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Field Crops Research 122 (2011) 207–213

Contents lists available at ScienceDirect

Field Crops Research

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elationship between grain filling duration and leaf senescence of temperate ricender high temperature

unwhan Kima, Jiyoung Shona, Chung-Kuen Leea, Woonho Yangb, Youngwhan Yoona, Won-Ha Yanga,uon-Gyu Kima, Byun-Woo Leec,∗

National Institute of Crop Science, RDA, Suwon 441-857, Republic of KoreaRural Development Administration, Suwon 441-707, Republic of KoreaDepartment of Plant Science, Seoul National University, Seoul 151-921, Republic of Korea

r t i c l e i n f o

rticle history:eceived 10 September 2010eceived in revised form 22 March 2011ccepted 23 March 2011

eywords:icerain filling durationeaf senescenceemperature

a b s t r a c t

High temperature during grain filling period has been reported to decrease the grain filling duration,leading to the lower grain weight and yield of rice. Two experiments in the phytotron and field werecarried out to test the hypothesis that the leaf senescence of rice plants may determine the grain fillingduration under high temperature. In the phytotron experiment in 2008, rice plants of a japonica cultivar“Ilpumbyeo” were subjected to three minimum/maximum (mean) temperature regimes of 11/19 (15),17/25 (21), and 23/31 ◦C (27 ◦C). In the field experiment, rice seedlings of the same rice cultivar weretransplanted on May 6th and June 19th in 2009 and the mean temperatures during the grain fillingperiod were 24.4 and 21.9 ◦C, respectively. Both experiments revealed consistently that high temperatureincreased the rates of grain filling and leaf senescence while it reduced the durations of them. However,grain filling was terminated earlier than complete leaf senescence, the time gap being greater at higher

temperature. In addition, the fraction of dry matter partitioning to the leaf sheath + culm resumed toincrease following the termination of grain filling under high temperature, indicating that leaves werestill maintaining photosynthetic capacity and supplying assimilates into the other plant tissues exceptgrain even after the termination of grain filling. These findings suggest that an early termination of grainfilling in temperate rice under high temperature was not resulted from the lack of assimilate owing tothe early leaf senescence but from the loss of sink activity owing to the earlier senescence of panicle.

. Introduction

Supra-optimal high temperature during grain filling period isetrimental to grain yield and quality of crop and is expected toe one of the major limitations of crop yield and quality in theuture climate. The ongoing climate change in Korea is projectedo decrease rice yield and deteriorate apparent grain quality in theuture as the projected high temperature especially during grainlling period will reduce grain filling duration and grain weightYun, 1990; Shin and Lee, 1995; Chung et al., 2006). The optimumemperature for grain filling in japonica rice was reported to be inhe range of 21–22 ◦C in the average temperature during 40 daysfter heading and the high temperature above this optimum tem-erature to decrease the grain weight of rice (Murata, 1964; Kim,

983). Tashiro and Wardlaw (1991) also reported that the graineight of japonica rice cultivars was reduced obviously when dailyean temperature exceeded 26 ◦C during grain filling period. The

∗ Corresponding author. Tel.: +82 2 880 4544; fax: +82 2 873 2056.E-mail address: leebw@snu.ac.kr (B.-W. Lee).

378-4290/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.fcr.2011.03.014

© 2011 Elsevier B.V. All rights reserved.

decrease of grain weight due to high temperature has been ascribedto the shortening of grain filling duration (Nagato and Ebata, 1965;Kim, 1983; Tashiro and Wardlaw, 1989). In addition to grain yielddecrease, chalky rice was reported to increase as the daily meantemperature rose above 27 ◦C at the early grain filling stage of rice(Wakamatsu et al., 2007).

