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RICE

Shading during the Early Grain Filling Period Does Not Affect Potential Grain Dry Matter Increase in Rice

Dep. of Agronomy, Faculty of Life and Environ. Sci., Shimane Univ., Matsue, 690-8504, Japan

kobata{at}life.shimane-u.ac.jp

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Shortage of C assimilate supply to rice grain during approximately the first 10 days after heading (DAY₁₀) has been believed to reduce potentiality of grain dry matter increase (G_P) and decrease the final grain weight (G_F). However, we suspect that assimilate shortage during DAY₁₀ does not determine G_F, if assimilate supply during the rest of the grain filling period (G_PERIOD) meets subsequent requirements for realizing G_P. Our objective was to determine if G_P is in fact affected by shortage of assimilate supply caused by shading during DAY₁₀. Plots of rice (Oryza sativa L.) grown under paddy field conditions were subjected to three strengths of shading for DAY₁₀. After the shades were removed, the plots were divided into two groups. Plants in the first group were thinned to half the plant population density, while the second group was left at normal spacing. G_F in the unthinned group was reduced depending on the strength of the shade, but weights in the thinned group almost reached those of nonshaded control plants, except in the most heavily shaded plots, where weights were slightly reduced. In pot experiments simulating crop stand, G_F all reached those of control plants when the pots were exposed to abundant radiation after being subjected to heavy shading during either the early or mid-G_PERIOD. These results suggest that rice plants are capable of recovering from early reduction in grain growth rate at least until the mid-G_PERIOD, and shortage of available assimilate during DAY₁₀ does not determine G_P.

Abbreviations: DAY₁₀, first 10 days after heading • G_F, final grain weight • G_P, potentiality of grain dry matter increase • G_PERIOD, grain filling period

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
RICE GRAIN FILLING and ripening are affected by many environmental factors, including water, temperature, radiation, and soil nutritional conditions (Yoshida, 1981). Grain dry matter increase is supported by available assimilate, as defined by C assimilation during the grain filling period plus assimilate reserve stored in the straw (Cock and Yoshida, 1972; Weng et al., 1982). Shortage of assimilate supply due to inhibition of photosynthetic processes is one of the major factors determining grain filling (Matsushima and Wada, 1958; Yoshida, 1981; Evans, 1996; Egli, 1998). Inhibition of photosynthesis during the grain filling period due to environmental stresses such as shading or water deficit can result in a major reduction in grain dry matter in rice (Kobata and Takami, 1986; Kobata and Moriwaki, 1990; Takami et al., 1990). These studies showed clearly that potential grain growth was not realized when available assimilate failed to meet the assimilate requirement. Other work suggests that potential grain growth rate (i.e., ability of the grain to fill, or sink strength) is influenced by change in environmental conditions (e.g., radiation, temperature, and fertilizer application) during the early phase of grain filling (Tanaka and Matsushima, 1963; Yoshida, 1981; Tashiro and Wardlaw, 1990; Sumi et al., 1996; Egli, 1998).

It has been shown that shortage of available assimilate caused by shading during the early grain filling period (approximately the first 10 d after heading) restricts final grain weight at the fully ripe stage, even if the shading is removed during the remainder of the grain filling period (Tanaka and Matsushima, 1963; Nagato and Chaudhry, 1970; Nagato et al., 1971). Assimilate supply during the first 10 d after heading, coupled with varied planting dates or fertilizer conditions, also affects the final percentage of grain filled (Tsukaguchi et al., 1996; Horie et al., 1997). In both wheat and rice, reductions in grain weight or filling percentage (decline in potential of grain growth) from shortage of assimilate supply are believed to be due to reduction in the number or size of cells in the endosperm (Singh and Jenner, 1984; Nakamura et al., 1992; Horie et al., 1997). In the first 10 d after flowering, cell division and expansion in the endosperm of most grains ends and starch deposition begins (Hoshikawa, 1967; Egli, 1998).

