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Published in Agron J 100:726-734 (2008)
DOI: 10.2134/agronj2007.0169
© 2008 American Society of Agronomy
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RICE

Postanthesis Moderate Wetting Drying Improves Both Quality and Quantity of Rice Yield

Hao Zhanga, Shenfeng Zhanga, Jianchang Yanga,*, Jianhua Zhangb and Zhiqin Wanga

a Key Lab. of Crop Genetics and Physiology of Jiangsu Province, Yangzhou Univ., Yangzhou, Jiangsu, China
b Dep. of Biology, Hong Kong Baptist Univ., Hong Kong, China

* Corresponding author (jcyang{at}yzu.edu.cn).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A major challenge in rice (Oryza sativa L.) production in China is to cope with a declining availability of fresh water without compromising grain yield and grain quality. This study was designed to determine if alternate wetting and moderate soil drying during grain filling could maintain grain yield and grain quality. Two rice cultivars, Zhendao 88 (japonica) and Shanyou 63 (indica), were field-grown at Yangzhou, China. Three irrigation treatments, alternate wetting and moderate soil drying (WMD, rewatered when soil water potential reached –25 kPa at 15–20 cm depth), alternate wetting and severe soil drying (WSD, rewatered when soil water potential reached –50 kPa), and conventional irrigation (CI, continuously flooded), were conducted from 6 d after heading to harvestable maturity. Root oxidation activity, the photosynthetic rate of the flag leaf, and activities of key enzymes in sucrose-to-starch conversion in grains during the late grain-filling stage were significantly increased under WMD, whereas they were significantly reduced under WSD. The grain yield was increased by 9.3 to 9.5% under WMD, while it was reduced by 7.5 to 7.8% under WSD, when compared with that under CI. Water applied to WMD was 44% and to WSD was 25% of the amount applied to CI. The WMD significantly improved milling, appearance, and cooking qualities, while WSD decreased these qualities. We conclude that a moderate wetting drying regime during the grain-filling phase of rice holds great promise to both increase yield quantity and quality and also could save precious fresh water resources.

Abbreviations: AWD, alternate wetting and drying irrigation • CI, conventional irrigation • DAH, days after heading • RVA, rapid viscoanalyzer • WMD, moderate soil drying • WSD, severe soil drying


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication May 21, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
RICE is the most important staple food in Asia, providing 35 to 80% of the total calorie intake (IRRI, 1997). Rice is also the greatest consumer of water among all crops and consumes about 80% of the total irrigated fresh water resources in Asia (Guerra et al., 1998; Bouman and Tuong, 2001). Fresh water, however, is becoming increasingly scarce because of population growth, increasing urban and industrial development, and the decreasing availability resulting from pollution and resource depletion (Guerra et al., 1998; Belder et al., 2005). Decreasing water availability for agriculture threatens the productivity of irrigated rice ecosystem, and ways must be sought to save water and maintain grain yield of rice (Guerra et al., 1998; Belder et al., 2004).

The dominant system of paddy rice production in Asia is transplanting or direct-seeding in a field that is kept continuously flooded with 5 to 10 cm throughout the growing season (Bouman and Tuong, 2001). To reduce water use in irrigated rice, water-saving regimes can be introduced, such as alternate wetting and drying (AWD) irrigation (Bouman and Tuong, 2001; Belder et al., 2004), continuous soil saturation (Borrell et al., 1997), internal drainage (Singandhupe and Rajput, 1987; Ramasamy et al., 1997), rice cultivation on raised beds (Ockerby and Fukai, 2001), aerobic rice system (Bouman et al., 2005), and nonflooded mulching cultivation (Liu et al., 2003, Tao et al., 2006). In AWD, water inputs can be reduced and water productivity increased by introducing periods of nonsubmerged conditions of several days throughout the growing season, unless cracks are formed through the plow sole (Bouman and Tuong, 2001). It has been reported that AWD can maintain or even increase grain yield (Tuong et al., 2005; Won et al., 2005; Yang et al., 2007), and this technology is being adopted in countries of East Asia such as Bangladesh, India, and Vietnam (Kukal et al., 2005; Tuong et al., 2005; Bouman et al., 2007). On the other hand, there are reports that AWD often reduces, rather than increases, grain yield when compared with continuously submerged conditions (Mishra et al., 1990; Tabbal et al., 2002; Belder et al., 2004). Obviously, it remains a major challenge to reduce water input without compromising yield and to optimize scarce water in rice production.

