Agronomy Journal 94:102-109 (2002)
© 2002 American Society of Agronomy
RICE
Carbon Remobilization and Grain Filling in Japonica/Indica Hybrid Rice Subjected to Postanthesis Water Deficits
Jianchang Yanga,
Jianhua Zhang*,b,
Lijun Liua,
Zhiqing Wanga and
Qingsen Zhua
a College of Agric., Yangzhou Univ., Yangzhou, Jiangsu, China
b Dep. of Biol., Hong Kong Baptist Univ., Kowloon Tong, Hong Kong, China
* Corresponding author (jzhang{at}hkbu.edu.hk)
Received for publication January 2, 2001.
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ABSTRACT
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Poor grain filling, as in unfilled grains, and unused carbohydrate in straws are two major problems in japonica/indica (J/I) hybrid rice (Oryza sativa L.). This study investigated if an early senescence, induced by controlled postanthesis soil drying, could enhance the remobilization of reserved C to the grains, and therefore improve the grain filling. Two J/I hybrids and one indica/indica (I/I) hybrid were field-grown. Three levels of soil water potential (
soil)—well watered (soil = 0), moderate water deficit (soil = -0.025 MPa), and severe water deficit (soil = -0.05 MPa)—were imposed 9 d after anthesis to maturity. As a result of water deficit treatment, chlorophyll concentrations and photosynthetic rates of flag leaves declined more quickly, indicating an enhanced senescence. The remobilized C reserve from the culm and sheath during grain filling was increased by 47 to 61% for the J/I hybrids and 12 to 26% for the I/I hybrid. Partitioning of 14C from the flag leaves to the grains was increased by 18 to 28%. Active grain-filling period was shortened by 2.7 to 8.7 d, and grain-filling rate increased by 0.14 to 0.36 mg d-1 grain-1. Water deficits reduced the grain yield of the I/I hybrid by 2.9 to 16.4% but increased that of the two J/I hybrids by 4.4 to 13.3%. We conclude that if water deficit is properly controlled for J/I, the gain from enhanced remobilization of stored C may outweigh the loss due to shortened photosynthetic duration, leading to a fast grain filling and high grain yield in cases where senescence is unfavorably delayed.
Abbreviations: Chl, chlorophyll • DAA, days after anthesis • I/I, indica/indica • J/I, japonica/indica • MD, moderate water deficit • NSC, nonstructural carbohydrates • PR, photosynthetic rate • SD, severe water deficit • WW, well watered • soil, soil water potential
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INTRODUCTION
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THE HYBRID of japonica/indica (J/I) rice possesses a greater heterosis in biomass production than other hybrid rice varieties (Yang and Zhao, 1959; Ceng et al., 1980; Yuan, 1994; Peng et al., 1999). It has been predicated that J/I hybrids may have a 30% yield advantage over the best existing indica/indica (I/I) hybrids (Yuan, 1994). Too many poorly filled grains, however, hinder the exploitation of such heterosis (Yuan, 1994, 1998) and have been attributed to many factors (e.g., Lu et al., 1994; Yuan, 1994, 1998; Peng et al., 1999; Yang et al., 2000a). Slow grain filling, resulting from delayed senescence (the plants remain green when the grains are due to ripe or plants are too vigorous and keep green for too long), is considered the main cause for the poor grain filling (Zhuang et al., 1994; Yuan, 1997; Zhu et al., 1997; Yang et al., 1997, 1999; Wang et al., 1998).
Plant senescence in monocarpic plants such as rice is the final stage in growth and development (Okatan et al., 1981; Nooden, 1988; Gan and Amasino, 1997). It is an active, ordered process that involves remobilization of stored food from vegetative tissues to grains (Nooden et al., 1997; Ori et al., 1999). Delayed senescence results in much nonstructural carbohydrate left in the straw and leads to a low harvest index. Extensive studies have demonstrated that water deficits result in an early senescence in annual plants (Gallagher et al., 1976; Johnson and Moss, 1976; Jones and Rawson, 1979; Austin et al., 1980; Lauer and Simmons, 1985; Nicolas et al., 1985a; Kobata et al., 1992; Palta et al., 1994; Zhang et al., 1998; Gebbing and Schnyder, 1999). Early senescence caused by water deficits, however, reduces photosynthesis, shortens the grain-filling period, and finally results in the reduction of grain weight (Bidinger et al., 1977; Nicolas et al., 1985b; Brown et al., 1991; Palta et al., 1994; Zhang et al., 1998). Our earlier work (Yang et al., 2000b, 2001) has shown that remobilization of prestored C reserves in wheat (Triticum aestivum L.) is promoted by water stress. Our results demonstrated that water deficits imposed during grain filling enhanced plant senescence, accelerated grain filling, and improved yield in cases where senescence in wheat is unfavorably delayed by heavy use of N.
