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Nitrogen accumulation and partitioning by three grain legumes in response to soil water deficits
Field Crops Research, 22 (1989) 33-44
33
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
N i t r o g e n Accumulation and Partitioning by Three Grain Legumes in Response to Soil Water Deficits 1
J.D. DEVRIES, J.M. BENNETT 2, K.J. BOOTE, S.L. ALBRECHT and C.E. MALIRO
Agronomy Physiology Laboratory, Building no. 164, University of Florida, Gainesville, Florida 3261I (U.S.A.) (Accepted 10 January 1989)
ABSTRACT DeVries, J.D., Bennett, J.M., Boote, K.J., Albrecht, S.L. and Maliro, C.E., 1989. Nitrogen accumulation and partitioning by three grain legumes in response to soil water deficits. Field Crops. Res., 22: 33-44. Few experiments have been conducted to compare the partitioning and accumulation of nitrogen (N) in plant components of grain legumes grown under different soil water regimes. The objective of this study was to determine the effect of soil water deficit on N accumulation and partitioning in soybean (Glycine max L. Merr. ), pigeon pea (Cajanus cajan L. ), and peanut (Arachis hypogaea L.). In 1984, the three legumes were subjected in a field environment to either well-watered or water-stressed treatments. Nitrogen concentration, total N accumulation, and N partitioning were determined throughout the growing season by measuring N content and concentration in leaves, stems, pod walls, and seeds. Peanut accumulated more total N than either soybean or pigeon pea under both the well-watered and water-stressed treatments. Water stress decreased both N concentration and total N accumulation, especially in soybean and pigeon pea. Less remobilization of N occurred in soybean leaves and stems during the seed-filling period in the stressed treatment because the water stress limited pod addition and subsequent seed demand for N. Loss of N from leaves during seed growth was greater in the crop with the most senescent growth habit (soybean), and lowest in the non-senescent but determinate crop (pigeon pea). Although peanut does not exhibit rapid leaf senescence during seed maturation as does soybean, considerable loss in leaf N was also observed in peanut leaves during the seed-filling period. Soybean, peanut, and pigeon pea differed in accumulation and partitioning of N under water-stressed and non-stressed environments. The partitioning and remobilization of N was dependent on the growth habit of the species and was significantly influenced by soil water deficits. 1Contribution of the Agronomy Department, Institute of Food and Agricultural Sciences, University of Florida and USDA-ARS, Gainesville, FL 32611, U.S.A. Florida Agriculture Experiment Stations Journal Series no. 8810. ~Author to whom correspondence should be addressed.
0378-4290/89/$03.50
© 1989 Elsevier Science Publishers B.V.
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J.D. DE VRIES ET AL.
INTRODUCTION Water deficits inhibit numerous physiological processes and reduce crop biomass and seed yields (Hsiao, 1973). The importance of nitrogen (N) for maximizing photosynthesis, dry-matter and seed yields of legume crops has been demonstrated (Sinclair and deWit, 1976; Cure et al., 1985; Muchow and Sinclair, 1986; Sinclair, 1986). Boote et al. (1978), Lugg and Sinclair ( 1981 ) and Boon-Long et al. (1983) reported that carbon exchange rates of soybean (Glycine max L. Merr. ) increased with increases in leaf-N concentration. Differences in partitioning and remobilization of N among different plant components have been shown to influence final yields in soybean genotypes by SaladoNavarro et al. (1985), who concluded that higher rates of N remobilization from leaves to seeds during reproductive growth can lead to earlier leaf senescence and a decrease in the length of seed-filling period. While decreased leaf photosynthesis and increased rates of senescence can be related to N depletion in leaves of certain legumes, the effect of water stress on limiting N2 fixation, N accumulation, partitioning, and remobilization is less well documented. In legumes, drought periods have been shown to affect total accumulation of N (Chapman and Muchow, 1985; Cure et al., 1985; Sinclair et al., 1987). However, Egli et al. (1983) found no consistent relationship between the level or timing of water stress and the contribution of remobilized N to seed N in soybean. They suggested that the amount of remobilization of N during seed-filling was related simply to the amount of N accumulated during the entire growing season. Data of Cure et al. (1985) indicated a more rapid decline in leaf N concentration when soil water deficits occurred during the mid seed-fill stage of soybean. Selemat and Gardner (1985), using non-nodulating and standard cultivars of peanut (Arachis hypogaea L. ), showed that N was redistributed from leaves to pods during periods of N stress, but no detectable remobilization was observed in the nodulated plants. Chapman and Muchow (1985) compared N accumulation in six grain legumes in response to water deficits, and concluded that the mean N accumulation rate, total N accumulated, and proportion of N partitioned to seed were generally decreased as water stress was imposed, although significant species X water-treatment interactions were observed. The specific mechanisms responsible for the movement of N within legume plants and the influence of soil water deficits on this process are not well understood or documented. Therefore, the objective of this study was to determine the effect of soil water availability on N accumulation and partitioning in soybean, pigeon pea (Cajanus cajan L. ), and peanut. MATERIALS AND METHODS
Cultural practices and experimental design Details of the cultural practices and experimental design used for the field study conducted in 1984 at Gainesville, Florida were given in DeVries et al.
NITROGEN IN LEGUMES IN RESPONSE TO WATER DEFICIT
35
(1989). To recount briefly, peanut (cv. 'Florunner'), soybean (cv. 'Bragg'), and pigeon pea (cv. '76W' ) were grown at the University of Florida Agronomy Farm on a Kendrick fine sand (loamy-siliceous, hyperthermic family of Arenic Paleudults). Bragg soybean has a determinate growth habit and exhibits rapid leaf senescence during seed maturation, whereas Florunner peanut is both indeterminate and non-senescing. The cultivar of pigeon pea used in this study has a determinate growth habit and is later in maturity than either soybean or peanut. The experimental area was fertilized with 40: 35:130 kg h a - i N: P: K and seeds of the three grain legumes were inoculated with the appropriate Bradyrhizobium strains before sowing. The crops were sown on 12 June and thinned after emergence to final stands of 9 plants m - 1 for peanut and pigeon pea and 13 plants m-1 for soybean. All species were sown in 76-cm row widths. Recommended insect and weed control procedures were applied and pests were controlled throughout the growing season. The experiment was a split-plot design with main plots consisting of wellwatered and water-stressed treatments replicated four times. Subplot treatments consisted of the three grain-legume species. Well-watered treatments received supplemental irrigation throughout the growing season as necessary to prevent water stress. The water-stressed treatment was subjected to naturally occurring mild and moderate stresses and one longer-term, severe stress during reproductive growth; the severe stress period was terminated by irrigation.
