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CROP PHYSIOLOGY & METABOLISM

Remobilization of Carbon Reserves Is Improved by Controlled Soil-Drying during Grain Filling of Wheat

^a College of Agric., Yangzhou Univ., Yangzhou, Jiangsu, People's Republic of China
^b Dep. of Biology, Hong Kong Baptist Univ., Kowloon Tong, Hong Kong, People's Republic of China
^c Agric. Bureau of Liangyungang, Jiangsu, People's Republic of China

jzhang{at}net1.hkbu.edu.hk

	ABSTRACT

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Heavy use of N fertilizers delays plant senescence in wheat (Triticum aestivum L.) and results in slow grain filling and a low harvest index. This study investigated whether senescence is enhanced by a controlled soil drying during the late grain-filling stage and whether such an enhancement can lead to better remobilization of reserved C to the grains. Two wheat cultivars were raised in pots and one was grown in the field. Two levels of nitrogen, either normal (NN) or high (HN) amounts, were applied at heading. Controlled soil drying was imposed 9 d after anthesis until maturity. Leaf water potential and conductance for the soil-drying treatments were lower during the day but completely recovered by the early morning. Photosynthetic rate and chlorophyll content in the flag leaves declined more quickly in the soil-drying treatments than in the well-watered ones, indicating that soil drying enhanced senescence. Nonstructural carbohydrate in the stem and sheath at maturity was greatly reduced, and the partitioning of fixed C from flag leaves into the grains and the final harvest index were significantly increased by soil drying under both nitrogen treatments for the three cultivars. Soil drying shortened the grain-filling period but grain-filling rate was substantially increased by all the soil drying treatments except one NN treatment with severe soil drying in the pot experiment. Soil drying actually increased the grain yield at HN in both experiments. We conclude that senescence induced by controlled soil drying during grain filling can promote the remobilization of prestored assimilates to the grains, accelerate grain filling, and improve yield in cases where senescence is unfavorably delayed by heavy use of nitrogen.

Abbreviations: DAA, days after anthesis • DAS, days after sowing • g_leaf, leaf conductance • HN, high nitrogen treatment • MD, mild soil drying • NN, normal nitrogen treatment • NSC, nonstructural carbohydrate • _leaf, leaf water potential • _soil, soil water potential • PAR, photosynthetically active radiation • Pr, photosynthetic rate • SD, severe soil drying • SE, standard error • WS, water stressed • WW, well watered

	INTRODUCTION

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REMOBILIZATION AND TRANSFER of the stored assimilates in vegetative tissues to the grain in monocarpic plants such as wheat require the initiation of whole plant senescence (Gan and Amasino, 1997; Nooden et al., 1997). Delayed senescence, which in practice is induced by either too much nitrogen fertilizer or an adoption of lodging-resistant cultivars that stay "green" for too long, results in much nonstructural carbohydrate left in the straw and leads to a low harvest index. Grain filling in wheat depends on C from two resources: current assimilation and remobilization of reserves stored in the stem and other parts (mainly the sheath) either pre- or postanthesis (Pheloung and Siddique, 1991; Kobata et al., 1992). Normally, preanthesis assimilate reserves in the stem and sheath of wheat contribute 25 to 33% of the final grain weight (Rawson and Evans, 1971; Gallagher et al., 1976; Hans, 1993; Gebbing and Schnyder, 1999). Remobilization of reserves to the grain is critical for grain yield if the plants are subjected to water stress during grain filling (Nicolas et al., 1985a; Palta et al., 1994; Ehdaie and Waines, 1996). We have observed that postanthesis soil drying accelerates the grain filling and increases harvest index (Zhang et al., 1998). Early senescence caused by postanthesis water deficit, however, reduces photosynthesis, shortens the grain-filling period, and finally results in reduction of grain weight (Bidinger et al., 1977; Brown et al., 1991; Palta et al., 1994; Zhang et al., 1998).

This investigation studied if a mild soil drying during grain filling could accelerate grain filling from preanthesis assimilate reserves, which may outweigh any loss of photosynthesis as a result of drying-induced early senescence. Wheat production in northern China suffers from a continental hot, dry wind usually at the end of the growing season. If crop maturation is unfavorably delayed, such wind may dehydrate the wheat rapidly and lead to reduced grain weight. If an applied soil drying accelerates senescence so that the wheat matures before the adverse weather occurs, grain filling and hence yield might be improved.

