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Published in Crop Sci. 43:2099-2108 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP PHYSIOLOGY & METABOLISM

Postanthesis Water Deficits Enhance Grain Filling in Two-Line Hybrid Rice

Jianchang Yanga, Jianhua Zhang*,b, Zhiqing Wanga, Lijun Liua and Qingsen Zhua

a College of Agriculture, Yangzhou Univ., Yangzhou, Jiangsu, China
b Department of Biology, Hong Kong Baptist Univ., Hong Kong, China

* Corresponding author (jzhang{at}hkbu.edu.hk).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Two-line hybrid rice (Oryza sativa L.) is a hybrid from a thermosensitive genetic male sterile line and its corresponding restoring line. This hybrid rice shows strong heterosis but a slow grain-filling rate, as a result of unfavorably delayed senescence. This study investigated the possibility that a moderate water deficit imposed during grain filling may enhance plant senescence, and therefore facilitate the remobilization of reserve C to the grains and improve grain filling. Two two-line indica hybrids were grown in both the pot and field. Three levels of soil water potential, well watered (WW), moderate water deficit (MD), and severe water deficit (SD), were imposed from 9 d after anthesis to maturity in both pot and field experiments. Results showed that Chlorophyll content and photosynthetic rate of the flag leaves declined more quickly as plants approached maturity for MD and SD plants than for WW plants, indicating the water deficits enhanced senescence. The remobilized C reserve and reallocation of pre-fixed 14C from stems to grains increased by 1.5- to 2-fold and 55 to 67%, respectively, for water deficit plants, when compared with WW plants. The water deficit treatments shortened the grain-filling by 5 to 17 d and increased the grain-filling rate by 0.09 to 0.27 mg d-1 grain-1. The grain yield of MD plants in both experiments was increased by 8.2 to 10% but that of SD plants in the pot was reduced. We conclude that if water deficit is properly controlled during the grain filling, whole-plant senescence is enhanced. The enhanced senescence can facilitate remobilization of C reserves, accelerate grain filling, and increase grain yield in the two-line hybrid rice.

Abbreviations: DAA, days after anthesis • Chl, chlorophyll • gleaf, leaf conductance • MD, moderate water deficit • NSC, nonstructural carbohydrate • {psi}soil, soil water potential • PGMS, photoperiod-sensitive genetic male sterile • Pr, photosynthetic rate • SD, severe water deficit • TGMS, thermo-sensitive genetic male sterile • WW, well watered


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
DEVELOPMENT OF TWO-LINE HYBRID RICE is an important innovation in rice breeding (Yuan, 1994, 1998). In comparison with the three-line or cytoplasmic male sterile system of hybrid rice, the two-line method, using photoperiod-sensitive genetic male sterile (PGMS) lines or thermo-sensitive genetic male sterile (TGMS) lines, has three advantages.

  1. PGMS or TGMS lines show pollen sterility under longer daylength or under higher temperature, and exhibit normal fertility under shorter daylength or under moderate temperature, and therefore the maintainer line is not needed.
  2. PGMS and TGMS genes are easily transferred into almost any rice line with desirable characteristics and, thus, the choice of parents in developing heterotic hybrids is greatly broadened. Grain yield of some two-line hybrids in experimental tests has been reported with an increase of 10 to 20% over the best existing three-line hybrids (Yuan, 1994).
  3. Some negative effects caused by the male sterile and dominant cytoplasmic genes in the three-line hybrid can be avoided. It is estimated that the two-line hybrids will replace 70% of the three-line hybrids in the first decade of the 21st century (Yuan, 1998).

Commercial adoption of the two-line hybrid in a large area has showed, however, that poor grain filling, as in unfilled grains, of this hybrid is a serious problem and also a major constraint in its production (Lu et al., 1994; Zhu et al., 1997; Chen, 2001, Gu and Tang, 2001). Causes of the poor grain filling have been investigated in some detail (Zhuang et al., 1994; Yuan, 1997; Wang et al., 1998; Chen, 2001, Gu and Tang, 2001) and are generally considered to be closely associated with the slower grain filling, resulting from a delayed senescence, e.g., plants are too vigorous and stay green for too long or plants remain green when the grains are due to ripen in comparison with three-line hybrid rice or conventional rice varieties (Zhu et al., 1997; Wang et al., 1998; Chen, 2001; Gu and Tang, 2001). Our earlier work (Yang et al., 2000, 2001b,c, 2002a) has shown that delayed senescence, which in practice is induced by either too much nitrogen fertilizer or an adoption of hybrids that are too vigorous, retards remobilization and can lead to poor grain filling and reduced grain weight. Our results demonstrated that water deficits imposed during grain filling can enhance plant senescence, promote the remobilization of prestored carbon reserves, accelerate grain filling, and improve yield in cases where senescence in wheat and rice is unfavorably delayed by heavy use of nitrogen fertilizer.