From previous studies, however, it is doubtful that assimilate supply to the grain during the early grain filling period alone defines potential grain growth and determines final grain weight. If assimilate supply to rice is restricted by shading or unfavorable cultivated conditions in the first 10 d of the grain filling period, the grain may be profoundly affected, as grain growth rate is generally highest within the 2 wk after heading (Yoshida, 1981). The larger part of the active phase of grain dry matter increase may thus encounter restricted assimilate supply. Assimilate shortfall during the rest of the grain filling period could also cause reduction of final grain weight. Past experiments (Tanaka and Matsushima, 1963; Nagato and Chaudhry, 1970; Nagato et al., 1971; Tsukaguchi et al., 1996; Horie et al., 1997) contained no positive trials in which available assimilate was increased after shading treatment or during varying cultivation conditions. In these cases, it is therefore not known if assimilate supply during the remainder of the grain filling period was sufficient to meet the requirements of grain dry matter increase. Kobata and Takami (1981) demonstrated that rice plants that suffered from severe deficiency of assimilate supply due to short-term soil desiccation during the early and middle grain filling periods exhibited grain dry matter increases as large as nonstressed plants did when the water deficit was redressed. It can thus be hypothesized that potential grain dry matter increase is not inevitably determined by shortage of assimilate supply during the early grain filling period when plants are grown under unfavorable environmental conditions.

Our objective was to test the hypothesis that shortage of assimilate supply to grain by shading during the early grain filling period of the first 10 d after heading does not reduce the rice grain weight at harvest if assimilate supply during the rest of the grain filling period is improved by thinning.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Field Experiment
Plant Materials
Plots of the rice cultivar Nipponbare were grown in silty clay loam in a paddy field at the Shimane University experimental farm, Matsue, Japan. Seeds were sown in seed beds (60 by 30 by 3 cm) containing seedling soil (Green Soil, Izumo Green Epoch Co., Izumo, Japan) on 22 Apr. 1997, and were grown on in a non-temperature-controlled greenhouse. On 21 May 1997, seedlings at the four-leaf stage were transplanted to a paddy field site on the farm, in rows 0.30 m apart, with a 0.15-m spacing between plants. Four grams N m^-2 [as (NH₄)₂SO₄], 12 g P m^-2 (as P₂O₄), and 12 g K m^-2 (as KCl) were applied at transplantation, and an additional 2, 4, and 4 g N m^-2 [as (NH₄)₂SO₄] were applied at the early tillering stage, at 3 wk before heading, and at the full heading date, respectively. The full heading date is defined as the date at which 90% of tillers bear ear heads. Plants were grown in a field plot 26.9 by 8.5 m in size. Flooded conditions were maintained throughout the growing season.

Shading Treatment and Thinning
Shading treatment was applied to 2.1- by 2.8-m areas on the full heading date (15 Aug. 1997). Steel frames 1.5 m high, covered with single or double thicknesses of black or white cheesecloth, were placed in the rice field. The cloths covered the tops and all sides of the frames. The experimental design was a randomized block with a split-plot arrangement, with three shading treatments as main plots and two densities as subplots. There were three replicates. The heavy shaded treatment (using double black cloth) reduces full sun radiation by 74.4%; reductions in moderate treatment (black cloth) and light treatment (white cloth) are 48.1 and 25.1%, respectively (Kobata and Moriwaki, 1990). The shade frames were removed 10 d after the full heading date. Half the plots were left untouched, and the other half were thinned to every other plant to reduce the plant density by half for the rest of the grain filling period. Unthinned control plots were grown under full sun conditions throughout the entire grain filling period.