Measurements Biomass accumulation in various plant parts (stems, leaves, pods, and seeds) was obtained from crop growth analysis conducted on each species in both water-management treatments. Plant samples were collected from a 0.9-m section of row at 7-day intervals throughout the entire growing season for each crop, and dry-matter accumulation and partitioning into various plant parts were determined. Seed yields were determined at maturity by harvesting an 8.2-m 2 area in each treatment. Plant samples from the growth analysis study were analyzed for N concentration of dried plant components using a Kjeldahl procedure (250-mg sample) where digestion was accomplished using an aluminum block digestion (Gallaher et al., 1975). Total N-content in each plant component was computed at 7-day intervals.
Statistical analyses All data were collected from the four field replicates and subjected to statistical analyses, appropriate for the experimental design, using the Statistical Analysis System (Anonymous, 1982). Following analysis of variance procedures, differences among main-plot treatment means for the same subplot treatment were determined using the least-significant-difference (LSD) comparison method.
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J.D. DE VRIES ET AL.
RESULTS AND DISCUSSION
Several periods of visible stress occurred in the three legumes during the growing season. Stress symptoms for soybean (25-26, 37, 61-65, and 78-99 days after planting (DAP)), pigeon pea (61-65, and 80-99 DAP), and peanut (26, and 82-99 DAP) were observed on different dates depending on their sensitivity to the soil water deficits. These intervals of water stress are identified by single arrows and arrows connected by bars on the x-axis on each figure. The duration and severity of the stress periods differed for each species; however, all species showed symptoms of visible water stress from 82 to 99 DAP. Plant water potentials and soil water potentials at various soil depths during the water-deficit periods were given by DeVries et al. (1989). For all three crop species, there was generally a gradual decline in the N concentration of both leaves and stems throughout most of the growing season (Fig. 1). The differing monocarpic senescence growth habits (senescing vs. non-senescing) of the three crops were quite evident in the changes in N-concentrations of leaves. Leaf N concentration of the determinate and senescing Bragg soybean declined 81%, from 5.7 to 1.1 g per 100 g dry-weight, between 37 and 127 DAP. This decline was particularly rapid during seed-filling after 92 DAP (water-stressed) and 113 DAP (well-watered). By contrast, only a slight change (20% decrease) in N concentration was observed in the non-senescing but determinate pigeon-pea crop. Soybean leaves had very low concentrations of N at the time of senescence. Peanut was intermediate in its loss of leaf N. Peanut leaf N-concentration declined from 5.4 to 2.7 g N per 100 g dry weight, a decrease of 50%. Williams (1979) observed that N concentrations in peanut leaves declined from 5 to 3 g per 100 g dry-weight during the seed-filling period. Hanway and Weber (1971) observed N concentrations near 2 g N per 100 g dry-weight at seed maturity for soybean leaves. Remobilization of N from leaves to seeds of soybean has been hypothesized as the cause of rapid leaf senescence during seed-filling (Sinclair and deWit, 1976). This rapid senescence has often been termed the 'self-destruct' mechanism associated with soybean (Sinclair and deWit, 1976). Even though peanut is considered to be less senescent and more indeterminate than Bragg soybean, considerable loss of N from leaves occurred during the seed-filling phase of peanut. In contrast, pigeon pea showed little evidence of significant loss of leaf N concentration or leaf senescence associated with seed-filling. Nitrogen concentrations in stem tissue of all three species were generally between 1 and 2 g per 100 g dry-weight and declined only gradually throughout the growing season, although more steeply during seed-filling for soybean. At harvest maturity, seed N concentrations for irrigated peanut, soybean, and pigeon pea were 5.0, 6.8, and 3.5 g per 100 g dry-weight, respectively. Similar concentrations were measured for the seeds in the water-stressed treat-
NITROGENIN LEGUMESIN RESPONSETO WATERDEFICIT
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Fig. 1. Nitrogen concentrations (g N per 100 g dry-weight) in stems and leaves of well-watered and water-stressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significant differences between the well-watered and water-stressed treatments according to the least significant difference (LSD) at the 0.05 probability level.
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J.D. DE VRIES ET AL.