The objective of this study was to determine if early senescence induced by a moderate water deficit during the grain filling could enhance carbon remobilization, and if such enhancement could improve grain filling in two-line hybrid rice.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials
Both pot and field experiments were conducted at a farm of Yangzhou University, Jiangsu Province, China (32°30'N, 119°25'E) during the rice growing season (May–mid October) of 2001. Two two-line hybrids, Pei-Ai 64S (TGMS, indica)/Yangdao 6 (indica) and Pei-Ai 64 S/E32 (indica), were used in both experiments. The two hybrids are currently used in local production because of their relatively higher grain yield, better grain quality, and pest-resistance compared with other hybrids. The normal time from sowing to maturity for the two hybrids is 155 to 158 d. Total leaves on the main stem are 18 (VN = 18) for each hybrid. Seeds (first generation) of the hybrids were harvested in the previous year. Seedlings from these seeds were raised in the field with a sowing date of 10 May. The soil of both experiments was sandy loam [Typic Fluvaquent, Etisols (U.S. taxonomy)] with 24.5 g kg-1 organic matter and available N-P-K at 106, 33.8, and 66.4 mg kg-1, respectively. The total precipitation during the growing season was 359 mm, 81% of which was in June and July. Air temperatures during grain filling (21 August–20 October), averaged every 10 d, were 27, 26.5, 25, 24, 23, and 21°C, respectively.

Pot Experiment
Each porcelain pot (30 cm in height and 25 cm in diameter, 14.7 L in volume) was filled with 20 kg soil and planted with three hills with one seedling per hill. On the day (9 June) of transplanting, 1 g N as urea, 0.3 g P as single superphosphate and 0.5 g K as KCl were mixed into the soil in each pot. N as urea was also applied at mid-tillering (0.5 g per pot) and panicle initiation (0.8 g per pot) stages (V9 and V14, respectively, refer to Counce et al., 2000). Both hybrids headed on 20 to 22 August (50% of plants), flowered on 22 to 24 August, and were harvested on 16 October. The water level in the pot was kept at 1 to 2-cm above the soil surface until 9 d after anthesis (DAA) when water-deficit treatments were initiated.

The experiment was a 2 by 3 (two hybrids and three levels of soil moisture) factorial design with six treatments. Each treatment had 80 pots as replicates. From 9 DAA to maturity, three levels of soil water potential ({psi}soil) were imposed by controlling water application. The well-watered (WW) treatment was kept at 1- to 2-cm water depth ({psi}soil = 0 MPa) in the pot by manually applying tap water, a moderate water deficit (MD) was maintained at –0.025 MPa, and a severe water deficit (SD) was maintained at –0.05 MPa. Soil water potential in the water deficit treatments was monitored in the 15- to 20-cm soil depth. A tension meter (Soil Science Research Institute, Nanjing, China) consisting of a sensor of 5-cm length was installed in each pot to monitor water potential. Tension meter readings were recorded every 4 h from 0600 to 1800. When the readings dropped to the desired value, 0.4 and 0.2 L of tap water per pot was added to MD and SD treatments, respectively. The pots were placed in a field and sheltered from rain by a removable polyethylene shelter during rain.

Field Experiment
Seedlings were transplanted on 10 June at a hill spacing of 0.20 by 0.16 m with one seedling per hill. N (60 kg ha-1 as urea), P (30 kg ha-1 as single superphosphate), and K (40 kg ha-1 as KCl) were applied and incorporated before transplanting. N as urea was also applied at mid-tillering (40 kg ha-1) and at panicle initiation (15 kg ha-1). Both hybrids headed on 23 to 25 August (50% of heading), and were harvested on 20 October. The water level in the field was kept 1 to 2-cm above the soil surface until 9 DAA when water-deficit treatments were initiated.

