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PLANT GENETIC RESOURCES

Seasonal Dynamics of Nonstructural Carbohydrate Partitioning in 15 Diverse Rice Genotypes

^a Dep. of Agronomy, Univ. of the Philippines at Los Baños, College, Laguna 4031, Philippines
^b Dep. of Entomology, Texas A&M Univ., College Station, TX 77843
^c USDA-ARS, Rt. 7, Box 999, Beaumont, TX 77713
^d Dep. of Plant and Soil Sciences, Box 9555, Mississippi State, MS 39762

* Corresponding author (s_samonte{at}lycos.com)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The presence of significant variation among rice (Oryza sativa L.) genotypes in total nonstructural carbohydrates (TNC) that is related to grain yield should be of interest to rice breeders. The effects of four important yield-determining traits (maximum number of tillers, grain weight, panicle node number, and panicle size) on the TNC concentrations of plant structures at heading and harvest were determined. Path analysis was used to determine the path coefficients of the effect of changes in TNC content ( TNC) in leaves or stems on panicle TNC at various stages of crop maturation. Fifteen rice genotypes were used in this study (‘Lemont’, ‘Teqing’, and 13 inbred lines obtained from a Lemont x Teqing cross) to represent the combinations of low and high levels of the four important yield determinants. Field experiments were conducted during the 1994 and 1995 cropping seasons at the Texas A&M University Agricultural Research and Extension Center, Beaumont, TX. Path coefficients for the significant direct effects of stem TNC on panicle TNC were -0.46 for the early- to late-heading period and -0.59 for the late-heading to grain hardening period. Significant genotype x developmental stage x plant structure interaction suggested the potential for selection of rice lines with high TNC concentration in stems at heading. Linear contrasts indicated that low-grain weight genotypes had higher stem TNC concentration at harvest, which in turn suggested for the selection of high-grain weight genotypes.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
TOTAL NONSTRUCTURAL CARBOHYDRATES are sugars, starches, and fructosans that can be accumulated and mobilized for metabolism or translocated to other plant structures (Henjum, 1980; Smith, 1981). For rice, estimates of the contribution by preheading carbohydrates accumulated in vegetative tissues to grain carbohydrates at maturity range from 24 to 27% (Cock and Yoshida, 1972) and 20 to 30% (Murthy, 1976). This contribution is lower at higher levels of fertilizer application and higher for longer-duration cultivars (Yoshida and Aha, 1968, as cited by Murata and Matsushima, 1975). Low temperatures and higher solar radiation tend to increase TNC (Akita, 1995). Preheading carbohydrate storage potentially serves as a buffer to support grain filling during unfavorable weather conditions such as low light intensity (Soga and Nozaki, 1957, as cited by Murata and Matsushima, 1975; Yoshida, 1972).

The TNC present in stems at heading can be a major contributor to grain TNC at harvest (Sato and Inaba, 1976; Weng et al., 1982). However, Akita (1995) emphasized the need to determine the optimum TNC concentration that produces the highest yield, because the presence of TNC beyond a specific limit indicates reduced allocation to structural components in the vegetative organs, which in turn may lead to increased susceptibility to lodging or premature senescence of leaves. The presence of significant variation among rice genotypes in TNC that is related to grain yield is of particular interest to rice breeders (Andrews, 1990). In this study, diverse rice lines obtained from a japonica x indica cross were studied. The effects of four important yield-determining phenotypic traits (tillering ability, grain weight, panicle node number, panicle size) on the concentrations of TNC in plant structures at heading and at harvest were determined. Path analysis was used to determine the developmental stages during which changes in TNC content in leaves or stems contributed significantly to changes in panicle TNC, and to measure the magnitude of significant direct effect(s) on changes in panicle TNC.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Experiment
The rice genotypes used in this study were Lemont, Teqing, and 14 lines derived from a Lemont x Teqing cross—LQ170, LQ49, LQ55, LQ267, LQ82, LQ348, LQ199, LQ287, LQ125, LQ214, LQ81, LQ338, LQ248, and LQ306 (Li et al., 1995). The LQ lines were F₉ in 1994 and F₁₀ in 1995. These recombinant inbred lines were selected from 300 F₈ LQ lines grown in 1993 to represent the combinations of low and high levels of four traits that are important yield determinants—tillering ability, grain weight, panicle node number, and panicle size (Table 1). The basis for the separation between the low and high level genotypes was the population mean for each trait measured from the 300 F₈ LQ lines. Lemont is a tropical japonica rice cultivar released in 1983; its characteristics include high yield, high milling percentage, semidwarf stature, and good ratoon potential (Bollich et al., 1985). Teqing is a high tillering, semidwarf indica rice cultivar from China with an extremely high yield potential (Wu et al., 1998).