ment, except for a slightly higher concentration in soybean (7.2 g per 100 g dry-weight). Water stress had little effect on N-concentration in either leaves or stems of peanut (Fig. 1 ), although there was a trend for slightly lower N concentrations in leaves of water-stressed plants. However, water stress decreased N concentrations in soybean leaves early in the growing season. Soybean responded sooner than peanut to the water-deficit periods imposed during vegetative growth phases (DeVries et al., 1989). The decrease in leaf N concentration during vegetative growth of soybean was associated with water-deficit periods (DeVries et al., 1989). Soybean was most sensitive to the water deficits and showed visible stress symptoms early during vegetative growth (25-26 and 37 DAP; Fig. 1). As a result of those stress periods, N concentrations in waterstressed soybean leaves were significantly lower than those in well-irrigated leaves at 65 DAP. Water-stressed soybean leaves remained lower in N concentration until later during the period of linear seed-filling when water-stressed leaves actually showed higher concentrations of leaf N. The severe stress period (78-99 DAB) occurred during the critical pod-addition period of soybean and significantly decreased the pod load in the water-stressed treatment. While the irrigated treatment achieved and maintained a full pod load, plants in the water-stressed treatment aborted many pods and did not enter the seed-filling phase with a full complement of pods. Since the more severe stress period occurred during a period which was very critical for establishing pod and seed numbers, the decreased sink size in the water-stressed treatment resulted in decreased N demand and thus decreased remobilization of leaf N. Because of lower N demand by seeds, N concentrations in the water-stressed soybean leaves remained higher than the concentrations in well-irrigated leaves during the last stages of seed-filling. A similar trend was also observed in pigeon pea, although the response was not quite as pronounced. These results agree with data presented by Chapman and Muchow (1985) suggesting that the timing of the onset of stress may differentially affect N partitioning. Accumulation of N in each plant component was totaled and total canopy N (g m -2 ) is shown for the three species and two water-management treatments in Fig. 2. Maximum N accumulation was observed at 113 DAP in peanut (33 g m - 2) and soybean (30 g m - 2 ) and 141 DAP in pigeon pea (28 g m - 2). Whereas pigeon pea continued to accumulate N between the 127 and 141-DAP sampling dates, peanut and soybean lost some N, presumably as a result of leaf abscission (or pod loss in the case of irrigated peanut). It is likely that some additional N accumulated in pigeon pea after 141 DAP (the last sampling date for N content) since some immature pods were still developing. Although statistically significant differences between the two water-management treatments were not generally observed until late in the growing season, the well-irrigated crop of both soybean and peanut had accumulated more total N than their respective stressed treatment on almost every sampling date.
NITROGEN IN LEGUMES IN RESPONSE TO WATER DEFICIT
39
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Fig. 2. Total canopy nitrogen accumulation (g m -2) in well-watered and water-stressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significant differences between the well-watered and water-stressed treatments according to the least significant difference (LSD) at the 0.05 probability level.
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J.D. DE VRIES ET AL.
Total N accumulation of soybean and pigeon pea was significantly decreased by water stress only late in the growing season. Two factors may have contributed to this phenomenon: 1 ) decreased N2 fixation during the severe drought period (DeVries et al., 1989); and 2) decreased pod-set during the drought period which reduced subsequent seed demand for N in both soybean and pigeon pea. To compare rates of N accumulation, slopes of the near-linear portion of the curves of N accumulation versus time (Fig. 2) were computed. Using data for N accumulated between 37 and 113 DAP, the well-watered peanut crop accumulated N at a higher rate (0.38 g m -2 day -1) than soybean (0.34 g m -2 day-1) or pigeon pea (0.22 g m -2 d a y - l ) . Water stress decreased the rates of N-accumulating by 10%, 31%, and 23% for peanut, soybean, and pigeon pea, respectively. The differential response in N-accumulation with water stress for the three species is consistent with their differential sensitivities as reflected by nitrogenase activity, leaf water status, stomatal conductance, and appearance of visible stress symptoms (DeVries et al., 1989). Chapman and Muchow (1985) also observed significant reductions in N accumulation in six grain legumes subjected to water deficits. They reported quite low rates of Naccumulation in pigeon pea when compared with other legumes. Pigeon pea in our study accumulated N at a rate of only about 60% that of peanut and soybean from 37 to 113 DAP. Pigeon pea was, however, later-maturing than the other species and accumulated N over a longer period of time, resulting in a similar total accumulation by 141 DAP (Fig. 2 ). Chapman and Muchow (1985) suggested that the lower rates of N accumulation observed for pigeon pea could be due to either a less-effective system for N2 fixation or less uptake of soil N. Since our study was conducted on a deep, well-drained sandy soil which is very low in N content, it is likely that the N2-fixation system of pigeon pea was simply less active than that of soybean or peanut. Brakke and Gardner (1987) have shown a slow rate of nodule mass accumulation for pigeon pea compared with soybean or cowpea ( Vigna unguiculata L. ). The three species differed in partitioning of N to leaves, stems and seeds during reproductive growth (Figs. 3 and 4). At harvest, peanut and pigeon pea retained a greater amount of N in leaves and stems than soybean. Almost all leaves of well-watered soybean had senesced and abscised by harvest maturity, whereas both peanut and pigeon pea retained considerable leaf area and N. Abscission of leaves by both soybean and pigeon pea in the water-stressed treatment increased the loss of N late in the growing season. In contrast, waterstressed peanut leaves were slow to abscise and remained as an N source after the water stress was relieved. Chapman and Muchow (1987) also observed that soybean redistributed more N to the seed than did the other legumes studied, and that pigeon pea retained considerable N in leaves at harvest maturity. Total N in pigeon-pea stems increased over the entire season, whereas N in peanut and soybean stems declined during reproductive growth. Soybean stem
N I T R O G E N IN L E G U M E S IN R E S P O N S E TO WATER D E F I C I T
41
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Fig. 3. Total nitrogen accumulation (g m - 2) in leavesand stems of well-watered and water-stressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significantdifferencesbetween the well-watered and water-stressed treatments according to the leastsignificantdifference (LSD) at the 0.05 probability level.
42
J.D. DE VRIES ET AL.
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Fig. 4. Total nitrogen accumulation (g m -2) in seeds and pod walls of well-watered and waterstressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significant differences between the well-watered and water-stressed treatments according to the least significant d i f f e r e n c e (LSD) at the 0.05 probability level.