Grain filling would be terminated by the activity loss of sourceand/or sink. Sato and Inaba (1973) and Morita et al. (2004) stud-ied the effect of temperature on rice organs and found that underhigh temperature during grain filling the panicle contributed tothe grain weight decrease rather than the vegetative organs. On thebasis of these results, they suggested that the grain weight decreaseresulted from an early loss of sink activity under high temperatureduring grain filling period. Similarly, slower panicle senescence wasobserved to allow grains to accumulate more assimilates (Debataand Murty, 1982). Early loss of sink activity at high temperaturemay result from a reduction of translocation ability and/or a loss of

activity of starch synthesis-related enzymes. The reduction of cellsize on the dorsal side close to the vascular bundles was reportedto be closely related to the decreased grain filling duration of riceunder high temperature (Morita et al., 2005). The sucrose synthase

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2 Research 122 (2011) 207–213

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ctivity of rice grain has been observed to be positively corre-ated with grain sink strength and starch accumulation (Caunce andravois, 2006; Mohapatta et al., 2009; Tang et al., 2009). Chevaliernd Lingle (1983) also reported similar results in barley and wheathat the duration of sucrose synthase activity in the endosperm wasmportant in determining the duration of grain filling.

Many evidences have been reported also on the positive con-ribution of the delayed leaf senescence, i.e., the lengthening ofource activity to grain filling and yield of crop. Delayed senes-ence of upper leaves was reported to be positively correlated withrain yield of rice varieties (Park and Lee, 2003). And Fu et al.2009) reported that a functional stay-green rice SNU-SG1 exhib-ted higher crop growth rate during the late grain filling period thanigh yielding varieties, resulting in higher grain filling percentagend non-structural carbohydrate re-accumulation in the culm athe final stage of grain filling. Several reports are available regard-ng the association of leaf senescence to grain filling duration. Seot al. (1981) reported that varieties with the characteristics of rapideaf senescence exhibited rapid panicle senescence in rice. The earlyenescence of flag leaf due to ozone-exposure negatively affectedhe grain filling duration and ultimately reduced grain yield inheat (Gelang et al., 2000). Similarly, Yang et al. (2001) observed

hat water stress or exogenous ABA treatment during the grain fill-ng period increased the remobilization of pre-stored carbohydrateo the grains, accelerated the grain filling rate, and shortened therain filling period.

At present, much information has been accumulated on thessociation of grain filling duration with not only leaf senescenceut also sink activity. However, it is still obscure which organ isesponsible for the shortening of grain filling duration of rice underigh temperature. In the present study leaf senescence and grainlling duration was observed in the phytotron and field conditionso clarify which organ determines the grain filling duration underigh temperature during grain filling period.

. Materials and methods

.1. Phytotron experiment

.1.1. Plant material and growth conditionFor exposing rice plants to different growth temperature

egimes during the grain filling periods, a pot experiment was con-ucted in the phytotron of National Institute of Crop Science (NICS),ural Development Administration, South Korea in 2008. A japon-

ca rice cultivar “Ilpumbyeo” was transplanted with 30-day oldeedlings in a 1/5000 Wagner pot filled with paddy soil and grownnder outdoor conditions with the fertilization rates of 1.0 g N, 0.5 g2O5, and 0.5 g K2O per pot. Panicles headed on the same day wereagged in order to take samples during the grain filling period. Atull heading stage, pots were transferred to the three phytotronooms controlled sinusoidally to the minimum/maximum (mean)emperature regimes of 11/19 (15), 17/25 (21), and 23/31 ◦C (27 ◦C)nder natural daylight condition.

.1.2. Sampling and chemical analysisFifteen plants headed on the same day were sampled 9 times at

n interval of 5–7 days from full heading to physiological matu-ity. Sampled plants were separated into panicles, leaves, andulms including leaf sheaths. All the samples were immediatelyrozen with liquid nitrogen, lyophilized, weighed, and powderedor chemical analysis. Leaf chlorophyll concentration was analyzed

ccording to the method of Hiscox and Israelstam (1979) using 80%thanol. The nitrogen concentration of the leaf sample was deter-ined using a CN elementary analyzer (Vario Max CN, Germany).t physiological maturity, another ten panicles were harvested to

Fig. 1. Daily mean temperature (A) and solar radiation (B) during the grain fillingperiod in field experiment. Early and late indicate the grain filling periods at thetransplanting on May 6th and June 19th 2009, respectively.

determine grain weight and ripening ratio. Panicles were hand-threshed and then husked. The husked grains were separated intomature and immature grains using a 1.6 mm sieve for measuringripening ratio.