This research was designed to test the hypothesis that if water deficit during the grain-filling period is controlled properly, the early senescence induced by water deficit would promote the reallocation of assimilates and grain filling would gain from an enhanced remobilization of C reserves. This gain may outweigh the possible loss of photosynthesis due to a shortened period. This practice may improve the grain filling in J/I hybrid rice where slow grain filling is a problem.
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MATERIALS AND METHODS
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Plant Materials
The experiment was conducted at a farm of Yangzhou University, Jiangsu Province, China (32°30' N, 119°25' E) during the rice growing season (May to October) of 1998 and was repeated in 1999. Two J/I hybrids, Ce 03 (japonica with wide compatibility genes)/Yangdao 4 (indica) and PC 311 (japonica with wide compatibility genes)/Zaoxiandang 18 (indica), were grown in a paddy field. An I/I hybrid, Shanyou 63 (Zhenshan 97 A/Minghui 63), was used as a control. The three hybrids are currently used in local production because of their relatively better grain quality and pest resistance compared with other hybrids. The normal days from sowing to maturity for the three hybrids are 150 to 152 d. Total leaves on the main stem are 17 (VN = 17) for each hybrid. Seeds (first generation) of the hybrids were harvested in the previous year. Seeds were sowed on 10 and 11 May and seedlings from these seeds were raised in the field and then transplanted on 10 and 11 June at a hill spacing of 0.20 by 0.16 m, with one seedling per hill. The soil was sandy loam [Typic fluvaquents, Etisols (U.S. taxonomy)] with 24.5 g kg-1 organic matter and 105, 33.5, and 66.0 mg kg-1 available N, P, and K, respectively. Nitrogen {60 kg ha-1 as urea [(NH2)2CO]}, P (30 kg ha-1 as single superphosphate), and K (40 kg ha-1 as KCl) were applied and incorporated before transplanting. Nitrogen as urea was also applied at midtillering (40 kg ha-1) and at panicle initiation (15 kg ha-1). All of the hybrids headed on 17 to 19 August (50% of heading) and were harvested on 6 to 8 October. Except for drainage at the end of tillering (V12, refer to Counce et al., 2000), the field was kept at a 1- to 2-cm water level until 9 d after anthesis (DAA) when water deficit treatments were initiated. Air temperatures, averaged every 10 DAA (19–21 August) to harvest, were 26.7, 26.4, 25.1, 24.1, and 22.8°C, respectively. The minimum temperature averaged 24 h during grain filling was 19.6°C (4 Oct. 1999).
Water Deficit Treatments
The experiment was a 3 x 3 (three hybrids and three levels of soil moisture) factorial design with nine treatment combinations. Each of the treatments had three plots as repetitions in a complete randomized block design. Plot dimension was 4 by 3.2 m, and plots were separated by a ridge (40 cm wide) wrapped with plastic film. From 9 DAA to maturity, three levels of soil water potential (soil) were imposed on the plants by controlling water application. The well-watered (WW) treatment was kept at a 1- to 2-cm water depth (soil = 0) in the field by manually applying tap water every day. A moderate water deficit (MD) was maintained at -0.025 MPa, and a severe water deficit (SD) was maintained at -0.05 MPa. The soil in water deficit treatments was monitored with tension meters buried at a 15- to 20-cm soil depth. Five tension meters were installed in each plot. Tension meter readings were recorded twice a day at 1000 h and 1600 h. When the readings dropped to the designed value, 120 and 60 L of tap water per plot were added manually to the MD and SD treatments, respectively. A rain shelter consisting of a steel-frame covered with plastic sheet protected the plots during rains. The rain hours during the treatment period were about 11 in 1998 and 14 in 1999.