Besides rice yield, quality of rice is a determinant factor in economic returns for farmers. Trends in preferences of consumers are toward high quality of rice in China, because of the growing economy and increased buying power of a considerable part of the population. Rice quality is evaluated using several characteristics including milling, appearance, cooking and eating, and nutrient qualities (Han et al., 2004). It is generally believed that rice quality is determined both genetically and environmentally (Borrell et al., 1997; Cheng et al., 2003; Han et al., 2004). Soil water status, especially during the grain-filling period, has a dramatic influence on grain quality of rice (Dingkuhn and Gal, 1996). There are reports that water deficit stress during grain filling often decreases grain quality (Kobata and Takami, 1981; Cheng et al., 2003; Zheng et al., 2003). However, little is known whether and how AWD affects quality of rice.

Grain filling is the final growth stage in rice when fertilized ovaries develop into caryopses, and grain quality is mainly formed during this period. Although irrigation is usually stopped about 1 or 2 wk before rice harvest in some regions (Singandhupe and Rajput, 1987; Ramasamy et al., 1997; Cheng et al., 2003), it is not known if less water can be applied earlier after heading without compromising yield quantity and quality.

As starch in rice endosperm contributes about 90% of the final dry weight of an unpolished grain (Yoshida, 1972), the grain filling is actually a process of starch accumulation. It is generally accepted that four enzymes play a key role in this process: sucrose synthase (SuS, EC 2.4.1.13), adenosine diphosphate glucose pyrophosphorylase (AGP, EC 2.7.7.27), starch synthase (StS, EC 2.4.1.21), and starch branching enzyme (SBE, EC 2.4.1.18) (Nakamura et al., 1989; Preiss et al., 1991; Kato, 1995; Yang et al., 2003). It is therefore important to know if and how activities of these four enzymes change under AWD during grain filling.

Photosynthesis in rice plants during the grain-filling period contributes 60 to 100% of final grain C content (Yoshida, 1981). The remainder is made up from remobilized storage carbohydrate in leaf sheaths and culms laid down before anthesis (Yoshida, 1981). To achieve higher yield and better quality, metabolic activity within the grain (sink) must coincide with maximum activity of source leaves. Usually, water stress during the grain-filling period reduces photosynthesis and shortens the grain-filling period, leading to many unfilled grains and low grain weight (Bidinger et al., 1977; Kobata and Takami, 1981; Zheng et al., 2003). It would be possible, however, that mild soil drying during grain filling would not inhibit photosynthesis. Instead, it would enhance remobilization of C from vegetative tissues to grains, leading to an increased harvest index and grain yield.

Roots are involved in acquisition of nutrients and water, synthesis of plant hormones, and anchorage of plants, and are assumed to mutually interact with shoots (Osaki et al., 1997; Yang et al., 2004). Although studies on the linkage of drainage during rice growing season with root activity, the uptake of nutrients, and yield formation have been reported (Patel et al., 1984; Ramasamy et al., 1997; Yang et al., 2004), no information is available with regard to changes in root activity under AWD during the grain-filling period.