2.2. Field experiment

2.2.1. Plant material and experiment siteTo subject rice plants to different temperature and solar radi-

ation conditions during their grain filling period, a transplantingdate experiment was conducted at the experimental field of NICS(37◦16′N, 126◦59′E) in 2009. Thirty-day old seedlings of a ricecultivar “Ilpumbyeo” were manually transplanted on May 6th(early-season planting) and June 19th (late-season planting) at ahill spacing of 0.14 m × 0.30 m with 3 seedlings per hill. Fertil-izer was applied at the rates of 90 kg N, 60 kg P2O5, and 65 kg K2Oper hectare. Experimental plots were laid out in a randomizedblock design with 3 replicates. The mean temperature and cumula-tive solar radiation during 30 days after full heading was 24.4 ◦Cand 474.3 MJ m−2 in the early-season planting, and 21.9 ◦C and499.0 MJ m−2 in the late-season planting, respectively (Fig. 1).

2.2.2. Sampling and chemical analysis

Five hills from each plot were sampled 7–8 times every 5–10

days from full heading to physiological maturity. Sampled plantswere separated into panicles, leaves, culms, and dead leaves, oven-dried at 70 ◦C for 7 days, and weighed. At physiological maturity,

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J. Kim et al. / Field Crops Research 122 (2011) 207–213 209

Table 1Coefficients of a logistic equation (Eq. (1)) describing the grain filling progress as a function of temperature during the grain filling period.

Temperature (◦C) A r tm R2 Probability ofregression

15 0.48 (0.04) 0.054 (0.01) 29.7 (4.4) 0.984 <0.000121 0.48 (0.02) 0.127 (0.02) 16.3 (1.2) 0.988 <0.0001

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anicles were counted and then hand-threshed. The filled grainsere separated by floating the threshed grains in tap water. Ripen-

ng ratio was calculated by the ratio of the submersed (filled) grainumber to the total grain number. Each plant organ was ground

or nitrogen analysis after weighing. The nitrogen concentration ofeaves was determined using a CN elementary analyzer (Vario MaxN, Germany).

.3. Data analysis

Because the fraction of dry matter partitioning to the panicleuring the grain filling period is an indirect indicator of grain fill-

ng progress, it was used to explain the grain filling progress aftereading for simplicity rather than grain weight. The fraction of dryatter partitioning to the panicle was calculated as the ratio of the

anicle dry weight to the total shoot dry weight (Goudriaan andan Laar, 1994). The following logistic equation (Verhulst, 1838)as used to fit the grain filling progress in relation to time after fulleading:

= A

[1 + exp{−(t − tm)r}] (1)

here F is the fraction of dry matter partitioning to the panicle, A ishe final fraction of partitioning to the panicle, r is growth rate, t ishe time (in days) after heading, and tm is the time when the fractioneaches the half of A (Yin et al., 2003) and daily grain filling rate ist maximum. A, r, and tm are the constants that are determined byon-linear regression analysis. From the logistic equation, the grainrowth duration was determined as the period from heading to theime reaching 95% of the maximum partitioning fraction (A).

The changes in leaf chlorophyll content and leaf nitrogen con-entration following full heading were fitted to the modifiedogistic equation (Black and Leff, 1983):

s = Nmin + (Nmax − Nmin)

1 + (t/B)Nr(2)

here Ns is the leaf nitrogen/chlorophyll concentration at the time, t is time in days after heading, r is the rate that determines theurvature of the equation pattern, Nmax is the maximum value ofs, Nmin is the minimum value of Ns, and B is the time when Ns

eaches the half of Nmax − Nmin. Nmin, Nmax, B, and Nr were deter-ined by non-linear regression analysis. Sigmaplot 11.2 was used

or non-linear regression analysis, and SPSS19 for one-way analysisf variance and Duncan’s multiple range test.