Biomass samples were obtained from one of five 0.7- by 0.3-m quadrats within the inner four rows (0.7-m width by 3.9-m length) of each plot. Sampling was conducted for each rice genotype approximately once every 12 to 14 d during the entire growth period for a total of six sampling dates for the earliest maturing line to 11 sampling dates for the latest maturing line. On each sampling date, plants were dug to a depth of 27.5 cm and washed to remove adhering soil. The plant samples were then divided into (i) main plants, MP; (ii) first and second tillers, T1T2; and (iii) third and fourth tillers, T3T4.

Within each tiller group, the plant samples were subdivided into stems (culms and leaf sheaths), leaf blades, panicles, and roots. To minimize wilting and respiration during the separation of tillers and plant structures, the plant samples were kept in ice-cold water. All samples were dried at 70°C in a ventilated oven. Dry weights were obtained after 48 h or until a constant weight was obtained. The samples were then ground using a cyclone sample mill (Udy Corporation, Fort Collins, CO) with a mesh size of 0.5 mm, after which they were sealed in plastic vials and stored at 4°C until analyzed for TNC concentration.

Heading date was estimated as the number of days from sowing to the exsertion of 50% of the panicles in each plot. Minimum and maximum daily temperature, daily rainfall, and solar radiation data were obtained from the Texas A&M University Research and Extension Center, Beaumont.

TNC Determination
The TNC determination method was modified from procedures developed by Hendrix (1993) and Tarpley et al. (1993). Sulfuric acid (4 mL 5 mM H₂SO₄) was added to 15-mL polypropylene tubes containing 0.15 g ground samples. The tubes were then autoclaved (20 min at 103.4 kPa at 121°C) to gelatinize the starch grains, hydrolyze the fructans, disrupt the membranes, and release the sugars into solution. To break down starch into maltose and oligosaccharides, 1 mL 100 mM HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid), pH 6.9, containing 100 EU porcine pancreas -amylase was added. These were then incubated at 38°C for 40 to 44 h and agitated twice per day. The carbohydrates in solution were separated from the structural residues 15 min after agitation by transferring 1.25 mL of the supernatant into 7-mL mini-vials. Maltose, a product of the previous step, was broken down into glucose by adding 0.25 mL 35 mM K₂HPO₄, 100 mM dithiothreitol, pH 7.5, containing 20 EU amyloglucosidase, followed by incubation for 45 min at 38°C. Absolute ethanol (4.5 mL) was added to prevent sugar adsorption to charcoal in the next step. Activated charcoal (0.1 g) was added to remove substances that could interfere with enzyme activities. The samples were then agitated and the charcoal was allowed to settle overnight while in cold storage (4°C).

The TNC content in the aliquots in the microtiter plate wells was determined by the methods of Hendrix (1993) for sucrose assay, except that the reaction mixtures were prepared by the modified method of Tarpley et al. (1993). The previous steps in sample processing had converted all TNC into sucrose, glucose, or fructose. The sucrose assay as used is designed to reduce iodonitrotetrazolium violet stoichiometrically to the hexose equivalents present in these sugars. After a 40-min incubation and consequent reduction of iodonitrotetrazolium violet absorbance at 490 nm was measured with an ELx-800 microtiter plate reader (Biotek Instruments, Inc., Winooski, VT). The content of hexose equivalents of TNC was determined by comparison with a glucose standard curve. The r² for a linear fit was typically 98%.