Measurements
Six hills were harvested from each replication every 10 d between the full heading date and 40 d after the full heading date. The sampled plants were divided into straw and spikelet, dried in an oven at 80°C for 48 h, and weighed. Fifty days after the full heading date, 15 plants from each replication in the control plot were harvested and measured for yield and yield components. Filled grain was selected by specific gravity (1.06 x 10³ kg m^-3). Selected spikelets were counted, oven-dried, and weighed. The husks were removed and oven-dried grain weights determined. Mean single husk weight was estimated by dividing the spikelet number into the weight differences between spikelets and grain in the control plants. The husk weight per hill for each sample was calculated by multiplying the mean husk weight by the number of spikelets, and the grain weight estimated by eliminating the husk weight from the spikelet weight. We ignored changes and differences between fertile and sterile grain in husk weight during the grain filling period, because dry matter increases in husk weight usually stop by 2 wk after flowering, and differences in husk dry weight between fertile and sterile grain is small (<1 mg per husk) (Seo and Ota, 1982). The grain yield (moisture content 140 g kg^-1) in the control plot was 596 ± 47 g m^-2 (mean ± standard error of three replications), where the number of spikelets was 28950 ± 2300, the percentage of filling was 90.7 ± 0.3, and the single filled grain weight was 0.0227 ± 0.0014 g. The grain yield and yield components are near the farm average over the past decade (Kobata et al., 1993).

Pot Experiment
Plant Materials
On 22 May 1996, seedlings of cultivar Nipponbare at the four-leaf stage were transplanted into pots 14 cm in diameter and 20 cm deep. The pots were filled with rice seedling mixture and sandy soil (a 1:1 ratio by volume), and two seedlings were placed in each. The methods used for sowing and growing the seedlings were the same as those used in the field experiments. At the time of transplantation, 0.2 g N pot^-1 [as (NH₄)₂SO₄] (5.3 g m^-2), 0.4 g P pot^-1 (as superphosphate of lime) (10.7 g m^-2), and 0.6 g K pot^-1 (as KCl) (16.0 g m^-2) were applied, and additional applications of 2.0, 4.0, and 3.0 g N pot^-1 [as (NH₄)₂SO₄] made at the early tillering, flower initiation, and full heading stages, respectively. Pots were placed in a 4.0- by 2.5- by 0.3-m water pan set in the rice paddy field at the experimental farm. Rice canopies of the same cultivar thus surrounded the pot plants. The pot plants were arranged in rows 0.21 m apart with 0.18 m spacing between plants, and plants were grown under flooded conditions. Three replications containing three hills (pots) from different locations in the pan were harvested for yield components 50 d after the full heading date. The grain yield (at a moisture content of 140 g kg^-1) of plants grown in the water pan was 568 ± 17 g m^-2 (mean ± standard error of three replications), where the number of spikelets was 24473 ± 659 m^-1, filling percentage was 96.6 ± 0.6%, and single filled grain weight was 0.0241 ± 0.0026 g. The yield in the pot experiment was similar to that in the field experiment, whereas the spikelet number was a little lower and the percentage of filling and the single grain weight were a little higher.

Shade Treatment during Early and Middle Grain Filling Periods
Between 0 and 10 d after the full heading date, and between 10 and 20 d, each group of 12 pots was taken from the water pan and divided into three groups for different shade treatments. The plants were shaded in 0.75- by 0.50- by 1.25-m steel frames covered on the top and all sides with three types of shade cloth, as in the field experiments. After shade treatment, the plants were placed in an isolated position along the side of the paddy field, so they could receive abundant radiation during the rest of the grain filling period.

Measurements
Shoots and roots were harvested at the full heading date and 10, 20, and 40 d later, oven-dried at 80°C, and weighed. Methods used for the measurement of grain weight were those used in the field experiments. Oven-dried straw at full heading date was powdered, and soluble carbohydrate content was estimated using the ß-hydroxybenzotic acid hydrazide method (Boehringer Mannheim, Mannheim, Germany), to determine assimilate reserve in the straw at heading. Total available assimilate during the grain filling period was estimated from the soluble carbohydrate in the straw plus dry matter increase (Yoshida, 1981; Horie et al., 1997).

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Field Experiments
Shading Effects in Grain and Shoot Dry Matter
The amount of grain dry matter in plants subjected to light shading for 10 d after the full heading date was the same as that of the control plants, whereas plants subjected to moderate or heavy shading contained less grain dry matter (Fig. 1) . After shading was removed, grain weight in shaded plants increased, but remained lower than the controls during the grain filling period and at grain maturity, particularly for the moderate and heavy shade treatments.

