences,Nanjing,China) was installed in each pot to monitor soil moisture.Tension meter readings were recorded every 4 h from 600 to 1800 h. When the reading dropped to the designated value,200,160, and 120 mL of tap water per pot were added to the WW,MD,and SD plants,respectively.The pots were placed in a field and sheltered from rain by a removable polyethylene shelter that was placed over them during rain.

2.3.Sampling

Three-hundred spikes that flowered on the same day were chosen and tagged in 30 pots of each treatment.Twenty to thirty tagged spikes from each treatment were sampled at 6, 18,and 30 DPA.All kernels from each spike were removed. Kernels on a spike were divided into two groups:superior and inferior kernels.The basal kernels in the middle spikelets(the fourth through twelfth spikelets)on a spike were taken as superior and the other kernels on the spike as inferior kernels[34].The sampled superior kernels and inferior ones were frozen in liquid nitrogen for 10 min and then stored at?70°C to determine the concentrations of free PAs(free Spd,free Spm, and free Put)and the activities of starch synthases(SSS and GBSS).Superior and inferior kernels then were also separated from sixty tagged spikes,as described above,at maturity.The mature superior and inferior kernels were ground to powder to determine amylose,amylopectin,and starch content and to extract starch granules after drying and weighing.

2.4.PAs determination

Free PAs fractions were extracted following the method of Flores and Galston[38]with modification.Free PAs were derivatized with benzoyl chloride and quantified by HPLC (Waters 2695 Separations Module;Waters,Milford,MA,USA)as described by DiTomaso et al.[39].PA levels were averages of three replications for each independent sample and expressed as nmolg?1DW.

2.5.Starch synthases determination

All chemicals and enzymes used for enzymatic measurements in kernels were from Sigma Chemical Company(St.Louis,MO, USA).The protein content was determined according to Bradford [40],using bovine serum albumin(BSA)as the standard.The activities of soluble starch synthase(SSS,EC2.4.1.21)and granule-bound starch synthase (GBSS,EC2.4.1.21)were determined as described previously[41].Enzymatic activities were expressed as nmol mg?1protein min?1.

2.6.Amylose,amylopectin and starch determination

The contents of amylose,amylopectin,and starch in wheat kernels were determined following He[42].The starch content was the sum of amylose and amylopectin.

2.7.Starch granule extraction and particle size analysis

Starch granules were extracted from the samples following the methods of Peng et al.[12]and Ji et al.[43]with some modifications.The particle size characteristics of the starch were determined using an MS-2000 Laser Particle Size Analyzer (Malvern,England).The starch granule size distribution at maturity was measured by the software in the instrument, and expressed as percentages.The starch granules were divided into small(＜5 μm),medium(5–50 μm),and large granules (＞50 μm)following our previously described method[11].

2.8.Chemical applications

The wheat cultivar NM13 was used for chemical application. Plants were grown in the field during the 2014–2015 growing season.Starting at 9 DPA,1 mmol L?1Spd,1 mmol L?1Spm, 2 mmol L?1Put, and 5 mmol L?1methylglyoxal-bis (guanylhydrazone)(MGBG,which inhibits Spd and Spmsynthesis by inhibiting SAM decarboxylase)(all from Sigma Chemical Company)were applied to spikes using a writing brush that had been dipped in the solution.The chemicals were applied daily for 4 days at a rate of 4 mL per spike at each application,with 0.5%(v/v)Teepol(Fluka,Riedel-de-Haen,Germany)as the surfactant.The same volume of deionized water containing the same concentration of Teepol was applied to control plants.Each chemical treatment was applied to an area of 5 m2with three replications.

Table 1–Kernel weight and starch contents of superior and inferior kernels in wheat cultivars YM16 and NM13 under various soil drying treatments in a pot experiment.

For all of the chemical treatments,kernels on a spike were also divided into superior and inferior kernels.The levels of free PAs(free Spd,free Spm,and free Put)and activities of SSS and GBSS were determined at 6,18,and 30 DPA and were then averaged.Kernel weight,contents of amylose,amylopectin andstarch,and starch granule size distribution were determined at maturity.The measurement methods for free PAs and starch content were the same as described above.Each sample was subjected to three measurements.

Fig.1–Changes in number(a–b),length(c–d),volume(e–f),and surface area(g–h)distribution ofstarch granules in superior(S)and inferior(I)kernels of wheat cultivars YM16(a,c,e,g)and NM13(b,d,f,h)at maturity under various soil drying treatments.WW, MD,and SD represent well-watered,moderate soil drying,and severe soil drying treatments,respectively,in a pot experiment during kernel filling.Vertical bars represent±standard error of the mean(n=4)where these exceed the size of the symbol.