The experiment was a 2 by 3 (two hybrids and three levels of soil moisture) factorial design with six treatment combinations. Each of the treatments had three plots as repetitions in a complete randomized block design. Plot dimension was 4 m by 3.2 m and plots were separated by a ridge (40 cm in width) wrapped with plastic film. From 9 DAA to maturity, three levels of {psi}soil, WW, MD and SD, were imposed on the plants by controlling water application. The treatment details were the same as those in the pot experiment. Five tension meters were installed in each plot to monitor water potential. Tension meter readings were recorded twice a day at 1000 and 1600 h. When the readings dropped to the desired value, 0.23 and 0.12 cm of irrigation per plot was added manually to the MD and SD treatments, respectively. A rain shelter consisting of a steel-frame covered with a plastic sheet placed over the plots to protect them during rains.

Radioactive Labeling
At the booting stage (12 August), plants of 15 pots from each treatment in the pot experiment were labeled with 14CO2. Flag leaves of main stems were labeled between 0900 to 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 was removed from the chamber and replaced by injection of 6 mL air containing 10 mmol L-1 CO2 at a specific radioactivity of 14C of 1.48 MBq L-1. The chamber was removed after 1 h.

Labeled plants were harvested at anthesis (12 d after labeling) and from 9 (the initiation of water withholding) to 45 DAA at 6-d intervals. 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 extracted by shaking for 30 min in 630 g L-1 [80%, (v/v)] boiling ethanol. The residue was extracted in 2:1 of 14.3 M HClO4 to 9.7 M H2O2 for 4 h at 60°C. The radioactivity of 14C in both the extracted aliquots was counted using a liquid scintillation counter (Beckman Instruments Inc., 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 potential of flag leaves was measured on clear days at predawn (0600 h) and midday (1130 h) on 0, 5, 12, 18, 24, and 29 d after withholding water. Well-illuminated flag leaves were chosen randomly for such measurements. A pressure chamber (Model 3000, Soil Moisture Equipment Corp., Santa Barbara, CA) was used for leaf water potential measurement with six leaves for each treatment in both experiments.

The photosynthetic rate (Pr), leaf conductance (gleaf), and chlorophyll (Chl) content of the flag leaves were measured on 0, 9, 15, 22, 28, 34, and 39 DAA. Pr and gleaf were measured with a gas exchange system (CID-PS CO2 Analyzer System, CID, Vancouver, WA) at 350 µL L-1 CO2, flow rate of 500 µmol s-1, and chamber temperature 28 to 30°C. Measurements were made during 0900 to1100 h when photosynthetically active radiation above the canopy was 1000 to1100 µmol m-2 s-1. Flag leaves were sampled for measurement of Chl content. Chl was extracted by shaking in methanol overnight and determined as described by Holden (1976). Six leaves were used for each treatment in each experiment.

Sampling and Harvesting
Four hundred panicles that headed on the same day were tagged for each treatment from both experiments. The flowering date of each spikelet on the tagged panicles was recorded. Twenty tagged panicles from each treatment for the pot experiment or 10 panicles from each plot for the field experiment were sampled every 3 d from anthesis to maturity. The sampled panicles from the pot experiment 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):

[1]

Grain filling rate (G) was calculated as the derivative of Eq. [1]:

[2]
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 above ground biomass was measured at both anthesis and grain maturity. At each harvest, plants from five pots (153–156 stems) from each treatment in the pot experiment and a 1-m2 area (243–252 stems) from each plot in the field experiment were sampled and separated into leaf blades, culms and sheaths, and panicles. All plant parts were dried at 80°C to constant weight and weighed.

Plants from three pots (93–96 stems) from each treatment in the pot experiment and 12 hills (94–99 stems) from each plot in the field experiment were sampled at 6-d intervals from anthesis to maturity for the measurement of stem (culm + sheath) dry weight and nonstructural carbohydrate (NSC) in culms, sheaths, and leaves. The method for extracting of NSC was modified from 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 cold water the tube was centrifuged at 3000 g for 20 min. The supernatant was collected and the extraction was repeated three times. The alcohol in the supernatant was evaporated in 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 to 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 30 min. Two milliliters of 9.36 M HClO4 was added to the tube after cooling in cold water. The solution was shaken further for 15 min. The extract was then made up to about 10 mL and centrifuged at 3000 g for 20 min. The supernatant was collected and 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 in 10 pots of each treatment in the pot experiment and a 4-m2 area (excluding border plants) in each plot in the field experiment were harvested at maturity for the determination of grain yield. Yield components, i.e., the panicles per pot or per square meter, the percentage of ripe grains and grain weight, were determined from the plants of 5 pots from each treatment in the pot experiment or from the plants of a 1-m2 area from each plot in the field experiment. The percentage of ripe grains was defined as the grains that sink in 1.52 M NaCl water (specific gravity = 1.06) as a percentage of total spikelets. The total spikelets were calculated from the grain yield, grain weight, and percentage of ripe grains, i.e., total spikelets = grain yield/(grain weight x percentage of ripe grains).