	Materials and methods

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Pot Experiment
A pot experiment was conducted at Yangzhou University, Yangzhou, China (32°30'N, 119°25'E). Each porcelain pot (30-cm height, 25-cm diameter, and 14.72-L volume) was filled with 18 kg sandy loam soil [Typic fluvaquents, Entisols (U.S. taxonomy)] which contained 24.5 g kg^-1 organic matter and available N-P-K at 105, 33.5 and 66.0 mg kg^-1, respectively. On the day of sowing (10 November), 1 g N as urea and 0.2 g P as single superphosphate were mixed into the soil of each pot. At 30 and 112 d after sowing (DAS), 0.4 g and 0.5 g N as urea were top-dressed into each pot.

Two semi-winter wheat cultivars (requirement of temperature and duration for vernalization is between winter and spring wheat), `Yangmai 158' and `Yangmai 931', were used. Sixteen seeds were sown in each pot. When plants reached the three-leaf stage, they were thinned to eight plants per pot (equivalent to a density of 163 plants m^-2). The plants were watered daily by hand to maintain a soil water content close to field capacity (soil water potential, _soil, at -0.01 to -0.02 MPa) until 9 d after anthesis (DAA), when soil-drying treatments were applied. Yangmai 158 and Yangmai 931 headed on 154 and 155 DAS, respectively, and flowered during 161 to 167 DAS. The daily temperatures, averaged per 10 d from anthesis to harvest (161–204 DAS), were 15.3, 17.1, 20.7, 21.2, and 24.6°C, respectively, for each 10-d period.

The crop was sheltered from rain by a removable polyethylene shelter. At the initial heading (10% of plants headed), the total number of plants was adjusted to 36 in each pot by removing 1 to 3 nonproductive tillers from the pot to minimize possible non-treatment effect.

The experiment was a 2 by 2 by 3 (two cultivars, two levels of N, and three levels of soil drying) design with 12-treatment combinations. Each treatment had 30 pots, of which 24 were used for destructive sampling during the experiment and six for harvest. When the main stem of wheat in pots was at 50% heading (154 DAS for Yangmai 158 and 155 DAS for Yangmai 931), two levels of N treatments were applied. Half the plants were top-dressed with either 0.5 g N (normal amount, NN) or 1.2 g N (high amount, HN) as urea per pot. From 9 DAA (163 DAS for both cultivars) to maturity, three levels of (soil drying were imposed to the plants of both NN and HN treatments. The well-watered (WW) treatment was maintained at -0.02 MPa, a mild soil drying (MD) was maintained at -0.04 MPa, and a severe soil drying (SD) was maintained at -0.06 MPa. Soil water potential was monitored at the 15- to 20-cm soil depth with a tension meter consisting of a sensor of 5-cm length was installed in each pot to monitor. Tension meter readings were recorded every 2 h from 0600 to 1800 h. When the reading dropped to the designed value, 100, 80, or 60 mL tap water was added into the pots for WW, MD, and SD treatments, respectively.

Field Experiment
A field experiment was conducted at Lianyungang Agricultural Research Institute, Lianyungang, China (34°50'N, 119°10'E), from November 1998 to June 1999. A winter wheat cultivar, `Yanfu 188', was used. The sowing date was 1 November, and the plant density was adjusted to 156 plants m^-2 at three-leaf age. The soil of the field was loamy clay [Typic agrudalfs, Alfisolfs (U.S. taxonomy)] with 20.1 g kg^-1 organic matter, available N-P-K at 103, 19.0 and 153.5 mg kg^-1, respectively. On the sowing day, 13.8 g N m^-2 as urea and 4.05 g P m^-2 as single superphosphate were applied into the soil. On 32 and 123 DAS, 6.9 g and 6.4 g N m^-2 as urea were top-dressed, respectively. The soil water content was maintained close to field capacity (_soil at -0.02 to -0.025 MPa) by manual watering until 10 DAA, when soil-drying treatments were applied. The cultivar headed on 166 DAS, and flowered from 174 to 181 DAS. The temperatures, averaged per 10-d from anthesis to harvest (174 to 217 DAS), were 16.8, 20.2, 20.6, 20.8 and 22.9°C, respectively, for each 10-d period.