The objective of this study was to investigate if an alternate wetting and moderate soil drying during grain filling could maintain grain yield and grain quality of rice. Three irrigation regimes and two cultivars were studied. The main quality traits of milling, appearance, cooking and eating, and starch paste viscosity were investigated. Root oxidation activity, photosynthetic rate of the flag leaf, and activities of key enzymes involved in the sucrose–starch conversion in grains were also determined.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials and Cultivation
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–October) of 2004 and 2005. The soil was a sandy loam [Typic fluvaquents, Etisols (U.S. taxonomy)] with 24.5 g kg–1 organic matter, 106 mg kg–1 alkali hydrolyzable N, 34.2 mg kg–1 Olsen-P, and 68.0 mg kg–1 exchangeable K. Alkali hydrolyzable N (NaOH) was analyzed using the method described by Cornfield (1960), and Olsen-P (0.5 M NaHCO3) and exchangeable K (NH4OAc) were analyzed using the method of Sparks et al. (1996). The field capacity soil moisture content, measured after constant drainage rate and made gravimetrically, was 0.189 kg kg–1, and bulk density of the soil was 1.34 g cm–3. The total precipitation during the gain-filling period was 44 mm in 2004 and 63 mm in 2005. The mean solar radiation during grain filling measured at weather station close to the experimental site was 18.1 MJ m–2 d–1 in 2004 and 17.6 MJ m–2 d–1 in 2005.

Two high-yielding rice cultivars currently used in local production, Zhendao 88 (japonica) and Shanyou 63 (indica hybrid, first generation), were grown in the paddy field. Seedlings were raised in the field with sowing date on 10 and 11 May and transplanted on 10 and 11 June at a hill spacing of 0.20 by 0.16 m with two seedlings per hill. 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 mid-tillering (40 kg ha–1) and at panicle initiation (25 kg ha–1). The heading date for both cultivars (50% of plants) was on 16 to 17 August, and plants were harvested on 5 to 6 October. Except for drainage at the end of tillering, the field was kept at 1- to 2-cm water level until 6 d after heading (DAH), when AWD irrigation treatments were initiated.

Treatments
The experiment was laid out in a complete randomized block design with three replicates. Plot dimension was 4.2 by 3.2 m and plots were separated by a 1-m wide alley using plastic film inserted into the soil to a depth of 50 cm. Three irrigation regimes (treatments), alternate wetting and WMD, alternate wetting and WSD, and CI, were conducted from 6 DAH to maturity. In WMD, plants were not rewatered until the soil water potential reached –25 kPa (soil moisture content 0.161 g g–1) at 15- to 20-cm depth. While in WSD, water was withheld till soil water potential reached –50 kPa (soil moisture content 0.136 g g–1) at 15- to 20-cm depth. The CI regime was continuously flooded with 2- to 3-cm water level in the plot. Soil water potentials of –25 kPa and –50 kPa were chosen as our earlier work (Yang et al., 2001) has shown that a mild soil-drying (soil water potential –25 kPa at 15–20 cm depth) during the grain-filling period did not seriously reduce grain yield, whereas a severe soil drying (soil water potential –50 kPa) did. Soil water potential was monitored at 15- to 20-cm soil depth with a tensionmeter consisting of a sensor of 5-cm length. One reason for the depth selected is that most of rice roots are concentrated down to 15-cm soil depth and the tensionmeters were placed immediately below this depth (Kukal and Aggarwal, 2003; Kukal et al., 2005). Also, most studies that have used tensionmeters in rice have monitored soil water status at this depth (Cai et al., 2002; Kukal et al., 2005; Belder et al., 2005; Tuong et al., 2005; Tao et al., 2006), and comparisons can be made between our data and others. Four tensionmeters were installed in each plot, and readings were recorded at 1200 h each day. When the reading reached the threshold, an amount of 15 mm water was applied to the plots. The irrigation water was applied via pipelines, and the amount of irrigation water was monitored with the flow meter installed in the irrigation pipelines. A rain shelter consisting of a steel-frame covered with plastic sheet was used in each block to protect the plot during rain.