View larger version (18K): [in this window] [in a new window]   Fig. 1 Whole shoot, grain, and straw dry matter in control rice plants (solid circles) and those subjected to three strengths of shading (open triangles, light; open diamonds, moderate; open squares, heavy) by differing shade cloths (light transmittance is 74, 48, and 25% of the control, respectively) for 10 d after full heading date, and subsequently grown under full sun conditions in the field. Data plotted is the mean and one standard error of three replications. The lateral bars indicate shading treatment periods

Under shaded conditions, straw weight decreased with suppression of dry matter production (Fig. 1). From 20 to 40 d after the full heading date, straw weight increased in plants undergoing heavy shade treatments. This may indicate an early stoppage in grain growth and deposition of dry matter in the straw during the late period of the grain filling, after assimilate supply decreased during the early grain filling period. This phenomenon has been observed previously (Tanaka and Matsushima, 1963; Nagato and Chaudhry, 1970; Nagato et al., 1971).

Effects of Thinning on Grain and Shoot Dry Matter after Shade Removal
Grain weights increased steeply when plant densities were halved after shade at 10 d. Grain weights in the light and moderate shaded plots at full maturity were almost the same as those in the control (Fig. 2) , whereas those in the heavy shaded plot were a little lower. At the full ripening period (40 d after the full heading), there was a statistically significant difference (LSD_0.05 = 1.81 g hill^-1) in grain dry weight between the control and the heavy, moderate, or light shaded treatments, but not between the control and the thinned moderate or thinned light shaded treatments. There was a statistically significant difference between the control and the thinned heavy shaded treatment.

View larger version (27K): [in this window] [in a new window]   Fig. 2 Whole shoot, grain, and straw dry matter in control rice plants (solid circles); in those subjected to three strengths of shading for 10 d after full heading, left unthinned, and grown under full sun conditions (open triangles, light; open diamonds, moderate; open squares, heavy); and in those thinned to half plant density after shading (solid triangles, solid diamonds, solid squares) and then grown under full sun conditions in the field. Data plotted is the mean and one standard error of three replications. The lateral bars indicate shading treatment periods

One piece of evidence that lack of sink activity may be due to shortage of assimilate supply during early grain filling is that dry matter is deposited in straw during the last part of the grain filling period, instead of being used for grain growth (Nagato and Chaudhry, 1970; Saitoh et al., 1993; Sumi et al., 1996). In our results, such deposition was also observed in moderate and heavy shade treatment plots, while the grain growth rate was lower than that of the control (Fig. 1). The deposition, however, could occur only if potentials of grain dry matter increase are maintained. Rates of increase in grain, shoot, and straw dry matter in the control and the shaded treatments are compared in Fig. 3 . Shading reduced the dry matter increase in shoots not only during the 10 d shading, but also until 20 d after full heading. Reduction was more marked in the plants subjected to heavy shade. Between 20 and 40 d after full heading, shoot dry matter increases in all shade treatments exceeded those of the controls. Shoot dry matter increases were followed by deposition of dry matter in the straw during the late grain filling period, 30 to 40 d after full heading (Fig. 2), because the rate of grain dry matter increase was already falling (Fig. 3). An excess of assimilate over capacity of the grain dry matter increase in the late grain filling period thus seems to result in a significant accumulation of dry matter in straw.