2.9.Statistical analysis

The results were analyzed for variance and correlation using the SPSS statistical analysis package(version 16.0,SPSS, Chicago,IL,USA).Data from each sampling date were analyzed separately.The means were tested by least significant difference at P=0.05(LSD0.05).

3.Results

3.1.Kernel weight and contents of amylose,amylopectin and starch

The kernel weight and contents of amylose,amylopectin and starch were much greater for superior than for inferior kernels when the soil moisture was the same(Table 1).The kernel weight and contents of amylose,amylopectin and starch in inferior kernels were significantly increased under MD compared with those under WW,and they showed no significant difference between the two treatments for superior kernels(Table 1).SD markedly reduced the kernel weight and contents of amylose, amylopectin,and starch for both superior and inferior kernels (Table 1).Similarly to starch content and kernel weight,kernel yield was significantly increased under MD,whereas it was significantly decreased under SD relative to that under WW(data not shown).The increased yield under MD was mainly due to the increased weight of inferior kernels.The two cultivars showed the same tendency in both growing seasons(Table 1).

3.2.Starch granule size distribution

The distributions of starch granule number,length,volume, and surface area in wheat kernels displayed a typical unimodal curve,a trimodal curve,a tetramodal curve,and a typical trimodal curve,respectively,for both superior and inferior kernels(Fig.1).The pattern of the granule size distribution was very similar in both superior kernels and inferior ones,even under different soil drying treatments(Fig.1).

3.3.Distribution of granule number,length,volume,and surface area

The proportions of the number and length of small(＜5 μm) starch granules in wheat kernels were in the range of 99.7–99.8%and 95.5–95.8%,respectively(Tables 2,3),indicating that the starch in wheat endosperm is almost made up of small granules,and that the total length of granules was determined mainly by small granules.No marked difference betweensuperior and inferior kernels in the proportions of the number and length of small and medium granules under various levels of soil drying was observed.However,the proportions of the number and length of large granules were much greater in inferior than in superior kernels.Compared with WW, MD significantly reduced the proportions of the number and length of large granules in inferior kernels,but showed no effect on those in superior kernels.SD significantly increased the proportions in both superior and inferior kernels(Tables 2 and 3).

Table 2–Proportion of starch granule number in superior and inferior kernels of wheat cultivars YM16 and NM13 at maturity under various soil drying treatments in the pot experiment.

Table 3–Proportion of starch granule length in superior and inferior kernels of wheat cultivars YM16 and NM13 at maturity under various soil drying treatments in a pot experiment.

The proportion of medium starch granule volume (5–50 μm)in wheat kernels was in the range of 80.2–90.0% (Table 4),suggesting that the volume of medium granules is the most important contributor to the total volume of starch granules.The surface area of small starch granules(＜5 μm) and medium granules(5–50 μm)accounted for 55.5–57.7%and 35.2–43.8%of the total surface area of starch granules,and only 0.21–7.79%for large granules(＞50 μm)(Table 5),implying that small and medium granules were the two main contributors to the total surface area of granules.No significant difference for the volume and surface area of small starch granules between superior and inferior kernels was observed. The volume and surface area of medium granules were much greater in superior than in inferior kernels,but much less for large granules.In contrast to WW,MD significantly increased the proportions of the volume and surface area of medium granules in inferior kernels,whereas SD decreased the proportions of the volume and surface area of medium granules and increased those of large granules in both superior and inferior kernels(Tables 4 and 5).

The contents of amylose,amylopectin,starch and kernel weight were significantly and positively correlated with the proportions of the volume and surface area of medium granules(r=0.74**to 0.84**,P=0.01,Table 6),and were very significantly and negatively correlated with the proportion of large granule size(r=?0.80**to?0.91**,P=0.01,Table 6).

3.4.PAs concentrations in kernels

The free Spd and free Spm concentrations were much higher in superior than in inferior kernels at 6 and 18 DPA,whereas this relationship was reversed at 30 DPA(Table 7).The free Put concentration was much higher in inferior than in superior kernels at 30 DPA.Compared with those under WW,both free Spd and free Spm concentrations in inferior kernels under MD were significantly increased at 18 DPA and decreased at 30 DPA,and showed no significant difference between the WW and MD treatments for superior kernels(Table 7).SD markedly reduced the free Spd and free Spm concentrations in both superior and inferior kernels.MD decreased the free Put concentration in inferior kernels,whereas SD increased the free Put level in both superior and inferior kernels at 18 and 30 DPA compared with the WW(Table 7).MD significantly increased the mean concentrations of free Spd and free Spm, whereas it decreased free Put concentration,in inferior kernels during kernel filling(Table 7).

Table 4–Proportion of starch granule volume in superior and inferior kernels of wheat cultivars YM16 and NM13 at maturity under various soil drying treatments in a pot experiment.