Physiological Measurements
At heading time, 80 to 100 panicles that headed on the same day were tagged in each plot. Ten to 15 tagged panicles from each plot were sampled at 6, 17, 28, and 42 DAH, respectively, when soil water potentials were –25 kPa in WMD and –50 kPa in WSD (refer to Fig. 1 ). All grains from each panicle were removed. The sampled grains were used to measure activities of SuS, AGP, StS, and SBE according to Yang et al. (2003). Briefly, 40 to 50 dehulled grains were homogenized with a pestle in a pre-cooled mortar that contained 8 mL frozen extraction medium: 100 mM HEPES-NaOH (pH 7.6), 8 mM MgCl2, 5 mM dithiothreital, 2 mM EDTA, 12.5% (v/v) glycerol, and 5% (w/v) insoluble polyvinylpyrrolidone 40. After being filtered through four layers of cheesecloth, the homogenate was centrifuged at 12,000 g for 10 min, and the supernatant was used for the enzyme assay. The SuS was assayed in the cleavage direction and analyzed as described by Ranwala and Miller (1998). The StS was determined according to the method Schaffer and Petreikov (1997). The AGP and SBE were assayed by the method of Nakamura et al. (1989).


Figure 1
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  		Fig. 1. Soil water potential of rice cv. (A) Zhendao 88 and (B) Shanyou 63 under various irrigation treatments. The abbreviations CI, WMD, and WSD indicate conventional irrigation, alternate wetting and moderate soil drying, and alternate wetting and severe soil drying, respectively, during grain filling (6–48 d after heading). Data are averages observed for the two study years. Vertical bars represent ± standard error of the mean (n = 24) where these exceed the size of the symbol.

 
The leaf water potential and photosynthetic rate of the flag leaves and root activity were measured on the aforementioned dates. Leaf water potentials of the flag leaves were measured at midday (1130 h). 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.

A gas exchange analyzer (Li-Cor 6400 portable photosynthesis measurement system, Li-Cor, Lincoln, NE) was used for the measurement of the photosynthetic rate during 0900 to 1100 h when photosynthetic active radiation above the canopy was 1000 to 1100 µmol m–2 s–1. Six leaves were used for each treatment.

Root activity was measured according to Ramasamy et al. (1997). The roots in soil were dug out by a spade (the soil volume around roots was 20 by 20 by 20 cm). The roots were carefully rinsed and detached from their nodal bases. Both fresh weight and dry weight (dried in an oven at 70°C) of roots were weighed. The oxidizing activity of the roots was determined by measuring oxidation of alpha-naphthylamine ({alpha}-NA). Two grams of fresh roots were transferred into a 150 mL flask containing 50 mL of 20 mg L–1 {alpha}-NA, The flaks were incubated for 2 h at room temperature in an end-over-end shaker. After incubation, the aliquots were filtered and 2 mL was mixed with 1 mL NaNO3 (1.18 mM) and 1 mL sulphanilic acid and the resulting color was measured by a spectrophotometer. Root activity was expressed as µg {alpha}-NA per gram dry weight (DW) per hour (µg {alpha} – NA g–1 DW h–1).

Harvesting and Grain Quality Measurements
All plants were harvested on 5 to 6 October. Grain yield was determined from all plants from a 4 m2 site (except border plants) in each plot and adjusted to a moisture content of 0.14 g H2O g–1 fresh weight. Aboveground biomass and yield components, that is, number of panicles per square meter, percentage of filled grains and grain weight, were determined from 50 plants (excluding the border ones) sampled randomly from each plot. The percentage of filled grains was defined as the filled grains (specific gravity ≥ 1.06 g cm–3) as a percentage of total number of spikelets. The total number of spikelets were calculated from the grain yield, grain weight (14% moisture content), and percentage of filled grains, that is, total number of spikelets = grain yield/(1000-grain weight x percentage of filled grains).

About 500 g of grains harvested from each plot were dried at 40°C in a forced-air oven for quality analysis. A 150-g sample of rice grains were twice passed through a dehusker, polished, then separated into broken and unbroken grains. The brown rice rate, milled rice rate, and head rice rate were expressed as percentages of total (150 g) rice grains. The length and breadth of 10 milled grains from each plot were measured using a vernier micrometer. Chalkiness was evaluated visually on 100 milled grains per plot. Grains containing 20% of more of white belly, white center, and white back or a combination of these were considered chalky. Gel consistency, alkali spreading value, and amylose content were measured according to Rice Quality Measurement Standards (Ministry of Agriculture, PR China, 1988).