. Results

.1. Phytotron experiment

As in Table 1, the ordinary logistic function (Eq. (1)) fitted therain filling data very well enough to compare the grain fillingttributes such as the rate of grain filling, the duration of grain fill-

ng, and the final partitioning rate to grain under three temperatureegimes. Grain filling rate (r) increased with the rise of temperature,he fraction of dry matter partitioning to panicle exhibiting steeperncrease with time after heading at higher temperature (Fig. 2A). On

10.1 (0.8) 0.982 <0.0001

the contrary, the duration of grain filling was reduced substantiallywith the rise of temperature during the grain filling period (Table 2).At the temperature of 27 ◦C grain weight was reduced significantlyas compared to that at the temperature of 21 ◦C because the sub-stantially reduced grain filling duration could not compensate theincreased rate of grain filling at higher temperature.

The fraction of dry matter partitioning to culm + leaf sheathdecreased after heading and more rapidly at higher temperature,indicating that the major sink was changed from culm + leaf sheathto panicle following heading and the speed of assimilate remobi-lization increased with temperature rise. However, the fraction ofdry matter partitioning to culm + leaf sheath stopped decreasingand rather resumed to increase after grain filling termination atthe temperature regimes of 21 and 27 ◦C (Fig. 2C), suggesting thatthe leaves could photosynthesize and accumulate the assimilate inculm + leaf sheath even after the termination of grain filling period.At low temperature of 15 ◦C the fraction of dry matter partition-ing to culm + leaf sheath did not exhibit the resumption of increaseafter grain filling termination possibly because of the reduced rateof photosynthesis at low temperature.

The decreases in the fraction of dry matter partitioning to leafand the leaf chlorophyll content are important indicators of leafsenescence. As seen in Fig. 2B and D, the fraction of dry matter par-titioning to leaf and the leaf chlorophyll content started to declinerapidly just after heading and about 10–15 days after heading,respectively. In order to compare the leaf senescence attributesamong temperature regimes the data of chlorophyll content afterheading was fitted to a modified logistic equation (Eq. (2)). As inTable 3 the modified logistic equation described the decreasingpattern of leaf chlorophyll content, i.e., the leaf senescence pat-terns very well enough to derive the leaf-senescence attributes forthe comparison among temperature regimes. Leaf senescence rate(Nr) that depicts both the pattern and the rate of decrease in leafchlorophyll content increased curve-linearly with the increase oftemperature following heading. The leaf chlorophyll content exhib-ited steeper decrease at higher temperature (Fig. 2D). Thus, thedurations to the complete leaf senescence that was calculated asthe period from heading to the time declining to 5% of the maxi-mum chlorophyll content increased with the rise of temperatureand were estimated as 71, 53, and 41 days at 15, 21, and 27 ◦C,respectively. As indicated in Fig. 2A, grain filling processes seemedto be terminated earlier than the completion of leaf senescence andthe time gap between grain filling termination and leaf senescencecompletion became greater at higher temperature.

3.2. Field experiment

A transplanting date experiment was conducted to confirm theresults of the phytotron experiment in the field condition. Eventhough temperature and solar radiation conditions were differentbetween early-season planting on May 6th and late-season planting

on June 19th, there were no significant differences in grain yieldand all the yield components. However, LAI at heading stage wasgreater at early-season planting compared to LAI at the late-seasonplanting (Table 4).

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Fig. 2. Changes in fractional dry matter partitioning to panicle (A), leaf (B), and culm + leaf sheath (C) and the chlorophyll content (D) with time after full heading of riceexposed to three temperature regimes during the grain filling period in the phytotron experiment. The curves in graphs A and D were fitted to the logistic equations as inTables 1 and 3, respectively. Vertical lines indicate the terminal date (95% of the final fraction of dry matter partitioning to panicle) of grain filling.