3.5.Activities of SSS and GBSS in kernels

The SSS and GBSS activities were much higher in superior than in inferior kernels at 6 and 18 DPA.Compared with WW, MD markedly increased the activities of SSS and GBSS in inferior kernels at 18 and 30 DPA,and exhibited no significant effect on those in superior kernels.SD significantly reduced activities of both enzymes in either superior or inferior kernels at 18 and 30 DPA(Fig.2).MD markedly increased the mean activities of SSS and GBSS in inferior kernels during kernel filling period(Fig.2).

3.6.Correlations of PA levels with kernel weight and granule size distribution The correlation of the mean free Put concentration with the contents of amylose,amylopectin,starch and kernel weight was insignificant(r=0.04 to 0.10,P＞0.05,Table 8).The mean concentrations of free Spd and free Spm and the ratio of Spd to Put(Spd/Put)or Spm to Put(Spm/Put)were significantly and positively correlated with the mean activities of SSS and GBSS during the kernel filling period and the contents of amylose,amylopectin,and starch and kernel weight at maturity(r=0.90**to 0.99**,P=0.01,Table 8).

PA levels were also very significantly and positively correlated with the proportions of the volume and surface area of medium granules(r=0.57**to 0.83**,P=0.01,Table 9), and very significantly and negatively correlated with the proportion of large granule size(r=?0.72**to?0.91**,P=0.01, Table 9).No significant correlation was observed between free Put concentration and starch granule size distribution (r=?0.25 to 0.29,P＞0.05,Table 9).

3.7.Effects of chemical application

To verify the roles of PAs in starch synthesis and the starch granule size distribution in wheat kernels,synthetic Spd, Spm,Put,or MGBG,which inhibits Spd and Spm synthesis by inhibiting S-adenosylmethionine decarboxylase(SAMDC), was applied to spikes during the early kernel filling period. As shown in Table 10,application of Spd or Spm significantly increased the mean concentration of free Spd or free Spm,and Spd/Put and Spm/Put,while application of Put increased the mean free Put concentration in inferior kernels.When MGBG, an inhibitor of Spd and Spm synthesis,was applied to spikes, the mean free Put was increased,whereas the mean free Spd and free Spm were reduced in inferior kernels(Table 10).The effects of MGBG on free PAs were eliminated by concomitant treatment with Spd(Table 10).Application of Spd or Spm significantly increased the activities of SSS or GBSS,kernel weight,starch content,and proportions of the volume and surface area of medium granules,whereas it markedly reduced the proportions of the volume and surface area of large granules,in inferior kernels(Table 10).There was no significant effect on the enzymatic activities and starch granule distribution when Put was applied.Application ofMGBG led to effects opposite to those in inferior kernels,in contrast to Spd or Spm application(Table 10).Except for MGBG,application of the chemicals showed no significant effect on free PAs in superior kernels(data not shown).

Table 5–Proportion of starch granule surface area in superior and inferior kernels of wheat cultivars YM16 and NM13 at maturity under various soil drying treatments in a pot experiment.

4.Discussion

The volume of A-type granules was significantly lower,whereas that of B-type granules was higher,in grains of middle spikelets than in those of upper and basal spikelets in wheat[37].The distribution of the starch granule volume in basal kernels and distal kernels on a wheat spike showed a three-peak and a two-peak curve,respectively[15].Superior kernels showed higher proportions of the volume and surface area of medium starch granules and smaller size of large granules than did inferior kernels(Tables 2–5,Fig.1).The correlation analysis showed that the contents of amylose,amylopectin,and starch and kernel weight were significantly correlated with the proportions of the volume and surface area of medium granules,and were highly significantly and negatively correlated with the proportion of large granule size(Table 6).The results indicate that variation in the volume and surface area of medium granules results,at least partly,in a difference in kernel weight between superior and inferior kernels,and suggests that increases in the volume and surface area of the medium granules increase kernel weight of inferior kernels in wheat.

Table 6–Correlations of the proportion of starch granule size distribution in wheat kernels with kernel weight and contents of amylose,amylopectin,and starch in kernels at maturity in a pot experiment.

Fig.2–Activities of soluble starch synthase(SSS,a–d)and granule-bound starch synthase(GBSS,e–h)in superior(S)and inferior(I)kernels of wheat cultivar YM16(a,c,e,g),and NM13(b,d,f,h)under various soil drying treatments in a pot experiment.WW,MD,and SD represent well-watered,moderate soil drying,and severe soil drying treatments respectively. Vertical bars represent±standard error of the mean(n=4)where these exceed the size of the symbol.The letters above the column indicate significance at the 0.05 probability level among the soil drying treatments within the same measurement date.

Table 8–Correlations of the mean concentrations of polyamines in wheat kernels with kernel weight and contents of amylose,amylopectin,and starch in kernels at maturity in a pot experiment.