Radioactive Labeling
At heading (R3, refer to Counce et al., 2000), flag leaves from six main stems in each treatment were labeled with 14CO2, between 0900 and 1100 h on a clear day, with photosynthetically active radiation at the top of the canopy ranging between 1000 and 1100 µmol m-2 s-1. The whole flag leaf was placed into a polyethylene chamber (25-cm length and 4-cm diam.) and sealed with tape and plasticine to maintain a gas-tight seal. Six milliliters of air in the chamber was drawn out, and the same volume of gas was injected into the chamber, which contained 10 mmol L-1 CO2 at a specific 14C radioactivity of 1.48 MBq L-1. The chamber was removed after 30 min.
The labeled plants were sampled at maturity. Each plant was divided into leaf blades, culms plus sheaths, and panicles (grains + branches and rachis). Samples were dried at 80°C to constant weight, ground into powder, and then extracted by shaking for 30 min in 630 g L-1 [80% (v/v)] boiling ethanol (C2H5OH). The exaction was repeated three times. The radioactivity of 14C in the extracted aliquots was counted by a liquid scintillation counter (Beckman Instruments, Fullerton, CA). Radioactivity distribution in each part of the plant was expressed as a percentage of total radioactivity remaining in the aboveground portion of the plant.
Physiological Measurements
Leaf water potentials of flag leaves were measured on clear days at predawn (0600 h) and midday (1130 h) on 0, 2, 5, 8, 12, 15, 19, 25, and 28 d after withholding water. Well-illuminated flag leaves were chosen randomly for measurements. A pressure chamber (Model 3000, Soil Moisture Equipment Corp., Santa Barbara, CA) was used for measuring leaf water potential, with six leaves for each treatment.
The photosynthetic rate (PR) and chlorophyll (Chl) content of the flag leaves were also measured on the same dates that leaf water potentials were measured. A gas-exchange analyzer (CID-PS CO2 Analyzer System, CID, Vancouver, WA) was used to measure PR. Measurements were made during 0900 and 1100 h when photosynthetically active radiation above the canopy was 1000 to 1100 µmol m-2 s-1. Flag leaves were sampled for measurement of Chl content. Chlorophyll was extracted by shaking in methanol (CH3OH) overnight and determined as described by Holden (1976). Six leaves were used for each treatment.
Sampling and Harvesting
Two hundred panicles that headed on the same day were tagged for each treatment. The flowering date and the position of each spikelet on the tagged panicles were recorded. Twenty tagged panicles from each treatment were sampled every 4 d from anthesis to maturity. The sampled panicles were divided into two groups (10 panicles each) as subsamples. Grains that developed from spikelets that flowered on the same day were removed, dried at 70°C to constant weight for 72 h, and weighed. The grain-filling process was fitted by Richards' (1959) growth equation as described by Zhu et al. (1988):
where W is the grain weight (mg); A is the final grain weight (mg); t is the time after anthesis (d); and B, k, and N are coefficients determined by regression. The active grain-filling period was defined as the days when W was from 5% (t1) to 95% (t2) of A. An average grain-filling rate during this period was therefore calculated from t1 to t2.
Total aboveground biomass was measured at both anthesis and grain maturity. At each harvest, 40 to 50 plants were sampled from each treatment and separated into leaf blades, culms and sheaths, and panicles. All plant parts were dried at 80°C to constant weight and weighed.
Fifteen to 20 plants were sampled from each treatment at 4-d intervals from anthesis to maturity to measure nonstructural carbohydrates (NSC) in culms, sheaths, and leaves. The method for extracting NSC was modified according to that described by Yoshida et al. (1976). The sample was dried in an oven and ground into fine powder. In a 15-mL centrifuge tube, 100 mg of ground sample was added with 10 mL of 80% ethanol (v/v) (density at 630 g L-1) and kept in a water bath at 80°C for 30 min. After cooling in cool water, the tube was then centrifuged at 2000 rpm for 20 min. The supernatant was collected, and the extraction was repeated three times. The alcohol in the supernatant was evaporated on a water bath at 80°C until most of the alcohol was removed and the volume was reduced to about 3 mL. The sugar extract was diluted into 25 mL with distilled water. The concentration of sugars in the extract was analyzed as described by Somogyi (1945).