Table 2Grain filling duration, ripening ratio and grain weight under three temperature regimes during grain filling period.

Temperature (◦C) Grain filling duration* (day) Ripening ratio (%) Grain weight (mg grain−1)

15 84 88.7a** 24.6b21 39 91.5a 26.4a27 25 85.9a 23.7b

*Grain filling duration was determined from the logistic equations in Table 1 as the days from heading to the time that the fractional dry matter partitioning to panicle (F)reaches 95% of the maximum partitioning fraction (A).**The same letters in a column are not significantly different at probability < 0.05 by Duncan’s multiple range test.

Table 3Coefficients of a logistic equation (Eq. (2)) describing the decrease of leaf chlorophyll content during the grain filling period.

Temperature (◦C) Nmin Nmax B Nr R2 Probability ofregression

15 −7.9 (5.7) 27.5 (2.5) 33.7 (5.9) 1.38 (0.37) 0.996 <0.000121 −0.0 (2.5) 24.9 (1.4) 25.3 (1.8) 3.97 (1.15) 0.980 0.000127 2.2 (1.8) 24.6 (1.1) 22.5 (1.3) 4.88 (1.34) 0.981 <0.0001

Values in parentheses are standard errors of parameters.

Table 4Mean temperature during growing season, yield and yield components at harvest, and LAI at full heading at two transplanting dates in the field experiment.

Transplantingdate

Mean temperature (◦C) Spikelets (No.m−2)

Ripening ratio(%)

Grain weight(mg grain−1)

Grain yield(t ha−1)

LAI

Rice growingseason

Grain fillingperiod

May 6th 22.7 24.4 36,386 ± 946a 91.4 ± 0.30 23.9 ± 0.83 7.48 ± 0.49 5.2 ± 0.04June 19th 23.4 21.9 35,584 ± 935 90.2 ± 0.26 24.0 ± 0.92 7.14 ± 0.32 4.3 ± 0.31

a Standard error (n = 3).

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Table 5Coefficients of a logistic equation (Eq. (1)) to describe the grain filling progress following heading in the field experiment.

Transplanting date A r tm R2 Probability ofregression

May 6th 0.52 (0.009) 0.174 (0.014) 10.3 (0.5) 0.997 <0.0001

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19th June 0.56 (0.007) 0.148 (0.009)

alues in parentheses are standard errors of parameters.

As in Table 5, the grain filling data were fitted to the ordinaryogistic function (Eq. (1)) very well enough to compare the grainlling attributes. The dates of grain filling termination that wasetermined as the date reaching 95% of the final fraction of parti-ioning to panicle were estimated as 25 and 39 days after heading athe early- and late-season planting, respectively (Fig. 3A). The frac-ional dry matter partitioning to culm + leaf sheath rapidly declinedntil the termination date of grain filling. Thereafter, it slightly

ncreased in the early-season planting while it slightly decreasedn the late-season planting (Fig. 3C). As shown in Fig. 1 and Table 3,he mean air temperature during grain filling period was 24.4 and1.9 ◦C in the early- and late-season planting, respectively. Theartitioning pattern in the early-season planting was similar tohose at the temperature regimes of 21 and 27 ◦C in the phytotronxperiment. However, it was different from those at the tempera-ure regimes of 21 and 27 ◦C in the phytotron experiment in theate-season planting that the mean air temperature was withinhis temperature but the daily mean temperature continuously

ropped after the termination of grain filling. This temperaturerop would have depressed photosynthesis, leading to the decreas-

ng fractional dry matter partitioning to culm + leaf sheath evenfter the termination date of grain filling.

ig. 3. Changes in the fraction of dry matter partitioning to panicle (A), leaf (B), and culm +n the field experiment. The curves in graphs A and D were fitted to the logistic equationshe final fraction of dry matter partitioning to panicle) of grain filling.