As starch paste parameters generated from a rapid viscoanalyzer (RVA) reflect starch gelatinization, disintegration, swelling and gelling ability, they are often used to evaluate the quality (Ryu et al., 1993; Guha et al., 1998; Allahgholipour et al., 2006). In this experiment, rice starch viscosity characters were evaluated using a RVA (Model RVA-3D; Newport Scientific, Sydney, Australia) as described by Han et al. (2004). Viscosity values were recorded as centipoises (cp). The original components of starch viscosity characters include peak viscosity, hot viscosity, and cool (final) viscosity. The secondary components, such as breakdown and setback, were calculated from the original components. A breakdown value was calculated by subtracting hot viscosity from peak viscosity, while a setback value was calculated by subtracting peak viscosity from cool viscosity.

Statistical Analysis
Analysis of variance was performed using SAS/STAT statistical analysis package (version 6.12, SAS Institute, Cary, NC). The statistical model used included sources of variation due to replication, year, cultivar, irrigation treatment, and interactions of year x cultivar, year x irrigation treatment, and cultivar x irrigation treatment. Data from each sampling date were analyzed separately. Means were tested by least significant difference at P = 0.05 [LSD (0.05)].


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Differences in Experimental Factors
Table 1 shows computed F values for the differences of grain yield and some main quality traits of rice between/among years, cultivars, and irrigation treatments. There existed significant differences between cultivars and among irrigation treatments. The differences in data across years and in interactions between year and cultivar, year and irrigation treatment, and cultivar and irrigation treatment were not significant (Table 1). Similar results were obtained in all other measurements (data not shown). Therefore data from both years were averaged.


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  		Table 1. Analysis of variance of F values of grain yield and some main quality traits of rice between/among years, cultivars, and irrigation treatments

 
Soil and Leaf Water Potentials
It took 5 to 8 d to reach soil water potential of –25 kPa and 11 to 16 d reach soil water potential of –50 kPa (Fig. 1A, B). Water application after heading was 110 mm to the WMD and 62 mm to the WSD treatment, which was only 44 and 25%, respectively, of that (250 mm) applied to CI treatments when the rainfall (44 mm in 2004 and 63 mm in 2005) during the treatment period was not calculated.

Figure 2 shows the changes in mid-day (1130 h) leaf water potentials under the three treatments. For plants grown under CI conditions, mid-day leaf water potentials decreased gradually during grain filling, from approximately –0.75 MPa at the beginning of measurements to about –1.30 MPa on 42 DAH. Leaf water potentials were reduced as soil water potential decreased, and ranged from –1.11 to –1.58 MPa when soil water potential was –25 kPa in WMD and –1.37 to –2.04 MPa when soil water potential was –50 kPa in WSD (Fig. 2A, B). As mid-day leaf water potentials lower than –1.60 MPa during grain filling are considered to be typical stress values for rice (Kobata and Takami, 1981; Yang et al., 2001; Cai et al., 2002), the results indicate that a mild soil-drying during the grain-filling period would not seriously affect plant water status.


Figure 2
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  		Fig. 2. Leaf water potential of rice cv. (A) Zhendao 88 and (B) Shanyou 63 under various irrigation treatments. The abbreviations CI, WMD, and WSD indicate conventional irrigation, alternate wetting and moderate soil drying, and alternate wetting and severe soil drying, respectively, during grain filling (6–48 d after heading). Measurements were made on the flag leaves at midday (1130 h) when soil water potentials were –25 kPa in WMD and –50 kPa in WSD. Data are averages observed for the two study years. Vertical bars represent ± standard error of the mean (n = 12) where these exceed the size of the symbol.

