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OIL & GRAIN CROPS

Comparison of Plant Measurements for Estimating Nitrogen Accumulation and Grain Yield by Flooded Rice

^a Rice Res. & Ext. Ctr., P.O. Box 351, Stuttgart, AR 72160 USA
^b Dep. of Crop, Soil & Environmental Sciences, Univ. of Arkansas, Plant Science 115, Fayetteville, AR 72701 USA
^c Agric. Statistics Lab, Univ. of Arkansas, AGRX 101, Fayetteville, AR 72701 USA

sixten{at}uaex.edu

Received for publication February 16, 1998.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Knowledge about N accumulation during the vegetative growth stage of flooded rice (Oryza sativa L.) may be useful in determining the need for topdressing fertilizer N at panicle differentiation (PD). In a 3-year field study, plant area, N concentration of the Y-leaf (most recently matured leaf blade) and the whole plant, and chlorophyll meter (SPAD) readings measured during vegetative and early reproductive growth stages were used to estimate total N accumulation. The techniques were then used to determine the growth stages that maximized correlation with grain yield. Five preflood (PF) N rates (0, 33.6, 67.2, 100.8, and 134.4 kg ha^-1), two PD N rates (0 and 67.2 kg ha^-1), and two cultivars (LaGrue and Lacassine) were used. The treatments were chosen to represent an array of dry matter and total N accumulations. No interactions of PF N rate x cultivar on grain yield and total N accumulation were observed. Plant area was linearly correlated (r = 0.84 to 0.93, P < 0.05) to dry matter accumulation, and accounted for >60% of the variation in total N accumulation every year. However, Y-leaf N concentration and SPAD readings accounted for <60% of the variation in total N accumulation in 1993 and 1994, and for >60% only in 1995. A combination of plant area and Y-leaf N concentration or SPAD readings accounted for more variation in total N accumulation than did individual plant measurements. Plant area and whole-plant N concentration was the best combination, accounting for 80 to 90% of the variation in total N accumulation. The maximum variation in grain yield accounted for by the measured traits was 50% at PD for plant area, and 37% at 2 wk after the PF N application for Y-leaf N concentration and SPAD reading. These low correlations of grain yield with plant measurements during the vegetative stage confirm that environmental and other conditions prevailing during later growth stages profoundly influenced grain yield of rice.

Abbreviations: CV, cultivar • PD, panicle differentiation • PF, preflood • YR, year

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
NITROGEN FERTILIZATION of rice (Oryza sativa L.) represents a major input to the crop, and high grain yields can be obtained if an adequate amount of N is accumulated in the rice plant throughout the growing season. Nitrogen absorbed by rice during the vegetative growth stage contributes to growth during the reproductive and grain-filling growth stages through translocation (Bufogle et al., 1997; Mae, 1986; Norman et al., 1992; Shoiji et al., 1986).

Recommendations for fertilizer N applications are often developed from empirical N rate studies based on cultivars grown, soil properties, and previous crop (Wells et al., 1989). In the dry-seeded, delayed flood system of growing rice, the proper amount of N is applied at the 4- to 5-leaf growth stage immediately prior to permanent flooding, after which the need for topdressing N at PD is estimated. Rice cultivars grown in the late 1970s and early 1980s were tall and leafy, and thus suffered from lodging and mutual shading if too much N was applied preflood (Wells and Johnston, 1970; Guindo et al., 1994). Consequently, the N application had to be split-applied, with 50% applied PF and 50% near PD. These older cultivars were dependent on PD N applications to supply the plant N needs during reproductive and grain-filling growth stages. Current cultivars are shorter, stiffer strawed, not as leafy as the older cultivars and were bred to be more responsive to the PF N and less responsive to the PD N (Moldenhauer et al., 1997; Norman et al., 1995, 1996, 1997). Guindo et al. (1994) found that the shorter-stature semidwarf cultivar Lemont did not suffer near as much from mutual shading when high N rates were applied PF as did the older, taller, leafy cultivar Lebonnet. In addition, Lemont was more efficient at translocating N from vegetative tissue to the panicle during grain filling. Since these newer short-stature cultivars respond to PF N and require more N during the vegetative growth stage to reach their full yield potential, the efficient use and management of the PF N is more important in determining their grain yield (Norman et al., 1995, 1996, 1997). Therefore, knowledge of the amount of N accumulated during vegetative growth can represent the basis for determining the need for topdressing N fertilizer at PD. Some cultivars have a larger capacity to accumulate N than others. Teo et al. (1995) attributed the maximum capacity to accumulate N to the extensive rice root system and not to kinetic uptake parameters.