The residue left over after extracting sugars in the centrifuge tube was dried at 80°C for starch extraction. Two milliliters of distilled water was added to the tube containing the dried residue. The tube was then shaken in a boiling water bath for 25 min. Two milliliters of 9.36 M perchloric acid (HClO4) was added to the tube after cooling in cool water. The solution was shaken further for 15 min. The extract was then made up to about 10 mL and centrifuged at 2000 rpm for 20 min. The supernatant was collected and a further 2 mL of 4.68 M HClO4 was added to the residue. The extraction was repeated as above. The supernatants were combined and made up to 50 mL with distilled water. The starch was analyzed by the method of Pucher et al. (1948).
Plants from a 2-m2 site (excluding border plants) in each plot were harvested at maturity for the determination of grain yield. Yield components, i.e., panicles per square meter, spikelets per panicle, the percentage of ripened grains, and grain weight, were determined from 50 plants sampled randomly from each plot. The percentage of ripened grains was defined as the grains that sink in salt water (specific gravity = 1.06) as a percentage of total spikelets.
The results were analyzed for variance using ANOVA. Data from each sampling date were analyzed separately. Means were tested by least significant difference at P = 0.05 (LSD0.05). The results of both years were very similar; therefore, data were averaged from both years.
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RESULTS
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Leaf Water Potential
Figure 1
illustrates the changes in leaf water potential during the first 28 d after withholding water. The leaf water potential at midday exhibited a gradual decrease during grain filling for all of the hybrids within the three treatments. The lower the soil was, the faster the decline of leaf water potential. The two J/I hybrids exhibited a similar trend of leaf water potential at both predawn and midday, but the midday leaf water potential of the I/I hybrid decreased faster than that of the two J/I hybrids even though soil was kept at the same level. The two J/I hybrids had similar predawn leaf water potential for the three water treatments, indicating that plants of the two J/I hybrids could rehydrate overnight. For the I/I hybrid, predawn leaf water potential of WW treatment was slightly different from that of MD treatment but significantly higher than that of the SD by the end of water-withholding period, suggesting a lost ability to rehydrate overnight under SD treatments.
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Fig. 1. Changes in leaf water potential of (A) an indica/indica (I/I) hybrid rice Shanyou 63 and two japonica/indica (J/I) hybrids, (B) Ce 03/Yangdao 4 and (C) PC 311/Zaoxiandang 18, during the first 28 d after withholding water. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit, respectively, during grain filling. Measurements were made on the flag leaves at predawn (0600 h) and at midday (1130 h). Vertical bars represent ± standard error of the mean (n = 6) where these exceed the size of the symbol.
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Chlorophyll Content and Photosynthetic Rate
Chlorophyll contents in the flag leaves for WW plants gradually declined during grain filling (Fig. 2)
. Water deficits enhanced such declines. The greater the water deficit was, the faster the Chl decreased, indicating that water deficits promoted the leaf senescence. Compared with the I/I hybrid, the Chl content of the two J/I hybrids declined slower, even when the soil was the same. Compared with the two J/I hybrids, the I/I hybrid senesced faster under both WW and water deficit conditions.
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Fig. 2. Changes in chlorophyll (Chl) content in the flag leaves of (A) an indica/indica (I/I) hybrid rice Shanyou 63 and two japonica/indica (J/I) hybrids, (B) Ce 03/Yangdao 4 and (C) PC 311/Zaoxiandang 18, during the first 28 d after withholding water. Treatment details are the same as in Fig. 1. Vertical bars represent ± standard error of the mean (n = 6) where these exceed the size of the symbol.
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Similar to Chl content, the PR of the flag leaves declined during grain filling, and the water deficit accelerated this trend (Fig. 3)
, further suggesting that water deficit enhanced plant senescence. The PR of the two J/I hybrids decreased more slowly than the I/I hybrid, under the same level of soil, and correlated with their slower decline of Chl.