Statistical Analyses
Path analyses as described by Li (1975) were used to organize and present the causal relationships between predictor variables (leaf and stem TNC) and response variable (panicle TNC) through a path diagram. This analysis partitioned the correlation coefficient between leaf and stem TNC content (g m^-2) and panicle TNC content (g m^-2) into its components—the path coefficient which measures the direct effect of a predictor variable upon its response variable; and the indirect effect(s) of a predictor variable on the response variable through other predictor variables (Dewey and Lu, 1959). Since each cultivar reached harvest at different dates, the biomass sampling of the 15 lines did not correspond with the same stage of phenological development. In addition to the TNC content at the harvest stage, the TNC content for each of four sampling dates prior to harvest which were chosen to represent the pre-heading, early heading, late heading, and grain hardening stages (49, 37, 26, and 15 d before harvest, respectively) was interpolated. Harvest is defined in this paper as the point when approximately 80% of grains in the panicles are straw-colored and the grains in the lower portions of the panicle are in the hard-dough stage (De Datta, 1981). The change in TNC content between successive stages was computed for each structural part. Four periods were obtained from the five stages. Within each period, path coefficients served as the estimate of the direct effects on panicle TNC content caused by the changes in leaf and stem TNC content.

Analyses of variance were conducted to determine the significance of year, genotype, developmental stage (heading and harvest), tiller group, and plant structure (leaf, stem, and panicle) on rice biomass (g m^-2) and TNC concentration (g kg^-1). For dry weights (g m^-2) and TNC concentrations (g kg^-1) of plant structures, contrasts were performed between the low and high levels for the four phenotypic categories. When the field sampling did not exactly coincide with the heading date, the dry weights and TNC concentrations of plant structures at heading were interpolated. Roots were excluded from the analyses because the field root data were not separated into tiller groups. In addition, the whole plant root TNC concentration was generally low at around 10 g kg^-1 dry weight.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Seasonality of Biomass and TNC Concentration
The seasonal patterns for main plant biomass and TNC concentration for the earliest (LQ49) and latest (LQ306) heading rice lines are presented in Fig. 1 and 2. Heading for LQ49 occurred at 77 d from planting in 1994 and 78 d from planting in 1995. In contrast, LQ306 headed at 114 d from planting in 1994 and 118 d from planting in 1995. In both years, leaf dry weights of both lines increased to a maximum around heading then declined afterwards. This pattern was in general the same for all 15 genotypes. Green leaf TNC was relatively stable (compared to stem and panicle TNC) averaging 68.5 g kg^-1 in 1994 and 48.5 g kg^-1 in 1995 for the 15 genotypes. In both years, stem dry weights increased to a maximum just after heading in lines LQ49 and LQ306 then declined during grain filling. Published research showed that TNC accumulation in the leaf sheath and culm reaches a maximum at or before heading (Perez et al., 1971; Murata and Matsushima, 1975; Rowland-Bamford et al., 1990). In this study, stem TNC of LQ49 peaked at around heading (405.8 g kg^-1 in 1994 and 306.0 g kg^-1 in 1995). In comparison, other studies reported maximum carbohydrate concentrations at 300 g kg^-1 for Lemont's leaf sheath (Andrews, 1990) and 240 g kg^-1 for IR8's stem (Yoshida and Aha, 1968, as cited by Murata and Matsushima, 1975). There was a reaccumulation of stem TNC in LQ49 for both years during the grain hardening stage until harvest, with the 1994 stem TNC at harvest being even greater than that during heading. Saitoh et al. (1991) and Akita (1995) stated that high-yielding cultivars exhibit reaccumulation of TNC in leaf sheath and culm at harvest. Andrews (1990) and Turner and Jund (1993) explained TNC reaccumulation as characteristic of rice lines with improved ratoon potential. However, in this study, although LQ49 was the earliest maturing line, it was a low-yielding line ranking 13th among the 15 lines. Hence, TNC reaccumulation indicates either a limitation in translocation of assimilates into the grain or a limited grain storage capacity (Murata and Matsushima, 1975). Other genotypes that exhibited reaccumulation of TNC were LQ287, LQ125, LQ214, LQ81, and Teqing.

View larger version (36K): [in this window] [in a new window]   Fig. 1. Dry weight and TNC concentration of leaves, stems and panicles of rice genotype LQ49 main plants in 1994 and 1995. Arrows indicate heading date.

