Variation in nitrogen fixation and yield in landraces of bambara groundnut (Vigna subterranea L.)

ELSEVIER

Field Crops Research 48 (1996) 57-64

Field Crops Research

Variation in nitrogen fixation and yield in landraces of bambara groundnut ( Vigna subterranea L.) B.D. Kishinevsky *, M. Zur, Y. Friedman, G. Meromi, E. Ben-Moshe, Chaya Nemas Departmentof Agronomy and Natural Resources, ARO, The Volcani Center, Bet Dagan 50250, Israel Received 26 April 1995; accepted 19 March 1996

Abstract Field experiments were performed in 1993 and 1994 on soil free of Vigna subterranea-nodulating rhizobia to study the nodulation and nitrogen fixation of 23 indigenous bambara groundnut landraces (bunchy and spreading types) from Malawi. Inoculation with a mixture of two Bradyrhizobium strains (280A and 100M) resulted in abundant nodulation for most of the accessions tested. As estimated over the total experiment, there was no significant increase in the number and weight of nodules between 68 and 105 days after sowing (82-94 nodules per plant respectively), but nitrogenase activity in nodules increased in this period from 20 to 43 p~mol C2H 4 g-1 dry weight nodules h - I . Landraces varied significantly in yield and N-harvest index, and the total amount of nitrogen fixed was not a clear guide to high pod and seed yields. Statistically significant correlations were found between harvest index per single plant and pod and seed yields of the plant. High-yielding landraces were found only among the accessions with a bunchy growth habit. The amount of symbiotic N measured in the shoots of 130 day old plants (landrace 3C I) was 1.2 g per plant, which was 80% of the total N accumulated in the plants.

Keywords: Bambara groundnut; Vigna subterranea L.; Bradyrhizobium; Inoculation; N2-fixation 1. Introduction Bambara groundnut (Vigna subterranea L.), formerly known as Voandzeia subterranea L., is indigenous to tropical Africa (Rachie and Silvestre, 1977), where it ranks third in importance as a grain legume after groundnut (Arachis hypogaea) and cowpea (Vigna unguiculata) in terms of production and consumption (Doku and Karikari, 1971; Anonymous, 1992). The crop has spread to other regions and is now also used in Malaysia, Java and South America (Allen and Allen, 1981). This ' p o o r m a n ' s crop' thrives on poor soils and is relatively tolerant of * Corresponding author.

drought, pests and diseases (Hepper, 1963). In many countries, bambara groundnut is grown mostly on soils that are too poor for groundnut, although high yields can be achieved on fertile soils with adequate rainfall ( 9 0 0 - 1 2 0 0 mm). Dadson et al. (1988) and Kishinevsky et al. (1993) demonstrated benefits to bambara groundnut plants from inoculation with effective Bradyrhizobium strains. In the present study, attempts were made: (a) to determine nodule development, nitrogenase activity, nitrogen accumulation and yield of irrigated bambara groundnut grown under field conditions; and (b) to identify bambara groundnut landraces potentially superior in seed yield and N 2 fixation. The trials, which provided preliminary information

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B.D. Kishinevskyet al. / Field Crops Research 48 (1996) 57-64

58

on the potential of the landraces in Israel, were directed at gaining insight into their development and at finding useful screening methods for use in Malawi.

2. Materials and methods

2.1. Bambara groundnut landraces An outline of some features of the landraces tested is presented in Table 1. Sixteen landraces were sown in 1993 and 20 in 1994. With the exception of 21A, 24A and 66A, which gave extremely low yields of pods and seeds in 1993, all the lines used in 1993 were also utilized in the second field experiment. In addition, six lines ( I l A 3, l l B , 16B 2, 47A, 79C and 88A) previously used in a greenhouse study were selected for field testing in 1994. All landraces had been collected in 1991-92 from different areas in Malawi and seeds were obtained from Dr. Spider Mughogho, Bunda College, Lilongwe, Malawi.