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Fig. 3. Photosynthetic rates (PR) of the flag leaves of (A) an indica/indica (I/I) hybrid rice Shanyou 63 and two japonica/indica (J/I) hybrids, (B) Ce 03/Yangdao 4 and (C) PC 311/Zaoxiandang 18, during the first 28 d after withholding water. Treatment details are the same as in Fig. 1. Vertical bars represent ± standard of the mean (n = 6) where these exceed the size of the symbol.
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Remobilization of Carbon Reserve
Concentration of NSC in the culm and sheath during grain filling was very different between the I/I and the two J/I hybrids under WW treatments (Fig. 4)
. Nonstructural carbohydrates in the culm and sheath of the two J/I hybrids initially decreased from 7 to 21 DAA but increased thereafter. Nonstructural carbohydrate concentrations at maturity were nearly the same as at anthesis for both J/I hybrids.
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Fig. 4. Nonstructural carbohydrate (NSC) concentrations in the culm and sheath of (A) an indica/indica (I/I) hybrid rice Shanyou 63 and two japonica/indica (J/I) hybrids, (B) Ce 03/Yangdao 4 and (C) PC 311/Zaoxiandang 18, during the grain filling. Treatment details are the same as in Fig. 1. Arrows in the figure indicate the start of water deficit treatments. Vertical bars represent ± standard error of the mean (n = 6) where these exceed the size of the symbol.
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For the I/I hybrid, NSC in the culm and sheath decreased sharply from 7 to 32 DAA and slowly thereafter. Water deficits substantially reduced NSC in the culm and sheath of all three hybrids. The more severe the water deficit was, the more that NSC were reduced. Under MD and SD treatments, the NSC patterns were similar for both I/I and J/I hybrids (Fig. 4). For the I/I hybrid, the remobilized C reserve from the culm and sheath during grain-filling period was 64, 76, and 89% for WW, MD, and SD treatments, respectively (Table 1). The remobilized C reserve of the two J/I hybrids was 53 to 61% and 67 to 74% for MD and SD treatments, respectively, which was a 47 and 61% increase, respectively, over the WW treatment (7–14%). Water deficits significantly increased the contribution of prestored C to the grain and the transfer ratio and greatly reduced the residue of NSC in the culm and sheath (Table 1). Compared with the WW treatment, the MD and SD treatments increased harvest index by 10 and 15%, respectively, for the I/I hybrid and by 17 and 24% and 29 and 38%, respectively, for the two J/I hybrids (Table 1). Water deficits benefited harvest index for the J/I hybrids more than the I/I hybrid.
The total radioactivity remaining in the aboveground portion of the plant was 6.5 to 7.2 kBq plant-1, and no significant difference was found among the hybrids and water treatments (data not shown). At maturity, partitioning of 14C from the flag leaves to the grains increased by 18 and 28%, respectively, for the MD and SD treatments of the I/I hybrid and by 22 and 26% and 34 and 43%, respectively, for the MD and SD treatments of the two J/I hybrids compared with WW treatments (Table 2). Results indicate that prestored C remobilization to the grains, enhanced by water deficits during the grain-filling period, was more in J/I hybrids than in the I/I hybrid.
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Table 2. Partitioning of 14CO2 in rice plants subjected to various moisture treatments. Labeling was conducted at heading.
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Grain Filling and Grain Yield
The active grain-filling period was shortened by a range of 2.7 to 8.7 d, and grain-filling rate increased by 0.14 to 0.36 mg d-1 grain-1 by water deficits across the three hybrids compared with WW treatments (Table 3). Greater water deficit led to a more severely shortened grain-filling period and greater grain-filling rate, and differences were more significant in the two J/I hybrids than in the I/I hybrid (Table 3).
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Table 3. Grain-filling rate and grain yield of three rice hybrids subjected to various soil moisture treatments. Active grain-filling period and rate were calculated according to Richards' (1959) equation.