A 2 2 (two levels of nitrogen and two levels of soil drying) design was conducted. Each of the treatments had three plots as repetitions in a complete randomized block design. Plot dimension was in 5 by 10 m and plots were separated by a ridge (20 cm in width) covered by plastic film. At heading (166 DAS), 7.5 and 15 g N m^-2 as urea were applied to the NN and HN treatments, respectively.

The _soil at 15-cm soil depth was kept at -0.025MPa (WW) and -0.1 MPa (water deficit stressed, WS) in the respective plots from 10 DAA (174 DAS) to maturity. The _soil was monitored every 4 d during the treatment period, and plots were watered manually. When the soil reached the designated moisture level, 10 to 12 L of water per m² was applied to WW treatments, and 5-6 L per m² to WS ones. WW plots were watered seven times and WS plots three times during the treatment period. Soil columns (5.4-cm diameter) at 12- to 18-cm depth were extracted with a soil borer, and the samples were assayed for their volumetric water content (V). Water potential was calculated according to the moisture retention equation between V (g cm^-3) and _soil that was obtained with a pressure plate. A rain shelter made of a steel frame and covered with a removable plastic sheet was used to exclude rain from the MD + SD plots.

Physiological Measurements
Leaf water potential (_leaf) and leaf conductance (g_leaf) were measured at 2-h intervals 20 and 21 DAA for both Yangmai 158 and Yangmai 931, and 24 DAA for Yanfu 188 when sky was clear. Well illuminated flag leaves were chosen randomly for such measurements. A pressure chamber (Model 3000, Soil Moisture Equipment Corp., Santa Barbara, CA, USA) was used for leaf water potential measurement with six leaves for each treatment. Leaf conductance of the flag leaves was measured with a gas exchange analyzer (CID-PS CO₂ Analyzer System, CID, Vancouver, WA, USA).

The gas exchange system was also used to measure the photosynthetic rate (Pr) of the flag leaves on 9, 15, 21, and 27 DAA for the pot experiment and on 10, 16, 21, and 30 DAA for the field experiment. Measurements were made between 0900 to 1100 h when PAR above the canopy was 850 to 1050 µmol m^-2 s^-1. The flag leaves were sampled on the above dates for the measurement of chlorophyll content. Chlorophyll was extracted by shaking in methanol overnight and determined as described by Holden (1976). Six leaves were used for each treatment.

Radioactive Labeling
In the pot experiment, flag leaves from six main stems from each treatment were labeled with ¹⁴CO₂ at heading. Labeling was between 0900 and 1100 h on a clear day when PAR on the top of the canopy ranged between 900–1100 µmol m^-2 s^-1. The whole flag leaf was placed in a polyethylene chamber (25-cm length and 4-cm diameter) and sealed with tape and plasticine to form 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 specific radioactivity of ¹⁴C at 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, stems plus sheaths, and panicles. Samples were dried at 80°C to constant weight, ground into powder, and then extracted by shaking in 630 g L^-1 boiling ethanol. The radioactivity of ¹⁴C in the extracted aliquots was counted by a liquid scintillation counter (Beckman Instruments Inc., Fullerton, CA, USA). Radioactivity distribution in each part of the plant was expressed as a percentage of total radioactivity remaining in the aboveground portion of the plant.

Sampling and Harvesting
Two hundred spikes that headed on the same day were tagged for each treatment. The flowering date and the position of each floret on the tagged spikes were recorded. Sixteen to 18 tagged spikes from each treatment were sampled every 3 d from anthesis to maturity. The sampled spikes were divided into two groups (8–9 spikes each) as subsamples. Kernels that developed from florets 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 kernel weight (mg), A is the final grain weight, 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 that when W was from 5% (t1) to 95% (t2) of A. The average grain filling rate during this period was calculated from t1 to t2.

Total above ground 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, stems and sheaths, and spikes. All plant parts were dried at 80 to constant weight and weighed.