The results were analyzed for variance by means of SAS statistical analysis package (SAS, 1995). Data from each sampling date were analyzed separately. Means were tested by least significant difference at P0.05 level (LSD0.05).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Physiological Parameters
Figure 1 illustrates the progression of leaf water potentials during the first 29 d after withholding water. Both hybrids exhibited a similar trend of leaf water potential changes at predawn (0600 h) and at midday (1130 h) in both experiments (Fig. 1). When plants were well watered, midday leaf water potential decreased gradually during grain filling. Water deficit treatments substantially reduced midday leaf water potential. The differences in predawn leaf water potentials between WW and MD plants grown in pots (Fig. 1A and B) or among WW, MD and SD plants grown in the field (Fig.1 C and D) were not significant, indicating that plants subjected to MD in the pot and both MD and SD in the field rehydrated overnight. The leaf water potential of SD plants in the pot experiment was significantly lower than that of either WW or MD plants from 18 d after withholding water, suggesting a lost ability to rehydrate overnight for these plants.



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  		Fig. 1. Changes in leaf water potentials of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A and C) and Pei-Ai 64S/E 32 (B and D), in the pot (A and B) and field (C and D) experiments during the first 29 d after withholding water. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Measurements were made on the flag leaves at pre-dawn (0600 h) and at midday (1130 h). Vertical bars represent ± SE of the mean (n = 6) where these exceed the size of the symbol. Both experiments were conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 
At the same level of {psi}soil, plants grown in the field had higher leaf water potential than those grown in the pot, indicating that plants grown in the field had a greater capacity than those grown in the pot to extract water from the soil. Presumably, the plants grown in the field possess a deeper root system that may increase the volume of soil from which water is extracted compared to plants in pots.

Similar to the case of leaf water potential, changes in gleaf also varied among treatments (Fig. 2). Plants with high leaf water potential had a high gleaf.



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  		Fig. 2. Changes in leaf conductance in the flag leaves of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A and C) and Pei-Ai 64S/E 32 (B and D), in the pot (A and B) and field (C and D) experiments during the grain filling period. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Measurements were made during 0900 to 1100 h. Arrows indicate the start of water deficit treatments. Vertical bars represent ± SE of the mean (n = 6) where these exceed the size of the symbol. Both experiments were conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 
Chl contents in the flag leaves of WW plants slowly declined with the aging of leaves (Fig. 3). The rate of decline was increased after plants were exposed to water deficits. The greater the water deficit, the faster the Chl content decreased, indicating that water deficits enhanced the leaf senescence. Plants grown in the field had higher Chl content than those grown in pots even at the same level of {psi}soil. Early senescence induced by the water deficits may be related to a reduction in N uptake from the soil (Yang et al., 1996) and an increase of abscisic acid (ABA) synthesis (Davies and Zhang, 1991).



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  		Fig. 3. Changes in chlorophyll content in the flag leaves of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A and C) and Pei-Ai 64S/E 32 (B and D), in the pot (A and B) and field (C and D) experiments during the grain filling period. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Arrows indicate the start of water deficit treatments. Vertical bars represent ± SE of the mean (n = 6) where these exceed the size of the symbol. Both experiments were conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 
In current experiments, both Chl and Pr were monitored as an objective way to quantify senescence. As expected, the water deficits significantly reduced Pr of the flag leaves during grain filling (Fig. 4). The reduction of photosynthesis in water deficit treatments could be attributed to the loss of Chl and early senescence (Kaiser, 1987; Siddique et al., 1999).