 
Root Activity, Photosynthetic Rate, and Enzymatic Activity in Grains
The root oxidation activity was significantly lower for WSD when compared to other treatments (Fig. 3A , B). The WMD significantly increased root oxidation activity at 28 and 42 DAH.


Figure 3
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  		Fig. 3. Root oxidation activity of rice cv. (A) Zhendao 88 and (B) Shanyou 63 under various irrigation treatments. The abbreviations CI, WMD, and WSD indicate conventional irrigation, alternate wetting and moderate soil drying, and alternate wetting and severe soil drying, respectively, during grain filling (6–48 d after heading). Measurements were made when soil water potentials were –25 kPa in WMD and –50 kPa in WSD. Data are averages observed for the two study years. Vertical bars represent ± standard error of the mean (n = 12) where these exceed the size of the symbol.

 
A similar observation was obtained on the photosynthetic rate of the flag leaf (Fig. 4A , B). The WSD significantly reduced the photosynthetic rate during the treatment period, while WMD significantly increased it at the late grain-filling stage (28 and 42 DAH) when compared to CI.


Figure 4
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  		Fig. 4. Photosynthetic rate of the flag leaf of rice cultivars (A) Zhendao 88 and (B) Shanyou 63 under various irrigation treatments. The abbreviations CI, WMD, and WSD indicate conventional irrigation, alternate wetting and moderate soil drying, and alternate wetting and severe soil drying, respectively, during grain filling (6–48 d after heading). Measurements were made when soil water potentials were –25 kPa in WMD and –50 kPa in WSD. Data are averages observed for the two study years. Vertical bars represent ± standard error of the mean (n = 12) where these exceed the size of the symbol.

 
Activities of the key enzymes involved in sucrose-to-starch conversion in grains, SuS, AGP, StS, and SBE, showed little difference at the early and mid-grain-filling stages (6 and 17 DAH) but differed markedly at the late grain-filling stage (Fig. 5A –H). The WMD significantly enhanced, whereas WSD inhibited, the activities of SuS, AGP, StS and SBE in the grain during the late grain–filling period.


Figure 5
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  		Fig. 5. Activities of sucrose synthase (SuS) (A, B), adenosine diphosphate glucose pyrophosphorylase (AGP) (C, D), starch synthase (StS) (E, F), and starch-branching enzyme (SBE) (G, H) in the grains of rice cv. Zhendao 88 (A, C, E, G) and Shanyou 63 (B, D, F, H) under various irrigation treatments. The abbreviations CI, WMD, and WSD indicate conventional irrigation, alternate wetting and moderate soil drying, and alternate wetting and severe soil drying, respectively, during grain filling (6–48 d after heading). Measurements were made when soil water potentials were –25 kPa in WMD and –50 kPa in WSD. Data are averages observed for the two study years. Vertical bars represent ± standard error of the mean (n = 12) where these exceed the size of the symbol.

 
Grain Yield and Harvest Index
As the irrigation treatments started after heading, neither the panicle number per m–2 nor the spikelets per panicle were influenced by the treatments, and the differences in sink size among the three treatments were insignificant (Table 2 ). However, grain yield varied significantly with the treatments. The percentage of filled grains, grain weight, and grain yield under WMD were increased by 4.4 to 5.2%, 3.4 to 4.5%, and 9.3 to 9.5%, respectively, and were significantly higher than those under CI (P < 0.05). However, they were all markedly reduced under WSD. The harvest index was significantly increased under both WMD and WSD, with the latter increased more than the former (Table 2), indicating that soil drying during grain filling enhances the partitioning of assimilates form vegetative tissues to grains.


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  		Table 2. Effects of alternate wetting and soil drying during grain filling on grain yield and harvest index of rice.