NITROGEN IN LEGUMES IN RESPONSE TO WATER DEFICIT
43
N decreased sharply after 100 DAP and continued to decline until seed maturity. Remobilization of N {Fig. 1 ) and leaf abscission in the irrigated soybean treatment appeared nearly complete by 127 DAP; however, there was significantly more total N present in leaves of water-stressed plants on this data. This increase was due to both higher concentrations (Fig. 1) of N in leaves coupled with less leaf senescence and abscission resulting from lower seed demand for N. A similar response was apparent for N in stems of water-stressed soybeans. Seed yields of the well-irrigated treatments at harvest maturity were 480, 359, and 283 g m -e for peanut, soybean, and pigeon pea, respectively. Seed yields in the water-stressed treatments were 412, 188, and 185 g m -e, respectively, representing 14%, 48%, and 35% yield reductions. Irrigated soybeans and peanut accumulated similar amounts of N in seeds but considerably more than pigeon pea (Fig. 4). Although soybean yielded less than peanut, a higher seed-N concentration resulted in total N yield in seeds being similar in both species. Pigeon pea had both low seed yields and low seedN concentrations, resulting in low total amounts of N in seeds of both irrigated and water-stressed treatments. By 141 DAP, accumulation of N in water-stressed soybean was 36% less than in the irrigated crop, while total N accumulation in peanut seed was unaffected by the water stress. Total seed N in pigeon pea was decreased 57% by the water stress. Total N in pod walls was quite low in all three species, ranging between 1 and 3 g m -z, and was quite stable during the growing season. SUMMARYAND CONCLUSIONS This study demonstrated that soybean, peanut, and pigeon pea differed in accumulation of N under both well-watered and water-stressed environments. Total N accumulation was significantly decreased by water stress, although decreases were more severe in soybean and pigeon pea than in peanut. Pigeon pea had lower rates of N accumulation compared to soybean and peanut, although these lower rates continued for a longer period of time before seeds were mature. These results also suggest that the partitioning and remobilization of N are related to the monocarpic senescence traits of the species and are significantly influenced by soil water deficit. Remobilization of N from soybean leaves was greater than from either peanut or pigeon pea leaves, and increased with increasing tendenc~e's for canopy senescence. In well-watered plants, N concentration in soybean leaves at seed maturity was very low and most leaves had abscised, whereas moderate concentrations of N remained in peanut and pigeon pea leaves and considerable leaf area was retained. In this study, N-remobilization in soybean and pigeon pea was decreased in response to a water deficit which inhibited pod set. However, water deficits which occur at later
44
J.D.DEVRIESET AL.
stages of seed-filling m a y e n h a n c e r e m o b i l i z a t i o n , as r e p o r t e d b y C u r e et al. (1985). I t is c o n c l u d e d t h a t in a d d i t i o n to N2 f i x a t i o n a n d a c c u m u l a t i o n , p a r t i t i o n i n g a n d r e m o b i l i z a t i o n of N p l a y s a n i m p o r t a n t role in yield f o r m a t i o n of legume crops. A d d i t i o n a l studies to e x a m i n e N a c c u m u l a t i o n , p a r t i t i o n i n g , a n d r e m o b i l i z a t i o n d u r i n g w a t e r deficits s h o u l d lead to a b e t t e r u n d e r s t a n d i n g of t h e role of N in yield f o r m a t i o n .
REFERENCES Anonymous, 1982. SAS User's Guide: Statistics. SAS Institute Inc., Cary, NC, 584 pp. Boon-Long, P., Egli, D.B. and Leggett, J.E., 1983. Leaf nitrogen and photosynthesis during reproductive growth in soybeans. Crop Sci., 23: 617-620. Boote, K.J., Gallaher, R.N., Roberston, W.K., Hinson, K. and Hammond, L.C., 1978. Effect of foliar fertilization on photosynthesis, leaf nutrition, and yield of soybeans. Agron. J., 70: 787791. Brakke, M.P. and Gardner, F.P., 1987. Juvenile growth in pigeon pea, soybean, and cowpea in relation to seed and seedling characteristics. Crop Sci., 27: 311-316. Chapman, A.L. and Muchow, R.C., 1985. Nitrogen accumulated and partitioned at maturity by grain legumes grown under different water regimes in a semi-arid tropical environment. Field Crops Res., 11: 69-79. Cure, J.D., Raper, C.D., Patterson, R.P., Robarge, W.P., 1985. Dinitrogen fixation in soybean in response to leaf water stress and seed growth rate. Crop Sci., 25."52-58. DeVries, J.D., Bennett, J.M., Albrecht, S.L. and Boote, K.J., 1989. Water relations, nitrogenase activity, and root development of three grain legumes in response to soil water deficits. Field Crops Res., 21: 215-226. Egli, D.B., Meckel, L., Phillips, R.E., Radcliffe, D. and Leggett, J.E., 1983. Moisture stress and nitrogen redistribution in soybean. Agron. J., 75: 1027-1031. Gallaher, R.N., Weldon, C.O. and Futral, J.G., 1975. An aluminium block digester for plant and soil analysis. Soil Sci. Soc. Am. Proc., 39: 803-806. Hanway, J.J. and Weber, C.R., 1971. N, P, and K percentages in soybean (Glycine max (L.) Merrill ) plant parts. Agron. J., 63: 286-290. Hsiao, T.C., 1973. Plant responses to water stress. Ann. Rev. Plant Physiol., 24: 519-570. Lugg, D.G. and Sinclair, T.R., 1981. Seasonal changes in photosynthesis of field-grown soybean leaflets. II. Relation to nitrogen content. Photosynthetica, 15: 138-144. Muchow, R.C. and Sinclair, T.R., 1986. Water and nitrogen limitations in soybean grain production. II. Field and model analyses. Field Crops Res., 15: 143-156. Salado-Navarro, L.R., Hinson, K. and Sinclair, T.R., 1985. Nitrogen partitioning and dry matter allocation in soybeans with different seed protein concentration. Crop Sci., 25: 451-455. Selemat, A. and Gardner, F.P., 1985. Nitrogen partitioning and redistribution in nonnodulating peanut related to nitrogen stress. Agrom J., 77: 859-862. Sinclair, T.R., 1986. Water and nitrogen limitations in soybean grain production. I. Model development. Field Crops Res., 15: 125-141. Sinclair, T.R. and deWit, C.T., 1976. Analysis of the carbon and nitrogen limitations of soybean yield. Agron. J., 68: 319-324. Sinclair, T.R., Muchow, R.C., Bennett, J.M. and Hammond, L.C., 1987. Relative sensitivity of nitrogen and biomass accumulation to drought in field-grown soybean. Agron. J., 79: 986-991. Williams, J., 1979. The physiology of groundnut. II. Nitrogen accumulation and distribution. Rhodesian J. Agric. Res., 17: 49-57.