Various plant analyses can be used as indicators of the nutritional status of the rice crop. Plant measurements used to monitor the availability of N for the plant and ultimately the grain include N concentration, N accumulation, and dry matter accumulation. Sims and Place (1968) and Moore et al. (1981) reported that the amount of N accumulated generally paralleled dry matter accumulation and increased with plant age. Sahrawat (1983), however, cautioned that plant N accumulation is a better indicator of soil and fertilizer N availability than dry matter accumulation because plant growth is governed not only by N availability, but also by several other nutrients. Thus, when dry matter accumulation is used to estimate N accumulation, it is assumed that all other nutrients are in proper supply.

The problem with techniques used for estimating plant N status is that they are good estimators of N accumulation in some environments but not in others. For example, the critical concentration levels adequate for normal growth and yield are used only in water-seeded rice in California (Mikkelsen, 1970). Critical concentration at a particular growth stage represents the minimum tissue concentration required for maximum yield. This method does not work in the dry-seeded, delayed-flood rice culture used in the southeastern United States, due to rapid changes that occur in the critical N concentration with plant development and large sample variability within a field (Wells et al., 1993). Visual observations of plant size and color have been used to estimate N needs at PD. Wells et al. (1989) observed in Arkansas that the growth of the rice plant could be monitored by measuring the plant area and using that as an indication of N nutritional status. They developed the plant area method for estimating rice growth following N application, and found that the plant size increase after N application was a better indication of total N accumulation and its influence on rice grain yield than the other proposed methods. This plant area method resulted from observations by Moore et al. (1981) and Gilmour (1985), who indicated that dry matter produced during vegetative growth could be used to predict final grain yield.

The plant area method, which is only a means of quantifying visual observations, is based on the concept that each rice cultivar must develop sufficient growth at specific, key growth stages to produce maximum grain yields.

Other plant measurements that estimate N accumulation are (i) Y-leaf (most recently matured leaf blade) N concentration (Mikkelsen, 1970), (ii) whole-plant (stem + leaves) N concentration, and (iii) leaf greenness measured with a chlorophyll meter (Turner and Jund, 1994; Ladha et al., 1998). Although these measurements have been evaluated individually, they have not been compared with each other to determine their relative accuracy in estimating N accumulation in the plant before the PD growth stage. Furthermore, knowing how and when these measurements are best related to grain yield during the growing season may help to predict if adequate grain yield will be obtained at harvest, provided all other factors affecting grain filling and yield are at optimum.

Our objectives were to (i) determine which plant measurements or combination of plant measurements best correlate with total N accumulation between PF fertilizer N application and PD, and (ii) determine the proper growth stage to take each measurement in order to maximize correlation with grain yield.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
A 3-year study was conducted at the University of Arkansas Rice Research and Extension Center, near Stuttgart, AR. The soil was a Crowley silt loam (fine, smectitic, hyperthermic Typic Albaqualfs) with a pH of 5.1 and organic matter of 1.12%. Rice was grown on new ground each year on plots rotated with soybean [Glycine max (L.) Merr.]. A fertilizer mixture of 45 kg P ha^-1 and 67 kg K ha^-1 was incorporated prior to planting each year. Two rice cultivars, LaGrue and Lacassine, were drill-seeded on 18 May in 1993 and 1994, and on 17 April in 1995. LaGrue is a short-stature cultivar with a greener leaf color during vegetative growth than the semidwarf cultivar Lacassine. Rice was planted at a seeding rate of 112 kg ha^-1. Each plot had nine rows, 6.7 m long with an 18-cm row spacing. Plant population was approximately 200 plants m^-2. Management operations such as weed control and fertilizer application were timed according to DD50 predictions (Keisling et al., 1984).

A mixture of thiobencarb {S-[(4-chlorophenyl)methyl]diethylcarbamothioate} and propanil [N-(3,4-dichlorophenyl)propanamide] for weed control was applied at 3.5 L a.i. ha^-1 thiobencarb and 7 L a.i. ha^-1 propanil 5 d after emergence of the rice. The permanent flood (5–10 cm depth) was established at the 4- to 5-leaf growth stage and maintained until physiological maturity.

The experimental design was a split plot with four replications. The main-plot treatments were factorial combinations of five PF N rates (0, 33.6, 67.2, 100.8, and 134.4 kg ha^-1) and two PD N rates (0 and 67.2 kg ha^-1). The subplot treatments were the two cultivars. Treatments were chosen to provide a wide range of dry matter production, N concentrations, and total N accumulations. It was assumed that the fertilizer N applied represented the primary factor limiting rice growth. Different rates of PF fertilizer N were applied onto a dry soil at the 4- to 5-leaf growth stage and incorporated (on the day of application) in the soil with the flood water. Panicle differentiation N rates were superimposed on the PF N rates; thus, any additional growth response due to PD N could be evaluated. Fertilizer N response was determined by measuring dry matter accumulation, plant area, chlorophyll meter (SPAD) readings (Model SPAD 502, Minolta, Japan), Y-leaf N concentration, and whole-plant N concentration, as well as total N accumulation at intervals during the growing season and grain yield at maturity. Rice plants were sampled weekly, beginning 1 wk after the PF N application and ending at PD (4 wk after PF N application). At each sampling time, measurements of plant area and SPAD readings were taken and samples of Y-leaves and total aboveground plant biomass were collected.