 
Grain Quality
Similar to grain yield, milling quality traits studied, such as brown rice rate, milled rice rate and head rice rate, all increased significantly under WMD, whereas they were reduced under WSD, when compared with those under CI (Table 3 ). The WMD significantly reduced the percentage of chalky grains and chalkiness, and WSD showed an opposite effect (Table 4 ). The irrigation treatments had no significant effects on the kernel length (P > 0.05), but WSD significantly reduced the breadth of a kernel, and increased the ratio of the length to breadth as a result (Table 4). Except the gel consistency which was significantly reduced under WSD, neither alkali spreading value nor amylose content was influenced by the irrigation treatments (Table 5 ).


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  		Table 3. Effects of alternate wetting and soil drying during grain filling on milling quality of rice.

 

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  		Table 4. Effects of alternate wetting and soil drying during grain filling on appearance quality of rice.

 

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  		Table 5. Effects of alternate wetting and soil drying during grain filling on cooking and eating quality of rice.

 
The components of starch viscosity characters, that is, peak viscosity, hot viscosity, cool viscosity and values of breakdown and setback, varied significantly among the treatments (Table 6 ). The WMD significantly increased values of peak viscosity and breakdown, but reduced values of hot viscosity, cool viscosity and setback. The results were reversed when WSD was imposed during grain filling (Table 6).


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  		Table 6. Effects of alternate wetting and soil drying during grain filling on starch viscosity characters of rice (unit: centipoises).

 
Under the same irrigation treatment, the cultivar of Zhendao 88 exhibited shorter kernel length, wider breadth, and higher gel consistency, alkali spreading value, and amylose content than the cultivar of Shanyou 63 (refer to Tables 4 and 5). The difference in these quality traits between the two cultivars may be attributed to the genetic difference between japonica and indica rice.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In previous studies on AWD irrigation, grain yield of rice was increased (Tuong et al., 2005; Won et al., 2005; Yang et al., 2007) but reduced in others (Mishra et al., 1990; Tabbal et al., 2002; Belder et al., 2004) when compared with continuously submerged conditions. The discrepancies between the studies are probably attributed to the variations in soil hydrological conditions and the timing of the irrigation method applied (Belder et al., 2004). The results of the present study showed that grain yield significantly decreased when soil water potential was reduced to –50 kPa when AWD was imposed during the grain-filling period. However, grain yield was significantly increased when soil water potential was reduced to –25 kPa in AWD (Table 2). Our results indicate that the drying condition in AWD is the most important factor affecting yield, and soil drying to –25 kPa is beneficial during the grain-filling period. It is noteworthy, however, that the grain-filling phase in this study lasted for about 50 d. Potential water savings during the grain-filling phase might be reduced in areas where the grain-filling phase in considerably shorter due to higher temperatures during this phase. Moreover, the effect of drought stress on leaves usually results in a temperature increase of the leaves (Solfield et al., 1977; Chowdhury and Wardlaw, 1978). Under relatively cool conditions like September and October in eastern China this temperature increase will be less harmful than in a tropical growing region.

There are reports that the grain quality of rice is sensitive to soil water status during grain filling and even a mild water stress (soil water potential of –20 kPa at 15–20 cm depth) during this period would reduce the quality of both japonica and indica cultivars (Cai et al., 2002; Zheng et al., 2003). We observed that WSD did exhibit detrimental effect on grain quality (Tables 3–6GoGoGo). The WMD, however, significantly increased brown rice rate, milled rice rate, and head rice rate, while significantly reduced the percentage of chalky kernels and chalkiness (Tables 3 and 4). Though there was no significant effect of WMD on the cooking quality (gel consistency, alkali spreading value, and amylose content), WMD significantly affected the parameters of starch paste characters, and increased values of peak viscosity and breakdown. It reduced values of hot viscosity, cool viscosity, and setback (Table 6). It is proposed that higher values of peak viscosity and breakdown and lower values of hot viscosity, cool viscosity, and setback are associated with better taste of rice (Bason et al., 1994; Shu et al., 1998; Han et al., 2004). The results in this study demonstrate that WMD during grain filling can improve rice quality.