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  		Fig. 4. Photosynthetic rates of the flag leaves of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A and C) and Pei-Ai 64S/E 32 (B and D), in the pot (A and B) and field (C and D) experiments during the grain filling period. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Measurements were made during 0900 to 1100 h. Arrows indicate the start of water deficit treatments. Vertical bars represent ± SE of the mean (n = 6) where these exceed the size of the symbol. Both experiments were conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 
Carbon Remobilization
Water deficit treatments facilitated the reallocation of preanthesis assimilates from the stems to grains. At the start of water withholding (9 DAA), about 75% of 14C was found in the stems, and about 8% in the grains (Fig. 5). After 24 d (33 DAA), 14C in the stem was reduced to 35 to 37% for MD, 17 to 19% for SD, and 52 to 58% for WW plants. Opposite to that observed in the stem, the 14C in the grains increased to 45 to 46% for MD, 62 to 66% for SD, and only 19 to 22% for WW plants at 33 DAA.



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  		Fig. 5. Changes in 14C partitioning in the stems (A and B) and grains (C and D) of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A and C) and Pei-Ai 64S/E 32 (B and D), during the grain filling period (pot experiment only). WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Arrows indicate the start of water deficit treatments. Vertical bars represent ± SE of the mean (n = 6) where these exceed the size of the symbol. The experiment was conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 
Very similar to 14C reallocation, NSC in the stems declined more quickly for MD and SD plants than for WW plants in both experiments (Fig. 6). The more severe the water deficit, the more the NSC was reduced in the stem. From anthesis to maturity, the remobilized C reserve from the stem for water deficit plants was as 2.5- to 3.9-fold as that for WW plants (Table 1). The contribution of remobilized carbon reserves to the grain was increased by 10.3 to 12.8% for MD and 15.1to 17.3% for SD plants, when compared with the WW plants. In contrast, NSC residues in the stem at maturity were remarkably reduced by water deficits. The enhanced remobilization by water deficits led to a high harvest index (Table 1).



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  		Fig. 6. Changes in nonstructural carbohydrate (NSC) concentrations in the stems of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A and C) and Pei-Ai 64S/E 32 (B and D), in the pot (A and B) and field (C and D) experiments during the grain filling period. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Arrows indicate the start of water deficit treatments. Vertical bars represent ± SE of the mean (n = 3) where these exceed the size of the symbol. Both experiments were conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 

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  		Table 1. Remobilization of stored assimilates in straw of rice subjected to various soil moisture treatments. Two two-line indica hybrids, Pei-Ai 64S/Yangdao 6 and Pei-Ai 64S/E 32, were grown at Yangzhou University farm, Jiangsu Province, China in 2001.

 
Grain Filling and Grain Yield
The water deficit treatments greatly accelerated grain filling in both pot and field experiments (Fig. 7). The more serve the water deficit, the greater the grain filling rate and shorter the grain filling period. When compared with the WW plants, the average grain filling rates of MD and SD plants were increased by 21 to 22% and 42 to 54%, respectively, for the pot experiment, and 17 to 20% and 36 to 39%, respectively, for the field experiment. The active grain filling period, on average, was shortened by 5 to 6 d for MD plants and 12 to 17 d for SD plants, compared with WW plants in both experiments.



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  		Fig. 7. Grain filling process (A, B, C, and D) and grain filling rate (E, F, G, and H) of two two-line indica hybrids of rice, Pei-Ai 64S/Yangdao 6 (A, C, E, and G) and Pei-Ai 64S/E 32 (B, D, F, and H), in the pot (A, B, E, and F) and field (C, D, G, and H) experiments. WW, MD, and SD are well watered, moderate water deficit, and severe water deficit during grain filling. Grain filling rate was calculated according to the Richards (1959) equation (lines E-H). Arrows indicate the start of water deficit treatments. Vertical bars in the figure A, B, C and D represent ± SE of the mean (n = 2, for the pot experiment and n = 3 for the field experiment) where these exceed the size of the symbol. Both experiments were conducted at Yangzhou University farm, Jiangsu Province, China in 2001.

 
Because neither the panicles per pot or per square meter, nor the spikelets per panicle, were affected by the water deficit treatments in these experiments, effects of water deficits on the number of grains per pot, or per square meter were too small to be statistically significant (Table 2). However, the grain yield varied with the treatments and the experiments. Compared with those of WW plants, the percentage of ripe grains, grain weight, and grain yield of MD plants were increased by 4.4 to 5.1%, 4.3 to 5.2%, and 9.5 to 10%, respectively, for the pot experiment, and 3.9 to 4.4%, 4.3 to 4.7%, and 8.2 to 8.3%, respectively, for the field experiment, implying that the gain from accelerated grain-filling rate outweighed a shorter grain-filling period when plants were subjected to a mild water deficit during grain filling. The grain yield of SD plants for the pot experiment was decreased by 9.0 to 9.4% and was statistically less than that of WW plants. The reduction in grain yield was attributed to the decrease in grain weight. However, the difference in grain yield between SD and WW plants in the field experiment was not statistically significant.