View larger version (39K): [in this window] [in a new window]   Fig. 2. Dry weight and TNC concentration of leaves, stems and panicles or rice genotype LQ306 main plants in 1994 and 1995. Arrows indicate heading date.

Panicle dry weight increased rapidly for each of the 15 genotypes. As illustrated for LQ49 and LQ306, panicle weight was much greater in 1994 than in 1995 (Fig. 1 and 2). Panicle TNC concentration decreases around heading because of the increased respiration of anthesis (Cock and Yoshida, 1972), then increases rapidly during grain filling. The rate of increase in panicle TNC was higher in 1994 (37.5 g kg^-1 d^-1 for LQ49 and 37.2 g kg^-1 d^-1 for LQ306) than in 1995 (27.5 g kg^-1 d^-1 for LQ49 and 13.8 g kg^-1 d^-1 for LQ306). This compares with 31.5 g kg^-1 d^-1 in 1994 and 29.6 g kg^-1 d^-1 in 1995 when averaged across genotypes. Panicle TNC at harvest for LQ49 was 589.66 g kg^-1 in 1994 and 277.69 g kg^-1 in 1995, while for LQ306 it was 603.60 g kg^-1 in 1994 and 452.44 g kg^-1 in 1995. When averaged across genotypes, the panicle TNC concentration at harvest was 527.6 g kg^-1 in 1994 and 405.2 g kg^-1 in 1995. The decrease in panicle TNC at harvest has also been observed in other studies (Andrews, 1990).

Effects on Panicle TNC
The path analyses showed that changes in stem TNC content (g m^-2) had a negative direct effect on panicle TNC content during the early-heading to late-heading period and during the late-heading to grain hardening period (Fig. 3). Path coefficients for the direct effect of stem TNC on panicle TNC were -0.46 (P = 0.018) during the early-heading to late-heading period and -0.59 (P = 0.002) during the late-heading to grain hardening period. This suggests that the decrease in stem TNC content was caused by translocation of the accumulated stem TNC to panicles during these periods. The indirect effect of stem TNC on panicle TNC through leaf TNC content were near zero at -0.002 for the early-heading to late-heading period and -0.010 for the late heading to grain hardening period.

View larger version (14K): [in this window] [in a new window]   Fig. 3. Correlation (double-arrowheaded lines) between leaf and stem TNC content and their direct effects (single-arrowheaded lines) on panicle TNC of rice during the (A) pre-heading to early heading period, (B) early heading to late heading period, (C) late heading to grain hardening period, and (D) grain hardening to harvest period. Non-solid lines indicate non-significant correlation or path coefficients.

Contributors to Plant Structure Dry Weight and TNC
The diversity of rice genotypes achieved by selecting the low and high genotypes for tillering ability, grain weight, panicle node number, and panicle size was reflected in the analysis of variance for dry weight. All main effects (year, genotype, stage, tiller group, and plant structure) and most interactions significantly affected dry weight. Tiller group, plant structure, and stage x plant structure interactions accounted for 78.4% of the total variation for dry weight.

The factors that had significant effects on TNC concentration were year (Y); genotype (G); developmental stage (S); plant structure (P); and Y x G, G x S, Y x P, G x P, S x P, Y x S x P, and G x S x P interactions. Tiller group did not affect TNC concentration as a main effect nor as part of any interaction effect, indicating that TNC concentration did not differ among main plants or tillers even though tillers differed in dry weight. Stage x plant structure interaction contributed the largest to TNC variation. The significant G x S x P interaction, which primarily reflected differences among genotypes in the stem TNC decline and panicle TNC increase with developmental stage, indicated the potential for selection of rice lines with high TNC concentration in stems at heading. The TNC accumulated in stems at heading is one of two major contributors to grain TNC at harvest, the other being photosynthates produced after heading (Sato and Inaba, 1976). Table 2 shows the genotype means for dry weight and TNC concentration of stems at heading and of panicles at harvest.