Plant area was measured with a rice gauge (Fig. 1) , which has a horizontal and vertical ruler to measure the plant height and the spread of the leaves, on the assumption that the plant has a triangular shape (Wells et al., 1989). In drill-seeded rice, measurements are taken by centering the rice gauge on the drill rows, examining an individual row of rice, and measuring the average canopy width (left and right) and height. In broadcast-seeded fields, the rice gauge is centered on individual plants for measurement, and many measurements are taken to obtain a representative sample. Plant area was calculated as the area of a triangle, with height equal to the plant height and the width equal to the sum of the leaf spread on the left and right sides of a rice plant. A single plant area measurement was taken from the center row of each plot, approximately 1 m from the edge of the plot.

View larger version (175K): [in this window] [in a new window]   Fig. 1 Rice gauge for measuring plant area in the field

Total dry matter production was determined at each sampling time from samples of aboveground plant biomass (approximately 35 plants) collected by sampling 1-m section of a row in each plot. The 1-m sampling section was chosen in the second, fourth, sixth, and eighth rows for sampling times of 1, 2, 3, and 4 wk after PF N application, respectively. In addition, 40 Y-leaf blades were randomly collected from rice plants in the middle of the second and eighth rows in each plot. The Y-leaf samples and aboveground plant biomass samples were dried at 65°C to a constant mass, then ground in a Wiley mill to pass a 1-mm screen. A 0.1-g subsample of ground plant material was used to determine Y-leaf N concentration and whole-plant N concentration. The Y-leaf N and the whole-plant N concentrations were analyzed with a dry combustion technique (Sweeney and Rexroad, 1987) using a LECO analyzer (Model SP 428, LECO Corp., St. Joseph, MI). Total N accumulation (kg N ha^-1) was calculated as the product of whole-plant N concentration (g N kg^-1) and total dry matter accumulation (kg ha^-1). Grain yield was measured by harvesting the center four rows (3.65 m long by 71 cm wide) from each plot with a small plot combine. Grain yields are reported on a 120 g kg^-1 moisture basis.

Analysis of variance was performed with the GLM procedure in the SAS system (SAS Inst., 1985) to evaluate the treatment effects on the variables measured. Means separation was done by using the Fisher's protected least significant difference (LSD) at the 0.05 level of probability. Linear correlation coefficients (r) between variables were determined by the CORR procedure in the SAS system. Also, quadratic regression analysis was performed with the RSREG procedure in the SAS system. The dependent variables were the measured grain yield and total N accumulation during the 4 wk following PF N application on each single plot. The independent variables were the plant measurements taken at 1, 2, 3, and 4 wk after PF N application. Grain yield measured included the combined effect of PF and PD fertilizer N. Grain yield was then analyzed using a split-plot design, where the main plot was the PF N x PD N combination and the subplot was the rice cultivar. For all other variables measured before PD fertilizer N application, results were analyzed by considering PF N rate as the main plot treatment and cultivar as the subplot treatment.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Grain Yield and Total N Accumulation
Significant effects for grain yield were attributed to year (YR), PF N rate, and cultivar (CV), and to PF x PD, YR x PF, YR x PD, and YR x CV interactions (Table 1) . There was a significant (P 0.05) quadratic effect of applied PF N on grain yield over years. Because the YR x PF x CV interaction was not significant, results were averaged across cultivars for each year and PF N rate. The effects of PD N rate and cultivar on grain yield were significantly influenced by year. There were also significant PF x PD interactions on grain yield.

Plant Measurements
No significant YR x PF x CV and PF x CV interactions on Y-leaf N concentration (Table 2) , whole-plant N concentration (Table 3) , and SPAD readings (Table 4) measured at all sampling times were found. Linear and quadratic effects of PF N rates on all plant measurements at all sampling times were observed. The significant PF x CV interactions were noticed for plant area measured at 3 and 4 wk after PF N application (Table 5) . Results were averaged over cultivars and are presented by year because of the nonsignificant PF x CV interaction and the significant YR x PF interaction on most of the plant measurements. (In practice, however, in N fertilizer rate studies similar to this one, we determine the plant area thresholds for each rice cultivar. This results in the greatest accuracy for indicating with the plant area method the amount of N to topdress at PD for each cultivar.)