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  		Table 2. Grain-filling rate and grain yield of rice subjected to various soil moisture treatments. Active grain filling period and grain-filling rate were calculated according to Richards (1959) equation. Two two-line indica hybrids, Pei-Ai 64S/Yangdao 6 and Pei-Ai 64S/E 32, were grown at Yangzhou University farm, Jiangsu Province, China in 2001.

 
Much water was saved under either MD or SD treatments in both experiments. Water applied to MD and SD treatments from 9 DAA to harvest was 32 and 24 L per pot, respectively, in the pot experiment and 148 and 96 L m-2, respectively, in the field experiment, which was only 67 and 50% of the WW treatments (48 L per pot) in the pot experiment, and 52 and 34% of the WW treatments (282 L m-2) in the field experiment.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our results showed that water deficit during grain filling of rice induced early senescence, reduced photosynthesis, enhanced remobilization of prestored C reserves from stems to grains, and shortened the grain-filling period (Fig. 2–7, Table 1 and 2). Such responses are consistent with the previous reports on other crops (Jones and Rawson, 1979; Nicolas et al., 1985b; Lauer and Simmons, 1985; Ludlow and Muchow, 1990; Kobata et al., 1992; Machado et al., 1992; Souza et al., 1997; Zhang et al., 1998). Usually, water stress imposed during grain filling, especially at the early filling stage, results in a reduction in grain weight and leads to reduced grain yield (Ober et al., 1991; Setter, 1993; Mambelli and Setter, 1998). We found, however, that if water deficit stress during grain filling is controlled properly, the grain weight and grain yield could be increased for conditions in which plant senescence is otherwise unfavorably delayed.

The discrepancies between our results and previous reports are probably attributed to several reasons. First, we used the two-line hybrids as materials. One feature of two-line hybrid rice is poor grain filling which is associated with the slow grain filling, resulting from an unfavorably delayed senescence. Early senescence enhanced by moderate water deficit during grain filling would be beneficial to the grain filling of the hybrid. Second, The reduction in gain weight in response to water deficit during grain-filling period is mainly attributed to the lowered number of endosperm cells, thereby decreasing sink size per kernel (Singh and Jenner, 1982; Nicolas et al., 1985a; Michihiro et al., 1994). In our experiment, water deficit was initiated at 9 DAA at which time cell division is complete (Zhang et al., 1998; Yang et al., 2002b) and therefore the sink size may not be seriously affected. Third, the water deficit imposed in our experiments was rather mild and controlled properly such that plants could rehydrate overnight, and photosynthesis was not seriously reduced. Under such conditions, the gain from the accelerated grain-filling rate may outweigh the possible loss of photosynthesis as a result of a shortened grain-filling period, leading to an increase in grain yield of the two-line hybrids. A similar observation was made by Rubia et al. (2002) who reported that rapid leaf senescence was correlated with a high percentage of grain filling and high yield in new rice lines. Dwyer et al. (1995) have observed that in maize it is the fast reallocation of stem carbohydrate that is responsible for the high grain weight, rather than the "stay green" characteristics.

We observed in present and previous studies (Yang et al., 2001c) that moderate water deficit during grain filling could promote C remobilization from stems to grains and enhance grain filling rate not only in the two-line hybrid rice, but also in some high lodging-resistant cultivars. We suspect that a moderate water deficit during grain filling would also enhance C remobilization and grain filling in three-line hybrid rice. The mechanisms are not fully understood with which seed growth rate is increased in response to water deficit. Our previous work (Yang et al., 2001d) has shown that water stress increased the ABA accumulation greatly in the grains. The peak values of ABA in the grains were significantly correlated with the maximum grain filling rates. An altered hormonal balance in the grains by water stress during grain filling, especially the decrease in gibberellins and increase in ABA, enhanced the remobilization of pre-stored carbon to the grains. Such a regulation may shorten the grain filling period but accelerate grain filling rate that could be beneficial to cases where slow grain filling is a problem in rice production. Our earlier work (Yang et al., 2003) also showed that the activities of sucrose synthase and starch branching enzyme in rice grains were substantially enhanced by moderate water deficit and positively correlated with starch accumulation rate in the grains. These results suggest that the water deficit-increased remobilization and grain-filling rate are attributed to enhanced sink strength by regulating sucrose synthase and starch branching enzyme activities in rice grains when subjected to water stress during the grain-filling period.