The significance of the year main effect on TNC concentration partly explained the low rice grain yields attained in 1995. The decrease (P < 0.001) in plant dry weight from 148.4 g m^-2 in 1994 to 108.1 g m^-2 in 1995 was reflected by the decrease (P < 0.001) in TNC concentration from 192.3 g kg^-1 in 1994 to 156.0 g kg^-1 in 1995. Total nonstructural concentrations of leaves, stems, and panicles at heading and at harvest were generally lower in 1995 than in 1994, with the exception of stems at harvest. Sato and Inaba (1976) observed that when rice was grown at 35°C, the translocation rate of assimilates to grain declined and the grain filling period shortened resulting in TNC concentrations in stems that were relatively higher than in rice grown at 25°C. High daily maximum temperatures during July and August 1995 have been suggested as the cause for the dry weight and yield decreases (Wilson et al., 1996). In 1994, daily maximum temperatures exceeded 35°C for 1 d in July and 3 d in August, while in 1995, the daily maximum temperatures exceeded 35°C for 13 d in July and for 12 d in August. High temperatures increase maintenance respiration causing an increased loss of assimilates (Yamamoto, 1954, as cited by Murata and Matsushima, 1975; Murata, 1964; Yoshida, 1981).

Correlation analysis was conducted to determine whether high stem TNC at heading was due to relatively lower number of few spikelets or grains available to take up the TNC. The results indicated that stem TNC at heading was not significantly correlated with either number of spikelets at heading or number of grains at harvest during 1994, 1995, and the combined 1994-1995 data. This indicated that stem TNC at heading had no significant linear relationship between either the number of spikelets at heading and the number of fertile spikelets that developed into grains at harvest.

Sato and Inaba (1976) reported that the panicle to straw ratios for dry weight and TNC content were lower at high temperature (35°C maximum, 30°C minimum) than at favorable temperatures. In their study, panicle to straw TNC content ratios were 11.3 at favorable temperature and 4.7 at high temperature, while panicle to straw dry weights ratios were 1.2 at favorable temperature and 0.8 at high temperature. A similar trend was shown in this study wherein panicle-to-straw ratios were lower in 1995 which was relatively hotter during July and August in 1995 than in 1994. The mean panicle to straw ratio for TNC content of main plants at harvest was 15.6 in 1994 and 5.7 in 1995, while the mean panicle to straw ratio for dry weight was 1.35 in 1994 and 0.94 in 1995.

Wada (1969) estimated the ratio of shoot TNC content at heading to grain TNC content at harvest to be about 0.1 to 0.4. Since this study measured panicle TNC instead of grain TNC, the ratio of stem TNC content at heading to the panicle TNC content (which is the TNC content of grains and panicle branches) at harvest is expected to be greater. Nevertheless, applying this ratio to the TNC contents of main plants resulted in a 1994 mean ratio of 0.61 and a 1995 mean ratio of 1.04. This indicated the possibility that the 1995 growing season relied heavily on the accumulated stem TNC at heading which was translocated into the panicles. This implies the importance of stem TNC as a potential TNC buffer during grain filling when the weather is unfavorable, such as high maximum daily temperature. Only three genotypes had ratios that were lower in 1995 than in 1994: LQ170 (0.56 in 1994, and 0.50 in 1995), LQ49 (0.92 and 0.87), and Lemont (0.73 and 0.55).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The importance of the amount of TNC that accumulated in the stems at heading that was translocated to panicles after heading was established through the path analyses of TNC partitioning in rice. The significant negative direct effects of stem TNC on panicle TNC content were -0.46 for the early- to late-heading period and -0.59 for the late-heading to grain hardening period. The significant genotype x developmental stage x plant structure interaction indicated the potential for selection of genotypes with high TNC concentration in stems at heading. The low grain weight genotypes had higher stem TNC concentrations at harvest, which suggested the selection of higher grain weight genotypes to maximize translocation of stem TNC to the panicle and also to maximize grain storage capacity.

	ACKNOWLEDGMENTS

 
This research was supported in part by funding from the Texas Rice Research Foundation and the Texas Agricultural Experiment Station. The senior author appreciates the graduate fellowship provided to him by the Rockefeller Foundation, and the support of the University of the Philippines during his Ph.D. studies.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
This paper is part of the Ph.D. diss. of the senior author.

Received for publication June 2, 2000.

	REFERENCES

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