The variation in grain yield accounted for by plant measurements considered at their maximum correlation with grain yield was 50% for plant area and total N accumulation between 3 and 4 wk after PF N application, and 37% for Y-leaf N concentration and SPAD readings at 2 wk after PF N application (Table 13). The Y-leaf N concentration and SPAD readings measured at 2 wk after PF N application accounted for <50% of the variation in grain yield. Research conducted in Arkansas by Wells et al. (1993) has shown that N concentration in rice leaf tissue is subject to both large changes over relatively short time spans during late tillering and large sample-to-sample variability within the field. Elsewhere, Maume and Dulac (1954) found that a high level of Y-leaf N concentration was essential for high grain yields, but the correlation between Y-leaf N concentration and grain yield was low.

Estimation of Total N Accumulation with Plant Measurements
No significant YR x PF x CV interaction on total N accumulation and plant measurements at all sampling times was observed (Tables 1–6). Also, the YR x CV interaction on total N accumulation was never significant (Table 1). Therefore, results of total N accumulation and plant measurements were averaged over cultivars for each year, and regression equations and coefficients of determination (R²) of total N accumulation on plant measurements and sampling times were determined (Tables 14–16) . Sampling time was included in the regression equations as a continuous variable, to reduce the dominance of dry matter production over N concentration and SPAD readings with plant growth. Interactions of sampling time with plant measurements, which indicate that the slope changes at each sampling time, were kept in the initial regression equation when they were significant; otherwise, a reduced model was fit.

In this study, plant area was a good predictor of dry matter accumulation. Sims and Place (1968) reported that the amount of N accumulated generally paralleled dry matter production and increased with plant age. In the determination of total N accumulation by rice during the vegetative growth stage, plant area measurements are easier procedures than handling dry matter and determining N concentration. Plant area measurements have added advantages, in that they actually reflect plant size, are taken rapidly, provide an on-site analysis as well as recommendation, and are nondestructive to the rice plants.

A multiple regression of total N accumulation on a combination of plant area and each of the other methods of monitoring the rice plant N status showed an improvement of the estimation of total N accumulation. Indeed, the variation in total N accumulation was 69 to 90% in 1993 (Table 14), 66 to 82% in 1994 (Table 15), and 76 to 89% in 1995 (Table 16). The highest variations in total N accumulation (90% in 1993, 82% in 1994, and 89% in 1995) were accounted for by plant area with whole-plant N concentration every year. This is due to the high correlation between plant area and dry matter accumulation, and because total N accumulation is the product of dry matter and whole-plant N concentration. Compared with the estimation of total N accumulation by plant area alone, combinations of plant area with Y-leaf N concentration or SPAD readings were as good as combination of plant area with whole-plant N concentration in 1995. Because of greater plant growth during the vegetative growth stage in 1993 and 1994, dilution of N in the plant by dry matter accumulation was more pronounced in 1993 and 1994 than in 1995. Under such conditions, plant area may not be sensitive enough to provide a good estimation of dry matter accumulation because the response to N fertilization as translated by an increase in dry matter production is relatively low. Consequently, all diagnostic methods tested (plant area, Y-leaf N concentration, whole-plant N concentration, and SPAD readings) may lose sensitivity in estimating total N accumulation when there are larger than optimum total N accumulations, except to indicate no need for additional N. This in itself is the only information really needed in commercial rice production as concerns N application at PD.

	Conclusion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Plant area was a good estimator of dry matter and total N accumulation by rice. Moreover, plant area was a more reliable estimator of total N accumulation than N concentrations and SPAD readings. The combinations of plant area with N concentrations or SPAD readings were better estimators of total N accumulation than any individual plant measurement. The correlations of grain yield with plant measurements during the vegetative growth stage were low, which highlights the fact that environmental and other conditions prevailing during the reproductive and grain-filling stages of rice had a profound influence in determining grain yields at harvest. The above findings would be valid in any rice-growing system in Arkansas, because the rice plant must develop sufficient growth and accumulate enough N at key growth stages to produce maximum yields in drill-seeded, water-seeded, and broadcast-seeded rice.SAS Institute 1985

	ACKNOWLEDGMENTS

 
We greatly appreciate Robert Baser and Mike Courtney for their help in fabricating the rice gauge, the Soils crew at Stuttgart, AR, for their help in managing the rice plots and sampling, and Dr. Chuck Wilson, Dr. Nathan Slaton, and Arkansas county agents and rice farmers for their critical evaluation of the rice gauge in commercial fields.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Published with the approval of the Director, Arkansas Agric. Exp. Stn.

	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 Ntamatungiro, S. Articles by Wells, B. R. Search for Related Content PubMed Articles by Ntamatungiro, S. Articles by Wells, B. R. Agricola Articles by Ntamatungiro, S. Articles by Wells, B. R.