Fifteen to 20 plants were sampled from each treatment at 5-d intervals (for the pot experiment) and 6-d intervals (for the field experiment) from anthesis to maturity for the measurement of nonstructural carbohydrate (NSC) in stems, sheaths, and leaves. The method for extraction of NSC was modified according to the method 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 ethanol (density at 630 g L^-1) and kept in a water bath at 80°C for 30 min. The tube wan then centrifuged at about 900 x g for 20 min after cooling. The supernatant was collected and the extraction was repeated for 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 then diluted into 25 mL with distilled water. The concentration of sugars in the extract was then analyzed as described by Somogyi (1945).

The residue left 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 15 min. Two milliliters of 9.36 M HClO₄ was added to the tube after cooling. The solution was shaken for 15 min. The extract was then made up to about 10 mL and centrifuged at about 900 x g 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 in six pots of each treatment in the pot experiment and all the plants (except border) in the plots in the field experiment were harvested at maturity for the determination of grain yield. Yield components, i.e., the spikes per square meter or per pot kernels per spike kernel weight, were determined from plants in the six pots or from plants harvested from a 1-m² site (excluding the border plants) randomly sampled from each plot.

The results were analyzed for variance by ANOVA. Means were tested by least significant difference at P = 0.05 level (LSD 0.05).

	Results

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The _soil in the pots reached -0.04 MPa and -0.06 MPa in 10 to 12 h and 50 to 54 h, respectively, after withholding water (Fig. 1A and B) . The pots of the HN treatment reached lower _soil earlier than those of the NN treatment. The two cultivars were very similar. When _soil in 15- to 20-cm depth in the pot reached -0.04 and -0.06 MPa, the _soil in 5- to 10-cm soil layer was -0.11 and -0.19 MPa, respectively (Zhu et al., 1994). The _soil in 12- to 15-cm soil depth in the field took 9 d to reach -0.1 MPa in plots with HN and 11 d for plots with NN after withholding water (Fig. 1C). Faster soil drying under the HN treatment indicated that leaves with better nitrogen nutrition lost more water, probably because of high leaf conductance and great leaf area index. During the daytime, the _leaf was lower under soil-drying treatments (MD, SD, and WS) than under WW ones (Fig. 2) . Plants under HN had lower midday _leaf than those under NN, even though _soil was kept at the same level, again suggesting that the former might have lost water more quickly than the latter. The differences in _leaf between the soil-drying and WW treatments were small at predawn (0600 h) and in early morning in both pot and field experiments, indicating that plants subjected to controlled soil drying could rehydrate overnight.

View larger version (22K): [in this window] [in a new window]   Fig. 1 Soil water potential of semi-winter wheat cultivars Yangmai 158 (A) and Yangmai 931 (B) in the pot experiment and winter wheat cultivar Yanfu 188 (C) in the field experiment when controlled soil drying was applied. Symbols NN and HN indicate normal and high levels of N application at heading time, and WW, MD, SD, and WS are well watered, mildly dried, severely dried (for the pot experiment), and water stressed (for the field experiment) during the grain filling. Vertical bars represent ± SE of the mean where values exceed the size of the symbol

View larger version (22K): [in this window] [in a new window]   Fig. 2 Diurnal changes in flag leaf water potential of semi-winter wheat cultivars Yangmai 158 (A) and Yangmai 931 (B) in the pot experiment and winter wheat cultivar Yanfu 188 (C) in the field experiment after controlled soil drying was applied. Treatment details are the same as in Fig. 1. Measurements were made 20 DAA for Yangmai 158, 21 DAA for Yangmai 931, and 24 DAA for Yanfu 188. Vertical bars represent ± SE of the mean where values exceed the size of the symbol

View larger version (25K): [in this window] [in a new window]   Fig. 3 Diurnal changes in flag leaf conductance of semi-winter wheat cultivars Yangmai 158 (A) and Yangmai 931 (B) in the pot experiment and winter wheat cultivar Yanfu 188 (C) in the field experiment after controlled soil drying was applied. Treatment details are the same as in Fig. 1. Measurements were made 20 DAA for Yangmai 158, 21 DAA for Yangmai 931, and 24 DAA for Yanfu 188. Vertical bars represent ± SE of the mean where values exceed the size of the symbol