How could the volume and surface area of the medium granules be increased in inferior kernels?We observed that MD treatment imposed during kernel filling increased the volume and surface area of the medium granules in inferior kernels(Tables 4 and 5,Fig.1),and consequently,increase contents of amylose,amylopectin and starch and kernel weight in these kernels(Table 1).In contrast,SD treatment imposed during kernel filling reduced the volume and surface area of the medium granules and accordingly,decreased kernel weight of inferior kernels.The results show clearly that an irrigation regime can affect the starch granule size distribution and kernel weight in wheat,especially for inferior kernels.MD treatment during kernel filling of wheat can increase kernel weight through increasing the volume of medium-sized starch granules in kernels.

The physiological mechanism by which MD could increase the volume and surface area of medium granules and decrease the formation of large granules in inferior kernels is yet to be determined.The present results showed that MD markedly increased the mean concentrations of free Spd and free Spm, Spd/Put and Spm/Put,and the mean SSS and GBSS activities in kernels during the kernel filling period(Table 7,Fig.2).The Spd and Spm concentrations were highly significantly correlated with the proportions of the volume and surface area of medium granules(Table 8),and significantly correlated with SSS and GBSS activities in kernels during kernel filling period(Table 9). When Spd or Spm was applied to spikes at the early kernel filling stage,the mean concentrations of free Spd and free Spm and their ratios to free Put,the SSS and GBSS activities,the proportions of volume and surface area of medium granules, contents of amylose,amylopectin and starch,and weight of inferior kernels were all significantly increased,while the large granule size in inferior kernels was dramatically decreased (Table 10).Application of MGBG led to completely opposite effects for inferior kernels(Table 10).These results suggest that the elevated level of free Spd and free Spm in inferior kernels by the MD plays a key role in increasing the volume of medium granules and inhibiting the formation of large granules by enhancing the activities of SSS and GBSS.

The mechanism by which PAs regulate the distribution of starch granules is not clear.Exogenous Spd and Spm significantly enhanced the activities of key enzymes involved in starch synthesis in inferior spikelets of rice[26].Both SSS and GBSS are believed to be the key enzymes in starch synthesis and their activities are closely associated with starch accumulation rate[3,44,45].The results imply that PAs,especially Spd and Spm,are involved in the formation of the starch granule size distribution by regulating the activities or gene expressions of the key enzymes in starch synthesis in wheat kernels.MD during grain filling enhances the biosynthesis of higher PAs (Spd and Spm),and thereby increases the volume and surface area of the medium granules by enhancing activities of the key enzymes involved in starch biosynthesis.

It is notable that neither PA levels nor starch content and kernel weight of superior kernels were markedly affected by MD treatment(Tables 1 and 7).Similar observations were also made by Chen et al.[28]who reported that post-anthesis soil drying treatment markedly affected PAs and ethylene levels and kernel weight of inferior spikelets of rice,but exerted little effect on those of superior spikelets.The mechanism involved is unclear.A plausible explanation is that earlier-flowering superior kernels dominate over later flowering inferior kernels in hormonal levels and kernel filling [28,46],and that MD treatment in this study could not markedly alter the PAs biosynthesis in superior kernels and, consequently,could not appreciably affect the distribution of starch granule size and kernel filling in wheat.Further research is needed to identify the mechanism by which superior and inferior grains in cereals respond differently to soil drying.

5.Conclusion

The volume of medium granules was much greater,whereas that of large granules was much smaller,in superior than in inferior wheat kernels.The MD imposed during kernel filling increased starch content in inferior kernels and kernel weight. The increase in kernel weight under MD was due mainly to the increase in the volume of medium granules and smaller size of large granules.PAs may mediate the effect of post-anthesis soil drying on starch biosynthesis in wheat kernels by regulating key enzymes in starch synthesis. Elevated PA levels under MD could increase the volume of medium granules and kernel weight of inferior kernels.

Acknowledgments

This work was supported by grants from the National Basic Research Program of China(No.2012CB114306),the National Natural Science Foundation of China (Nos.31461143015, 31271641,and 31471438),the National Key Technology R&D Program of China(Nos.2012BAD04B08 and 2014AA10A605),the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD),the Top Talent Supporting Program of Yangzhou University(No.2015-01),and Jiangsu Creation Program for Post-graduation Students(No.KYZZ15_0364).

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*Corresponding author.Tel.:+86 514 87324276;fax:+86 514 87979317.

E-mail address:jcyang@yzu.edu.cn(J.Yang).

Peer review under responsibility of Crop Science Society of China and Institute of Crop Science,CAAS.

http://dx.doi.org/10.1016/j.cj.2016.05.004

2214-5141?2016 Crop Science Society of China and Institute of Crop Science,CAAS.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Starch granule size distribution Soil drying

Triticum aestivum L.(wheat)