12.0 (0.4) 0.998 <0.0001

On the contrary to the phytotron experiment, leaf nitrogen con-centration was used to evaluate the leaf senescence instead ofleaf chlorophyll content as they showed good linear relationshipas in Fig. 4. The leaf nitrogen concentration at the full heading inthe late-season planting was higher than that in the early-seasonplanting. Similar to the pattern of leaf senescence in the phytotronexperiment, the fractional partitioning to leaf and leaf nitrogenconcentration declined rapidly following full heading and from10 to 15 days after full heading, respectively (Fig. 3B and D). Thestart of leaf senescence as represented by the decline of leaf nitro-gen concentration coincided with tm, the time that the fractionreaches the half of the final fraction of partitioning to panicle.The values of tm were estimated as 10.3 and 12.0 days in early-and late-season planting, respectively (Table 5). However, the leafsenescence rates (Nr) in the field experiment showed differentresponses to temperature from those in the phytotron experiment.That is, Nr in the late-season planting was greater than that inthe early-season planting even though the daily mean temperature

during grain filling was higher in the early-season planting (Fig. 1,Table 4). In both planting seasons, leaf senescence continued evenafter the termination of grain filling similarly as in the phytotronexperiment.

leaf sheath (C) and the nitrogen concentration of leaves (D) with time after headingas in Tables 5 and 6, respectively. Vertical lines indicate the terminal date (95% of

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212 J. Kim et al. / Field Crops Research 122 (2011) 207–213

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ig. 4. Relationship between leaf chlorophyll content and nitrogen concentrationuring grain filling period of rice grown in phytotron.

. Discussion

Leaf senescence was initiated from 10 to 15 days after fulleading under the temperature regimes of 21 and 27 ◦C in the phy-otron experiments (Fig. 2) and under two planting seasons in theeld experiment that showed mean air temperature of 21.9 and4.4 ◦C (Fig. 3), coinciding with the time of half grain filling (tm)hat showed the maximum daily grain filling rate (Tables 1 and 5).

urchie et al. (2002) also reported the similar observation that aapid decline of protein content in flag leaves began around the timef the rapid grain filling phase falling on 10–13 days after heading.urthermore, leaf senescence did not proceed or was delayed whenhe panicles were removed (Khan and Choudhuri, 1992; Nakanot al., 1995). These results imply that the rapid grain filling is asso-iated with the initiation of leaf senescence.

High temperature accelerated the rate of grain filling and short-ned the duration of grain filling in both phytotron and fieldxperiments (Tables 1, 2, and 5). Many corroborating results haveeen reported in the previous studies (Nagato and Ebata, 1965;im, 1983; Tashiro and Wardlaw, 1989). However, high temper-tures did not increase the leaf senescence rate as much as therain filling rate in the phytotron experiment (Fig. 5). Leaf senes-ence was not accelerated even by higher temperature in the fieldxperiment (Table 6). The leaf senescence rate in the late-seasonlanting was greater than in the early-season planting even thoughhe daily mean temperature during grain filling period was highern the early-season (Fig. 3, Table 6). These contrasting results sug-est that there might be another factor except for temperature thatontrolled the leaf senescence rate during grain filling period inhe field experiment. Generally, a large sink size has been reportedo accelerate leaf senescence (Wada et al., 1993; Wada and Wada,991). The sink/source ratio as evaluated by the ratio of spikelet

umber to leaf area index at heading was higher in the late-seasonlanting than in the early-season planting (Table 4), suggesting thatot only the sink/source ratio might have exerted a greater impactn leaf senescence than temperature within the range of 21–24 ◦C

able 6oefficients of a logistic equation (Eq. (2)) describing the decrease of leaf nitrogen concen

Transplanting date Nmin Nmax B

May 6th 1.08 (0.18) 2.25 (0.08) 29.0 (June 19th 1.14 (0.14) 2.58 (0.08) 23.6 (

alues in parentheses are standard errors of parameters.