It was reported that air temperature affects the mobilization of assimilates in wheat (Triticum aestivum L.) and rice, and a possible rise in temperature because of stomatal closure under water stress may account for the enhanced movement of carbon to the grains (Wardlaw, 1971; Solfield et al., 1977; Chowdhury and Wardlaw, 1978). In our study, it was observed that the average day time air temperatures in the canopy (the top 20 cm) of WW, MD and SD treatments during grain-filling period were 28.2, 28.5, and 28.9°C, respectively, in the pot experiment, and 26.9, 27.1, and 27.4°C, respectively, in the field experiment (data not shown). The air temperature of the pot experiment was a little higher than that of the field experiment. This may partially explain the greater decrease in gleaf and carbon remobilization in the pot experiment under water deficits than those in the field experiment even at the same level of {psi}soil (Fig. 2, 5, and 6). However, the difference in air temperatures among treatments within the same experiment was not so dramatic, possibly because the soil drying was relatively mild. It is concluded that such a small increase in the day temperature under MD and SD treatments within the experiment may not explain the observed enhancement of carbon remobilization and increase in the grain-filling rate.

Our results showed that the remobilization of prestored assimilates was closely associated with senescence (Fig. 3–6, Table 1). The more severe the senescence of plants (Fig. 2 and 3), the more and faster the preanthesis carbon assimilates were remobilized (Fig. 5 and 6, Table 1). It is proposed that the whole-plant senescence in monocarpic plants initiates the remobilization of assimilates from vegetative tissues to grains (Nooden et al., 1997; Ori et al., 1999). However, Sinclair and deWit (1975) stated that nutrient depletion from vegetative tissues to seeds or fruits leads to plant senescence. Biswas and Choudhuri (1980) suggested that senescence in rice was induced by ABA-like substances by a deprivational stress developed in the leaves as a consequence of maximum transport of metabolites to the grains. The present study indicates that senescence and remobilization are coupled processes in rice. These processes are accelerated by a water deficit stress imposed during grain filling, and probably attributed, or at least related, to an elevated ABA level, enhanced {alpha}-amylase activity, and the high activation state of sucrose-phosphate synthase in rice plants (Yang et al., 2001a,d, 2002c). Obviously, the mechanism of interaction of plant senescence and remobilization needs to be investigated further.

It is notable that a great amount of irrigation water is saved without any sacrifice in the grain yield under MD treatments in both pot and field experiments. The water saved may be attributed to the reduction in either transpiration rate and/or soil surface evaporation. Usually, decrease in transpiration rate by reducing gleaf will result in the loss of photosynthesis (Wong et al., 1979, 1985; Farquhar and Sharkey, 1982). We observed, however, that the extent to which Pr was reduced by water deficits was much less than the extent to which gleaf decreased (Fig. 2 and 4). We speculate that a mid-day decrease in gleaf (when evaporation demand was highest) would reduce daily transpiration more than photosynthesis.

In conclusion, if a water deficit is imposed and controlled properly during the grain filling in the two-line hybrid rice, the plant can rehydrate overnight and a whole-plant senescence is enhanced, whereas photosynthesis may not be inhibited too severely. The senescence induced by the water deficit can substantially facilitate the remobilization of carbon from vegetative tissues to grains, shorten the grain-filing period, accelerate the grain-filling rate, and improve the grain yield. This practice would not only be beneficial to deal with the delayed senescence, but also have important significance in the water-saving production of rice where water shortage is threatening the sustainability of agriculture.


   ACKNOWLEDGMENTS
 
The work was finaced by the Research Grant Council of Hong Kong (RGC 2052/00M), the Area of Excellence for Plant and Fungal Biotechnology in the Chinese University of Hong Kong, the National Natural Science Foundation of China (Project No. 30270778), and the State Key Basic Research and Development Plan (G1999011704).

Received for publication December 16, 2002.


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