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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
N i t r o g e n Accumulation and Partitioning by Three Grain Legumes in Response to Soil Water Deficits 1
J.D. DEVRIES, J.M. BENNETT 2, K.J. BOOTE, S.L. ALBRECHT and C.E. MALIRO
Agronomy Physiology Laboratory, Building no. 164, University of Florida, Gainesville, Florida 3261I (U.S.A.) (Accepted 10 January 1989)
ABSTRACT DeVries, J.D., Bennett, J.M., Boote, K.J., Albrecht, S.L. and Maliro, C.E., 1989. Nitrogen accumulation and partitioning by three grain legumes in response to soil water deficits. Field Crops. Res., 22: 33-44. Few experiments have been conducted to compare the partitioning and accumulation of nitrogen (N) in plant components of grain legumes grown under different soil water regimes. The objective of this study was to determine the effect of soil water deficit on N accumulation and partitioning in soybean (Glycine max L. Merr. ), pigeon pea (Cajanus cajan L. ), and peanut (Arachis hypogaea L.). In 1984, the three legumes were subjected in a field environment to either well-watered or water-stressed treatments. Nitrogen concentration, total N accumulation, and N partitioning were determined throughout the growing season by measuring N content and concentration in leaves, stems, pod walls, and seeds. Peanut accumulated more total N than either soybean or pigeon pea under both the well-watered and water-stressed treatments. Water stress decreased both N concentration and total N accumulation, especially in soybean and pigeon pea. Less remobilization of N occurred in soybean leaves and stems during the seed-filling period in the stressed treatment because the water stress limited pod addition and subsequent seed demand for N. Loss of N from leaves during seed growth was greater in the crop with the most senescent growth habit (soybean), and lowest in the non-senescent but determinate crop (pigeon pea). Although peanut does not exhibit rapid leaf senescence during seed maturation as does soybean, considerable loss in leaf N was also observed in peanut leaves during the seed-filling period. Soybean, peanut, and pigeon pea differed in accumulation and partitioning of N under water-stressed and non-stressed environments. The partitioning and remobilization of N was dependent on the growth habit of the species and was significantly influenced by soil water deficits. 1Contribution of the Agronomy Department, Institute of Food and Agricultural Sciences, University of Florida and USDA-ARS, Gainesville, FL 32611, U.S.A. Florida Agriculture Experiment Stations Journal Series no. 8810. ~Author to whom correspondence should be addressed.
0378-4290/89/$03.50
© 1989 Elsevier Science Publishers B.V.
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J.D. DE VRIES ET AL.
INTRODUCTION Water deficits inhibit numerous physiological processes and reduce crop biomass and seed yields (Hsiao, 1973). The importance of nitrogen (N) for maximizing photosynthesis, dry-matter and seed yields of legume crops has been demonstrated (Sinclair and deWit, 1976; Cure et al., 1985; Muchow and Sinclair, 1986; Sinclair, 1986). Boote et al. (1978), Lugg and Sinclair ( 1981 ) and Boon-Long et al. (1983) reported that carbon exchange rates of soybean (Glycine max L. Merr. ) increased with increases in leaf-N concentration. Differences in partitioning and remobilization of N among different plant components have been shown to influence final yields in soybean genotypes by SaladoNavarro et al. (1985), who concluded that higher rates of N remobilization from leaves to seeds during reproductive growth can lead to earlier leaf senescence and a decrease in the length of seed-filling period. While decreased leaf photosynthesis and increased rates of senescence can be related to N depletion in leaves of certain legumes, the effect of water stress on limiting N2 fixation, N accumulation, partitioning, and remobilization is less well documented. In legumes, drought periods have been shown to affect total accumulation of N (Chapman and Muchow, 1985; Cure et al., 1985; Sinclair et al., 1987). However, Egli et al. (1983) found no consistent relationship between the level or timing of water stress and the contribution of remobilized N to seed N in soybean. They suggested that the amount of remobilization of N during seed-filling was related simply to the amount of N accumulated during the entire growing season. Data of Cure et al. (1985) indicated a more rapid decline in leaf N concentration when soil water deficits occurred during the mid seed-fill stage of soybean. Selemat and Gardner (1985), using non-nodulating and standard cultivars of peanut (Arachis hypogaea L. ), showed that N was redistributed from leaves to pods during periods of N stress, but no detectable remobilization was observed in the nodulated plants. Chapman and Muchow (1985) compared N accumulation in six grain legumes in response to water deficits, and concluded that the mean N accumulation rate, total N accumulated, and proportion of N partitioned to seed were generally decreased as water stress was imposed, although significant species X water-treatment interactions were observed. The specific mechanisms responsible for the movement of N within legume plants and the influence of soil water deficits on this process are not well understood or documented. Therefore, the objective of this study was to determine the effect of soil water availability on N accumulation and partitioning in soybean, pigeon pea (Cajanus cajan L. ), and peanut. MATERIALS AND METHODS
Cultural practices and experimental design Details of the cultural practices and experimental design used for the field study conducted in 1984 at Gainesville, Florida were given in DeVries et al.
NITROGEN IN LEGUMES IN RESPONSE TO WATER DEFICIT
35
(1989). To recount briefly, peanut (cv. 'Florunner'), soybean (cv. 'Bragg'), and pigeon pea (cv. '76W' ) were grown at the University of Florida Agronomy Farm on a Kendrick fine sand (loamy-siliceous, hyperthermic family of Arenic Paleudults). Bragg soybean has a determinate growth habit and exhibits rapid leaf senescence during seed maturation, whereas Florunner peanut is both indeterminate and non-senescing. The cultivar of pigeon pea used in this study has a determinate growth habit and is later in maturity than either soybean or peanut. The experimental area was fertilized with 40: 35:130 kg h a - i N: P: K and seeds of the three grain legumes were inoculated with the appropriate Bradyrhizobium strains before sowing. The crops were sown on 12 June and thinned after emergence to final stands of 9 plants m - 1 for peanut and pigeon pea and 13 plants m-1 for soybean. All species were sown in 76-cm row widths. Recommended insect and weed control procedures were applied and pests were controlled throughout the growing season. The experiment was a split-plot design with main plots consisting of wellwatered and water-stressed treatments replicated four times. Subplot treatments consisted of the three grain-legume species. Well-watered treatments received supplemental irrigation throughout the growing season as necessary to prevent water stress. The water-stressed treatment was subjected to naturally occurring mild and moderate stresses and one longer-term, severe stress during reproductive growth; the severe stress period was terminated by irrigation.