The relationship between soil water potential and plant physiological processes has been discussed in the literature (Wopereis et al., 1996; Lu et al., 2000; Bouman and Tuong, 2001). However, the mechanisms that led to higher grain yield and higher grain quality of rice under the moderate soil drying regime are not well understood. We observed that both WMD and WSD increased harvest index (Table 2), suggesting that both treatments enhance the partitioning or remobilization of assimilates from vegetative tissues to grains. However, the mid-day leaf potential and photosynthetic rate were markedly reduced under WSD (Fig. 2 and 4), and the loss from photosynthesis could not compensate the gain from the partitioning of assimilates, leading to the reduction in grain yield. On the other hand, the mid-day leaf water potential was not seriously reduced and the photosynthetic rate significantly increased at the late grain-filling stage under WMD (Fig. 2 and 4). The enhancements in both assimilate production and harvest index may have contributed to the increase in grain yield under WMD.

Interestingly, we observed that WMD significantly enhanced activities of SuS, AGP, StS, and SBE in grains during the grain-filling period (Fig. 5). The SuS is proposed to be a key enzyme in sucrose–starch metabolic pathway, and its activity is regarded as a biochemical marker of sink strength (Kato, 1995). On the other hand, AGP, StS and SBE are considered as key enzymes involved in starch synthesis, and their activities are closely associated with the rate and quantity of starch synthesis (Hawker and Jenner, 1993; Ahmadi and Baker, 2001; Hurkman et al., 2003). We speculate that enhancement in activities of these enzymes under WMD may benefit grain filling and lead to higher weight and better quality of grains.

It is notable that activities of SuS, AGP, StS, and SBE in the rice grains exhibited a similar changing pattern during the grain-filling period (Fig. 5). A similar observation was also reported in wheat (Triticum aestivum L.) grains (Jiang et al., 2003). The maximum activities of the enzymes were concomitant with the maximum rate of starch accumulation in both rice and wheat grains (Yang et al., 2001; Jiang et al., 2003). The results imply that the four enzymes may relate to each other and changes in their activities coincide synchronously with starch synthesis. Obviously, the relationship between the enzymes and their roles in the formation of grain quality need to be elucidated.

How could the source capacity (the photosynthetic rate of the flag leaf) and sink strength (activities of the key enzymes involved in starch synthesis in grains) be enhanced under WMD? Our results showed that WMD significantly increased, whereas WSD reduced root oxidation activity at the late grain-filling stage (Fig. 3), indicating that a moderate soil drying in AWD benefits, while severe soil drying harms root activity. Increase in root oxidation activity in WMD may be attributed to the improvement in oxygen supply to the root system with potential advantages for nutrient uptake (Stoop et al., 2002; Yang et al., 2004). It is also possible that a greater photosynthetic rate of shoots in WMD secures high root activity by supplying a sufficient amount of photosynthates to roots. Conversely, an increase in root oxidation activity in WMD contributes to a greater source capacity and sink strength. On the other hand, the WSD reduces the yield and quality due to the decreases in source capacity and sink strength resulting form the low root activity.

In conclusion, the WMD imposed during the grain-filling period not only increased grain yield and saved water, but also improved grain quality. Increases in harvest index, photosynthetic rate, root activity, and activities of the key enzymes involved in the sucrose–starch metabolic pathway in grains contributed to the improvement of the yield and quality under such an irrigation system. On the other hand, the WSD reduced grain yield and quality due to the decreases in source capacity and sink strength.


   ACKNOWLEDGMENTS
 
We are grateful for grants from the National Natural Science Foundation of China (Project No. 30671225), the Natural Science Foundation of Jiangsu Province (BK2006069), Research Grant Council of Hong Kong (HKBU 2465/05M), Hong Kong University Grants Committee (AOE/B-07/99), and Hong Kong Baptist University Matching Research Fund.

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H. Zhang, Y. Xue, Z. Wang, J. Yang, and J. Zhang
An Alternate Wetting and Moderate Soil Drying Regime Improves Root and Shoot Growth in Rice
Crop Sci., October 22, 2009; 49(6): 2246 - 2260.
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