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There was no effect of water deficits on spikelets per square meter because neither the panicles per square meter nor the spikelets per panicle were influenced significantly by water deficits in this experiment (Table 3). Influence of water deficits on the percentage of ripened grains, grain weight, and grain yield varied among the treatments and the hybrids. Water deficits reduced the percentage of ripened grains, grain weight, and grain yield of the I/I hybrid, but the difference was not statistically significant between the WW and MD treatments. The percentage of ripened grains and grain yield increased under both MD and SD treatments for the two J/I hybrids. Changes in the percentage of ripened grains under water deficit treatments were mainly attributed to changes in the number of unfilled fertilized grains because the percentage of sterile or unfertilized grains (sterile grains as a percentage of total spikelets) was only slightly affected by water deficits (4–5% for the I/I hybrid and 6–7% for both J/I hybrids) under all water treatments (data not shown). The grain weight of both J/I hybrids was not affected by water deficits compared with the WW treatments (Table 3).
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DISCUSSION
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Under the same soil water deficit and atmospheric conditions, the J/I hybrids had a higher leaf water potential than the I/I hybrid, both at predawn and midday. The two J/I hybrids could be rehydrated fully overnight while the I/I hybrid could not under the SD treatments (Fig. 1), indicating J/I hybrids had a greater capability than the I/I hybrid to extract water from the soil. A probable explanation is that the J/I hybrids possess a deep root system (Harada et al., 1994; Kang et al., 1994) although it is possible that a faster decline in leaf water potential during grain filling was related to the early senescence in the I/I hybrid.
The senescence process, as indicated by the loss of Chl and a decline of photosynthesis, differed between the I/I and J/I hybrids (Fig. 1 and 2). Chlorophyll and PR of the flag leaves of the I/I hybrid declined faster during grain-filling period, both under WW and water deficit treatments. Chlorophyll and PR of the two J/I hybrids remained rather high at maturity under WW treatments (Fig. 1 and 2), suggesting that J/I hybrids had a stronger heterosis than the I/I hybrid. Chlorophyll and PR of the two J/I hybrids were significantly reduced by water deficit treatments but were higher at maturity relative to that of the I/I hybrid, indicating that photosynthesis of the two J/I hybrids was less inhibited by water deficits.
Under a WW condition, it is notable that the I/I hybrid senesced fast (loss of Chl and a decline of PR) and more prestored C was remobilized during grain filling while the J/I hybrids senesced slowly and more NSC in the culm and sheath remained at maturity (Fig. 3 and Table 1). When water deficit treatments were imposed, plant senescence was hastened, and remobilization was greatly enhanced (Fig. 3; Tables 1 and 2). Perhaps senescence and remobilization are coupled processes in rice. We speculate that poor translocation of assimilates to the grains in J/I hybrids is attributed to, or at least related to, the delayed senescence.
When plants such as the I/I hybrid senesced properly (plant senesced when it was due to become ripe), water deficits reduced the percentage of ripened grains and grain yield though grain-filling rate increased (Table 3), indicating that the gain from an enhanced remobilization could not compensate for the loss of photosynthesis shortened by water deficits during grain filling. We found, however, that the water deficit–induced senescence not only substantially enhanced remobilization of prestored C to the grains (Fig. 3; Tables 1 and 2), but also improved grain filling and grain yield of the two J/I hybrids (Table 3). Therefore, we conclude that if a water deficit is applied to induce senescence to a degree that overnight rehydration can be completed and photosynthesis is not too severely inhibited, the gain from an increased remobilization of prestored food reserve may outweigh the small loss of photosynthesis and lead to faster grain filling and higher grain yield.
Unfavorably delayed senescence has been a problem aggravated during the recent rice production in China, mainly as a result of the heavy use of N fertilizers (Ling et al., 1993; Yang et al., 1996), adoption of lodging-resistant cultivars with a longer growth period (Yuan, 1997; Zhu et al., 1997), or utilization of heterosis (Yuan, 1990, 1997; Gu et al., 1996; Wang et al., 1998). All of these cases result in a slow grain-filling period, with NSC left in the vegetative tissues at maturity. This leads to a low harvest index and reduced grain yield. Our results indicate that controlled water deficits can improve rice grain filling in cases where plant senescence is unfavorably delayed.
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ACKNOWLEDGMENTS
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We are grateful for grants from the Faculty Research Grant of Hong Kong Baptist University, the Research Grants Council of Hong Kong, the Area of Excellence Research Foundation of the Chinese University, and the National Natural Science Foundation of China (Project no. 39970424).
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