Measurements Biomass accumulation in various plant parts (stems, leaves, pods, and seeds) was obtained from crop growth analysis conducted on each species in both water-management treatments. Plant samples were collected from a 0.9-m section of row at 7-day intervals throughout the entire growing season for each crop, and dry-matter accumulation and partitioning into various plant parts were determined. Seed yields were determined at maturity by harvesting an 8.2-m 2 area in each treatment. Plant samples from the growth analysis study were analyzed for N concentration of dried plant components using a Kjeldahl procedure (250-mg sample) where digestion was accomplished using an aluminum block digestion (Gallaher et al., 1975). Total N-content in each plant component was computed at 7-day intervals.
Statistical analyses All data were collected from the four field replicates and subjected to statistical analyses, appropriate for the experimental design, using the Statistical Analysis System (Anonymous, 1982). Following analysis of variance procedures, differences among main-plot treatment means for the same subplot treatment were determined using the least-significant-difference (LSD) comparison method.
36
J.D. DE VRIES ET AL.
RESULTS AND DISCUSSION
Several periods of visible stress occurred in the three legumes during the growing season. Stress symptoms for soybean (25-26, 37, 61-65, and 78-99 days after planting (DAP)), pigeon pea (61-65, and 80-99 DAP), and peanut (26, and 82-99 DAP) were observed on different dates depending on their sensitivity to the soil water deficits. These intervals of water stress are identified by single arrows and arrows connected by bars on the x-axis on each figure. The duration and severity of the stress periods differed for each species; however, all species showed symptoms of visible water stress from 82 to 99 DAP. Plant water potentials and soil water potentials at various soil depths during the water-deficit periods were given by DeVries et al. (1989). For all three crop species, there was generally a gradual decline in the N concentration of both leaves and stems throughout most of the growing season (Fig. 1). The differing monocarpic senescence growth habits (senescing vs. non-senescing) of the three crops were quite evident in the changes in N-concentrations of leaves. Leaf N concentration of the determinate and senescing Bragg soybean declined 81%, from 5.7 to 1.1 g per 100 g dry-weight, between 37 and 127 DAP. This decline was particularly rapid during seed-filling after 92 DAP (water-stressed) and 113 DAP (well-watered). By contrast, only a slight change (20% decrease) in N concentration was observed in the non-senescing but determinate pigeon-pea crop. Soybean leaves had very low concentrations of N at the time of senescence. Peanut was intermediate in its loss of leaf N. Peanut leaf N-concentration declined from 5.4 to 2.7 g N per 100 g dry weight, a decrease of 50%. Williams (1979) observed that N concentrations in peanut leaves declined from 5 to 3 g per 100 g dry-weight during the seed-filling period. Hanway and Weber (1971) observed N concentrations near 2 g N per 100 g dry-weight at seed maturity for soybean leaves. Remobilization of N from leaves to seeds of soybean has been hypothesized as the cause of rapid leaf senescence during seed-filling (Sinclair and deWit, 1976). This rapid senescence has often been termed the 'self-destruct' mechanism associated with soybean (Sinclair and deWit, 1976). Even though peanut is considered to be less senescent and more indeterminate than Bragg soybean, considerable loss of N from leaves occurred during the seed-filling phase of peanut. In contrast, pigeon pea showed little evidence of significant loss of leaf N concentration or leaf senescence associated with seed-filling. Nitrogen concentrations in stem tissue of all three species were generally between 1 and 2 g per 100 g dry-weight and declined only gradually throughout the growing season, although more steeply during seed-filling for soybean. At harvest maturity, seed N concentrations for irrigated peanut, soybean, and pigeon pea were 5.0, 6.8, and 3.5 g per 100 g dry-weight, respectively. Similar concentrations were measured for the seeds in the water-stressed treat-
NITROGENIN LEGUMESIN RESPONSETO WATERDEFICIT
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Fig. 1. Nitrogen concentrations (g N per 100 g dry-weight) in stems and leaves of well-watered and water-stressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significant differences between the well-watered and water-stressed treatments according to the least significant difference (LSD) at the 0.05 probability level.
38
J.D. DE VRIES ET AL.
ment, except for a slightly higher concentration in soybean (7.2 g per 100 g dry-weight). Water stress had little effect on N-concentration in either leaves or stems of peanut (Fig. 1 ), although there was a trend for slightly lower N concentrations in leaves of water-stressed plants. However, water stress decreased N concentrations in soybean leaves early in the growing season. Soybean responded sooner than peanut to the water-deficit periods imposed during vegetative growth phases (DeVries et al., 1989). The decrease in leaf N concentration during vegetative growth of soybean was associated with water-deficit periods (DeVries et al., 1989). Soybean was most sensitive to the water deficits and showed visible stress symptoms early during vegetative growth (25-26 and 37 DAP; Fig. 1). As a result of those stress periods, N concentrations in waterstressed soybean leaves were significantly lower than those in well-irrigated leaves at 65 DAP. Water-stressed soybean leaves remained lower in N concentration until later during the period of linear seed-filling when water-stressed leaves actually showed higher concentrations of leaf N. The severe stress period (78-99 DAB) occurred during the critical pod-addition period of soybean and significantly decreased the pod load in the water-stressed treatment. While the irrigated treatment achieved and maintained a full pod load, plants in the water-stressed treatment aborted many pods and did not enter the seed-filling phase with a full complement of pods. Since the more severe stress period occurred during a period which was very critical for establishing pod and seed numbers, the decreased sink size in the water-stressed treatment resulted in decreased N demand and thus decreased remobilization of leaf N. Because of lower N demand by seeds, N concentrations in the water-stressed soybean leaves remained higher than the concentrations in well-irrigated leaves during the last stages of seed-filling. A similar trend was also observed in pigeon pea, although the response was not quite as pronounced. These results agree with data presented by Chapman and Muchow (1985) suggesting that the timing of the onset of stress may differentially affect N partitioning. Accumulation of N in each plant component was totaled and total canopy N (g m -2 ) is shown for the three species and two water-management treatments in Fig. 2. Maximum N accumulation was observed at 113 DAP in peanut (33 g m - 2) and soybean (30 g m - 2 ) and 141 DAP in pigeon pea (28 g m - 2). Whereas pigeon pea continued to accumulate N between the 127 and 141-DAP sampling dates, peanut and soybean lost some N, presumably as a result of leaf abscission (or pod loss in the case of irrigated peanut). It is likely that some additional N accumulated in pigeon pea after 141 DAP (the last sampling date for N content) since some immature pods were still developing. Although statistically significant differences between the two water-management treatments were not generally observed until late in the growing season, the well-irrigated crop of both soybean and peanut had accumulated more total N than their respective stressed treatment on almost every sampling date.