View larger version (21K): [in this window] [in a new window]   Fig. 4 Photosynthetic rates of flag leaves of semi-winter wheat cultivars Yangmai 158 (A) and Yangmai 931 (B) in the pot experiment and winter wheat cultivar Yanfu 188 (C) in the field experiment after controlled soil drying was applied. Treatment details are the same as in Fig. 1. Arrows in the figures indicate the start of soil drying treatments. Vertical bars represent ± SE of the mean where values exceed the size of the symbol

View larger version (19K): [in this window] [in a new window]   Fig. 5 Chlorophyll concentrations in the flag leaves of semi-winter wheat cultivars Yangmai 158 (A) and Yangmai 931 (B) in the pot experiment and winter wheat cultivar Yanfu 188 (C) in the field experiment after controlled soil drying was applied. Treatment details are the same as in Fig. 1. Arrows in the figure indicate the start of soil drying treatments. Vertical bars represent ± SE of the mean where values exceed the size of the symbol

View larger version (21K): [in this window] [in a new window]   Fig. 6 Nonstructural carbohydrate concentrations in the stem and sheath of semi-winter wheat cultivars Yangmai 158 (A) and Yangmai 931 (B) in the pot experiment and winter wheat cultivar Yanfu 188 (C) in the field experiment after controlled soil drying was applied. Treatment details are the same as in the Fig. 1. Arrows in the figure indicate the start of soil drying treatments. Vertical bars represent ± SE of the mean where values exceed the size of the symbol

	Discussion

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When a normal amount of N was applied, the kernel weight under the soil drying treatments was reduced compared with the plants under the well-watered treatment, indicating that the loss of photosynthesis could not compensate for the gain from increased remobilization of carbon reserves. When a high amount of N was applied, kernel weight was increased under water deficit (Table 3). The obvious explanation for such a result is that, when N was heavily used, delayed senescence led to a slow grain filling and a poor remobilization and partitioning of assimilates into the grain. The imposed soil drying accelerated plant senescence and improved remobilization of assimilates to grains. In addition, photosynthesis was not severely inhibited because the soil drying was controlled as relatively mild. We conclude that for high N application the gain from an accelerated grain filling and an increased remobilization of preanthesis assimilates outweighed the loss of reduced photosynthesis and early senescence as a result of soil drying.

The finding that early senescence induced by controlled soil drying can improve grain filling when senescence is unfavorably delayed has great significance to wheat production in China for several reasons. First, seasons for wheat production in China's main agricultural area, the north and middle of China, are limited by adverse weather conditions. Continental hot and dry wind (temperature >30°C, relative humidity <30%, and wind speed >3 m s^-1) is common in early June in this area and can dehydrate wheat in 1 or 2 d before it matures (Wang et al., 1978; WRGHDW, 1983). Controlled soil drying may accelerate senescence so that the wheat can mature before the adverse conditions occur. Second, the heavy use of N in China results in unfavorably delayed senescence and slow grain filling, leading to a low kernel weight. Early senescence induced by controlled soil drying could increase the rate of grain filling and improve kernel weight in this case. Third, some lodging-resistant wheat cultivars bred recently in China have low harvest index and poor grain filling because they stay "green" for too long and remobilize assimilates poorly to the grains (Peng et al., 1992). Controlled soil drying may induce these cultivars to mobilize more prestored assimilates to grains.

In addition, controlled soil drying may contribute to water saving in wheat production, which is urgently needed in developing a sustainable agriculture in many parts of China where the rapid depletion of water resources is threatening wheat production (Zhang et al., 1998). Wheat production is the largest underground water consumer in northern China and relies heavily on irrigation. Controlled soil drying in this area is applicable with some reduction of irrigation at the grain-filling period (Zhang et al., 1998). Johnson Moss 1976

	ACKNOWLEDGMENTS

 
We are grateful for the financial support from FRG (Faculty Research Grant) of Hong Kong Baptist University, RGC (Research Grant Council of UGC, Hong Kong), and a grant from the Area of Excellence Research Fund from Chinese University in Hong Kong. We also thank Mr. Junli Liu, Wei Wang, and Qimin Pan for their excellent technical assistance in this study.

Received for publication January 24, 2000.

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