View larger version (36K): [in this window] [in a new window]   Fig. 3 Rate of dry matter increase in grain, whole shoots, and straw in control plants (solid circles); in plants shaded for 10 d after full heading date and then left under full sun conditions (open squares); and in plants thinned to half density after shading (open triangles) and grown in full sun

View larger version (24K): [in this window] [in a new window]   Fig. 4 Comparison of rate of dry matter increase in grains grown under field conditions. Solid circles represent control plants; open squares, plants shaded for 10 d after full heading date and then left under full sun conditions; open triangles, plants thinned to half plant density after shading; open diamonds, plants thinned to half plant density every 10 d during the grain filling period to estimate potential grain growth rate for each grain growth period

View larger version (28K): [in this window] [in a new window]   Fig. 5 Filling percentages in the field experiment (defined by observed grain weight/potential grain weight) at the early grain filling period (10 d after full heading date) and at grain maturity (40 d), and dry matter increase between 0 and 10 d after the full heading date. The horizontal dotted line indicates the filling percentage of the control plants at maturity. Solid circles represent control rice plants. Open triangles, open diamonds, and open squares represent plants subjected to light, moderate, and heavy shading, respectively, for 10 d after full heading, left unthinned, then grown under full sun conditions. Solid triangles, solid diamonds, and solid squares represent plants thinned to half plant density after light, moderate, and heavy shading, respectively; plants were then grown under full sun conditions in the field. Data plotted are the mean ± standard error of three replicates

View larger version (37K): [in this window] [in a new window]   Fig. 6 Filling percentages of grain on primary and secondary rachis branches and whole branches 30 d after full heading, when grain filling has virtually ceased. Results are given for control plants (solid bars), for plants subjected to heavy shading treatment (shaded bars) for 10 d after full heading, and for plants thinned after shading (open bars). Data are the mean ± one standard error of three replicates. Vertical bars at the top of each column indicate LSD (P < 0.05) for filling percentage between controls and experimental groups

View larger version (19K): [in this window] [in a new window]   Fig. 7 Filling percentages in the pot experiment at the early grain filling period (open symbols, 10 d after full heading date) and at grain maturity (closed symbols, 40 d after full heading date), and available assimilate (dry matter increase plus soluble carbohydrates in straw) between 0 and 10 d (circles) and 10 and 20 d (squares) after the full heading date. Separate pots were shaded under three differing radiant conditions during the two terms, and subsequently received abundant radiation in open conditions. Each data point is the mean of two to four observations

The first 20 d after full heading, when pot experiments were shade-treated, may completely span the period of endosperm cell development in most rice grains (Hoshikawa, 1967). In our study, when rice plants experienced assimilate shortage during both the early and middle grain filling periods, their subsequent growth capacity was approximately equal to that of plants in the control plots; i.e., the endosperm could still deposit abundant starch, despite assimilate shortage in the endosperm cell development phase. In a study of wheat grown under shaded conditions, Takahashi and Kanazawa (1996) found that final grain weights were affected by the size of starch granules formed after completion of endosperm structural development, rather than by the number of endosperm cells.

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Rice grains are able to maintain capacity for dry matter increase, irrespective of shortage of assimilate supply from shading during the early grain filling periods. Final grain weights almost attain those of control plants if assimilate production is increased by thinning, and meets requirements for grain dry matter increase. Our results show that patterns in grain growth rate during the grain filling period are fairly stable, at least under differing radiant conditions. This suggests that a simple model for rice grain dry matter increase, in which the developmental pattern of grain growth rate during the grain filling period is fixed and only dry matter production and assimilate reserve in straw are variables (Kobata and Moriwaki, 1990; Takami et al., 1990), also applies to estimation of grain growth under fluctuating radiant conditions. Our results, however, were obtained under shaded conditions designed to restrict assimilate supply to the grain during the early ripening period. Growth potential of grains subjected to unfavorable conditions during the early ripening period needs to be established for other cases of environment stress conditions, such as unfavorable fertilizer or high temperature conditions.

	ACKNOWLEDGMENTS

 
Our thanks to the staff of the Crop Science laboratory, Shimane University, for their diligence and assistance in harvesting and analysis of plant materials for this study.

Received for publication August 20, 1998.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (11) Citing Articles via Google Scholar Google Scholar Articles by Kobata, T. Articles by Takatu, S. Search for Related Content PubMed Articles by Kobata, T. Articles by Takatu, S. Agricola Articles by Kobata, T. Articles by Takatu, S. Related Collections Rice Crop Physiology & Metabolism