Fig. 5. Rates of leaf senescence and grain filling as a function of temperature duringgrain filling period in the phytoron experiment.

in the field experiment but also leaf senescence is less sensitiveto temperature than grain filling. This difference in the sensitivityto temperature may be ascribable to the temperature differencebetween leaf and panicle. According to the studies on temperatureof rice plant, leaf temperature was lower than the air tempera-ture in the daytime because of transpiration cooling (Takai et al.,2010; Yan et al., 2008), and panicle temperature is higher than leaftemperature (Yan et al., 2008).

Grain filling was terminated earlier than leaf senescence at thetemperature above 21 ◦C in the phytotron experiment and also atboth planting seasons of field experiment with the mean temper-ature of above 21 ◦C during grain filling period (Figs. 2 and 3). Theleaf senescence continued even after the grain filling termination.And the fraction of dry matter partitioning to culm + leaf sheathresumed to increase after the grain filling termination similarlyas reported by Kobata and Uemuki (2004), Okawa et al. (2003),and Fu et al. (2009), suggesting that leaves were still maintainingphotosynthetic capacity and supplying assimilates into the otherplant tissues except grain even after the termination of grain filling.Therefore, the lack of assimilate supply owing to leaf senescence isnot a cause to terminate the grain filling process. Consequently,the shorter duration of grain filling at higher temperature is deter-mined by the earlier loss of sink activity rather than the earlier lossof source activity. Early loss of sink activity at high temperature mayresult from a reduction of translocation ability and/or a loss of activ-ity of starch synthesis-related enzymes in the grain. Dehydrogenaseactivity disappears earlier in the endosperm than in the vascularbundle of rachilla and pedicel (Seo et al., 1981). This result impliesthat the sink activity may be related to starch synthesis-relatedenzyme activity rather than the senescence of vascular bundle ofrachilla and pedicel. The sucrose synthase activity of rice grain hasbeen observed to be positively correlated with grain sink strengthand starch accumulation (Caunce and Gravois, 2006; Mohapattaet al., 2009; Tang et al., 2009). Chevalier and Lingle (1983) also

reported similar results in barley and wheat that the durationof sucrose synthase activity in the endosperm was important indetermining the duration of grain filling. Further investigation onstarch synthesis-related enzymes in rice grain would be required

tration following heading in the field experiment.

Nr R2 Probability ofregression

4.2) 2.83 (0.91) 0.973 0.00142.3) 3.22 (0.89) 0.986 0.0004

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J. Kim et al. / Field Crops

o overcome the high temperature-induced reduction of grain fill-ng duration that leads to the lower grain weight and yield of rice.ven though the role of leaves as a source does not affect grain fill-ng duration, it can affect the grain filling rate. The increased grainlling rate could not compensate the reduced grain filling dura-ion at high temperature, leading to low grain weight (Table 2) ashe assimilate supply rate failed to meet the daily requirementsor grain growth under high temperature conditions (Kobata andemuki, 2004).

. Conclusion

High temperature increased the rates of grain filling and leafenescence while it reduced the durations of grain filling and leafenescence. Grain filling was terminated earlier than the completeeaf senescence and the fraction of dry matter partitioning to theeaf sheath + culm resumed to increase following the terminationf grain filling under high temperature, indicating that leaves weretill maintaining photosynthetic capacity and supplying assimilatesnto the other plant tissues except grain even after the terminationf grain filling.

It could be concluded from these findings that an early termina-ion of grain filling in temperate rice under high temperature wasot resulted from the lack of assimilate owing to the earlier leafenescence but from the loss of sink activity owing to the earlierenescence of panicle.

cknowledgements

A part of this research was supported by Technology Devel-pment Program for Agriculture and Forestry, Ministry for food,griculture, Forestry and Fisheries, Republic of Korea and Agri-ultural Science & Technology Development Program (ATIS no.J0065072010), Rural Development Administration, Republic oforea.

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