NITROGEN IN LEGUMES IN RESPONSE TO WATER DEFICIT
39
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Fig. 2. Total canopy nitrogen accumulation (g m -2) in well-watered and water-stressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significant differences between the well-watered and water-stressed treatments according to the least significant difference (LSD) at the 0.05 probability level.
40
J.D. DE VRIES ET AL.
Total N accumulation of soybean and pigeon pea was significantly decreased by water stress only late in the growing season. Two factors may have contributed to this phenomenon: 1 ) decreased N2 fixation during the severe drought period (DeVries et al., 1989); and 2) decreased pod-set during the drought period which reduced subsequent seed demand for N in both soybean and pigeon pea. To compare rates of N accumulation, slopes of the near-linear portion of the curves of N accumulation versus time (Fig. 2) were computed. Using data for N accumulated between 37 and 113 DAP, the well-watered peanut crop accumulated N at a higher rate (0.38 g m -2 day -1) than soybean (0.34 g m -2 day-1) or pigeon pea (0.22 g m -2 d a y - l ) . Water stress decreased the rates of N-accumulating by 10%, 31%, and 23% for peanut, soybean, and pigeon pea, respectively. The differential response in N-accumulation with water stress for the three species is consistent with their differential sensitivities as reflected by nitrogenase activity, leaf water status, stomatal conductance, and appearance of visible stress symptoms (DeVries et al., 1989). Chapman and Muchow (1985) also observed significant reductions in N accumulation in six grain legumes subjected to water deficits. They reported quite low rates of Naccumulation in pigeon pea when compared with other legumes. Pigeon pea in our study accumulated N at a rate of only about 60% that of peanut and soybean from 37 to 113 DAP. Pigeon pea was, however, later-maturing than the other species and accumulated N over a longer period of time, resulting in a similar total accumulation by 141 DAP (Fig. 2 ). Chapman and Muchow (1985) suggested that the lower rates of N accumulation observed for pigeon pea could be due to either a less-effective system for N2 fixation or less uptake of soil N. Since our study was conducted on a deep, well-drained sandy soil which is very low in N content, it is likely that the N2-fixation system of pigeon pea was simply less active than that of soybean or peanut. Brakke and Gardner (1987) have shown a slow rate of nodule mass accumulation for pigeon pea compared with soybean or cowpea ( Vigna unguiculata L. ). The three species differed in partitioning of N to leaves, stems and seeds during reproductive growth (Figs. 3 and 4). At harvest, peanut and pigeon pea retained a greater amount of N in leaves and stems than soybean. Almost all leaves of well-watered soybean had senesced and abscised by harvest maturity, whereas both peanut and pigeon pea retained considerable leaf area and N. Abscission of leaves by both soybean and pigeon pea in the water-stressed treatment increased the loss of N late in the growing season. In contrast, waterstressed peanut leaves were slow to abscise and remained as an N source after the water stress was relieved. Chapman and Muchow (1987) also observed that soybean redistributed more N to the seed than did the other legumes studied, and that pigeon pea retained considerable N in leaves at harvest maturity. Total N in pigeon-pea stems increased over the entire season, whereas N in peanut and soybean stems declined during reproductive growth. Soybean stem
N I T R O G E N IN L E G U M E S IN R E S P O N S E TO WATER D E F I C I T
41
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Fig. 3. Total nitrogen accumulation (g m - 2) in leavesand stems of well-watered and water-stressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significantdifferencesbetween the well-watered and water-stressed treatments according to the leastsignificantdifference (LSD) at the 0.05 probability level.
42
J.D. DE VRIES ET AL.
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Fig. 4. Total nitrogen accumulation (g m -2) in seeds and pod walls of well-watered and waterstressed peanut, soybean, and pigeon pea. Asterisks below data points indicate significant differences between the well-watered and water-stressed treatments according to the least significant d i f f e r e n c e (LSD) at the 0.05 probability level.
NITROGEN IN LEGUMES IN RESPONSE TO WATER DEFICIT
43
N decreased sharply after 100 DAP and continued to decline until seed maturity. Remobilization of N {Fig. 1 ) and leaf abscission in the irrigated soybean treatment appeared nearly complete by 127 DAP; however, there was significantly more total N present in leaves of water-stressed plants on this data. This increase was due to both higher concentrations (Fig. 1) of N in leaves coupled with less leaf senescence and abscission resulting from lower seed demand for N. A similar response was apparent for N in stems of water-stressed soybeans. Seed yields of the well-irrigated treatments at harvest maturity were 480, 359, and 283 g m -e for peanut, soybean, and pigeon pea, respectively. Seed yields in the water-stressed treatments were 412, 188, and 185 g m -e, respectively, representing 14%, 48%, and 35% yield reductions. Irrigated soybeans and peanut accumulated similar amounts of N in seeds but considerably more than pigeon pea (Fig. 4). Although soybean yielded less than peanut, a higher seed-N concentration resulted in total N yield in seeds being similar in both species. Pigeon pea had both low seed yields and low seedN concentrations, resulting in low total amounts of N in seeds of both irrigated and water-stressed treatments. By 141 DAP, accumulation of N in water-stressed soybean was 36% less than in the irrigated crop, while total N accumulation in peanut seed was unaffected by the water stress. Total seed N in pigeon pea was decreased 57% by the water stress. Total N in pod walls was quite low in all three species, ranging between 1 and 3 g m -z, and was quite stable during the growing season. SUMMARYAND CONCLUSIONS This study demonstrated that soybean, peanut, and pigeon pea differed in accumulation of N under both well-watered and water-stressed environments. Total N accumulation was significantly decreased by water stress, although decreases were more severe in soybean and pigeon pea than in peanut. Pigeon pea had lower rates of N accumulation compared to soybean and peanut, although these lower rates continued for a longer period of time before seeds were mature. These results also suggest that the partitioning and remobilization of N are related to the monocarpic senescence traits of the species and are significantly influenced by soil water deficit. Remobilization of N from soybean leaves was greater than from either peanut or pigeon pea leaves, and increased with increasing tendenc~e's for canopy senescence. In well-watered plants, N concentration in soybean leaves at seed maturity was very low and most leaves had abscised, whereas moderate concentrations of N remained in peanut and pigeon pea leaves and considerable leaf area was retained. In this study, N-remobilization in soybean and pigeon pea was decreased in response to a water deficit which inhibited pod set. However, water deficits which occur at later
44
J.D.DEVRIESET AL.
stages of seed-filling m a y e n h a n c e r e m o b i l i z a t i o n , as r e p o r t e d b y C u r e et al. (1985). I t is c o n c l u d e d t h a t in a d d i t i o n to N2 f i x a t i o n a n d a c c u m u l a t i o n , p a r t i t i o n i n g a n d r e m o b i l i z a t i o n of N p l a y s a n i m p o r t a n t role in yield f o r m a t i o n of legume crops. A d d i t i o n a l studies to e x a m i n e N a c c u m u l a t i o n , p a r t i t i o n i n g , a n d r e m o b i l i z a t i o n d u r i n g w a t e r deficits s h o u l d lead to a b e t t e r u n d e r s t a n d i n g of t h e role of N in yield f o r m a t i o n .
REFERENCES Anonymous, 1982. SAS User's Guide: Statistics. SAS Institute Inc., Cary, NC, 584 pp. Boon-Long, P., Egli, D.B. and Leggett, J.E., 1983. Leaf nitrogen and photosynthesis during reproductive growth in soybeans. Crop Sci., 23: 617-620. Boote, K.J., Gallaher, R.N., Roberston, W.K., Hinson, K. and Hammond, L.C., 1978. Effect of foliar fertilization on photosynthesis, leaf nutrition, and yield of soybeans. Agron. J., 70: 787791. Brakke, M.P. and Gardner, F.P., 1987. Juvenile growth in pigeon pea, soybean, and cowpea in relation to seed and seedling characteristics. Crop Sci., 27: 311-316. Chapman, A.L. and Muchow, R.C., 1985. Nitrogen accumulated and partitioned at maturity by grain legumes grown under different water regimes in a semi-arid tropical environment. Field Crops Res., 11: 69-79. Cure, J.D., Raper, C.D., Patterson, R.P., Robarge, W.P., 1985. Dinitrogen fixation in soybean in response to leaf water stress and seed growth rate. Crop Sci., 25."52-58. DeVries, J.D., Bennett, J.M., Albrecht, S.L. and Boote, K.J., 1989. Water relations, nitrogenase activity, and root development of three grain legumes in response to soil water deficits. Field Crops Res., 21: 215-226. Egli, D.B., Meckel, L., Phillips, R.E., Radcliffe, D. and Leggett, J.E., 1983. Moisture stress and nitrogen redistribution in soybean. Agron. J., 75: 1027-1031. Gallaher, R.N., Weldon, C.O. and Futral, J.G., 1975. An aluminium block digester for plant and soil analysis. Soil Sci. Soc. Am. Proc., 39: 803-806. Hanway, J.J. and Weber, C.R., 1971. N, P, and K percentages in soybean (Glycine max (L.) Merrill ) plant parts. Agron. J., 63: 286-290. Hsiao, T.C., 1973. Plant responses to water stress. Ann. Rev. Plant Physiol., 24: 519-570. Lugg, D.G. and Sinclair, T.R., 1981. Seasonal changes in photosynthesis of field-grown soybean leaflets. II. Relation to nitrogen content. Photosynthetica, 15: 138-144. Muchow, R.C. and Sinclair, T.R., 1986. Water and nitrogen limitations in soybean grain production. II. Field and model analyses. Field Crops Res., 15: 143-156. Salado-Navarro, L.R., Hinson, K. and Sinclair, T.R., 1985. Nitrogen partitioning and dry matter allocation in soybeans with different seed protein concentration. Crop Sci., 25: 451-455. Selemat, A. and Gardner, F.P., 1985. Nitrogen partitioning and redistribution in nonnodulating peanut related to nitrogen stress. Agrom J., 77: 859-862. Sinclair, T.R., 1986. Water and nitrogen limitations in soybean grain production. I. Model development. Field Crops Res., 15: 125-141. Sinclair, T.R. and deWit, C.T., 1976. Analysis of the carbon and nitrogen limitations of soybean yield. Agron. J., 68: 319-324. Sinclair, T.R., Muchow, R.C., Bennett, J.M. and Hammond, L.C., 1987. Relative sensitivity of nitrogen and biomass accumulation to drought in field-grown soybean. Agron. J., 79: 986-991. Williams, J., 1979. The physiology of groundnut. II. Nitrogen accumulation and distribution. Rhodesian J. Agric. Res., 17: 49-57.