2.2. Preparation of inoculants and soil inoculation Bradyrhizobium strains 280A (from A. hypogaea) and 100M (from Macroptilium atropurpureum) (Kishinevsky et al., 1993) were used as inoculants. The strains were cultured in yeast-mannitol broth, (Vincent, 1970), aseptically injected into gammairradiated peat (Roughley and Pulsford, 1982) and kept at 28°C for 10 days. Inoculation was done at sowing by pouring a suspension of peat inoculant, containing both strains, over the seed in open furrows to provide 1 × 10 s Bradyrhizobium cells m row. The furrows were closed immediately after inoculation. In 1994, a comparison was also made between inoculated and non-inoculated field-grown bambara groundnuts (landrace 3C l) with reference to nodule development and nitrogen accumulation in plant tops. 2.3. Field experiments Field experiments were conducted in 1993 and 1994 on adjacent fields at the Besor Experiment

Table 1 An outline of some features of different bambara groundnutlandraces used for experimentsin 1993 and 1994 Landrace

Growth habit

Seed mass (mg)

Seed colour

Experiments

1Aj 1B1 3C l 8B l 11A3 11B 16B2 17A1 19A ~ 21A ~ 24A l 26D 30A 1 31A 1 47A 49A 57A l 57B l 66A I 75A 79C 88A 89C

slightly spreading spreading spreading bunched bunched bunched bunched spreading bunched spreading bunched spreading bunched bunched bunched bunched bunched bunched bunched slightly spreading bunched bunched spreading

316 330 253 259 400 375 459 443 532 387 530 262 366 478 432 400 530 436 360 336 355 434 476

light brown brown spotted brown black black purple black purple black speckled white reddish white with black spots brown brown spotted white light brown with black spots light brown speckled brown dark brown reddish purple black white light black white black

1993-94 1993-94 1993-94 1993-94 1994 1994 1994 1993-94 1993-94 1993 1993 1994 1993-94 1993-94 1994 1993-94 1993-94 1993-94 1993-94 1993-94 1994 1994 1993-94

B.D. Kishinevsky et al. / Field Crop.; Research 48 (1996) 57-64

59

Table 2 Monthly maximum and minimum temperatures (°C) at Besor Experiment Farm, 1993 and 1994 Month

Maximum

Minimum

1993

1994

1993

May June July August September

27 31 32 32 30

28 30 32 32 32

18 18 20 21 19

F a r m in the northwestern N e g e v (Israel), (31°19"N, 34°28"E, 100 m asl) on a sandy arenosol soil (Dan et al., 1976) typical o f that region. D u r i n g both years spring and s u m m e r temperatures w e r e close to the 16 y e a r a v e r a g e (Table 2). The e x p e r i m e n t a l fields were characterized by a natural d e f i c i e n c y in nitrogen and the absence o f rhizobia specific to b a m b a r a groundnut. Soil analyses in 1993 and 1994, respectively, r e v e a l e d the f o l l o w i n g physical and c h e m i c a l characteristics: sand, 91 and 9 2 % ; electrical conductivity, 0.105 and 0.106 dS m - l ; N O 3, 2.0 and 3.0 m g kg 1; Na, 5.0 and 6.2 m o l J l ; and p H (in water) 8.5 and 8.2. D u r i n g preparation o f the land, superphosphate and p o t a s s i u m sulphate w e r e applied at rates o f 33 kg P h a - ~ and 65 kg K ha-~ , respectively. The e x p e r i m e n t a l layout o f both e x p e r i m e n t s was a ran-

Maximum

Minimum

1994

1976-1992

1976-1992

15 18 20 21 21

28 31 32 32 30

14 17 20 20 19

d o m i z e d b l o c k design with four replicates o f w i d e l y spaced plants per treatment. Plots consisted o f a single row, 2 m (1993) and 5 m (1994) in length with a spacing o f 0.9 m f r o m adjacent s i n g l e - r o w plots. Thus, each plot was bordered by a different landrace; w i t h i n - r o w spacing b e t w e e n plants was 0.2 m. In both experiments, peanut surrounded the ends and outside rows o f the plots. B a m b a r a g r o u n d n u t seeds w e r e s o w n into m o i s t soil on 2 J u n e 1993 and 4 M a y 1994, and i m m e d i a t e l y afterwards the w h o l e field was g i v e n a light irrigation. Subsequently, the fields were irrigated by sprinklers e v e r y 8 to 10 days throughout the g r o w i n g season. In 1993, all plants f r o m each row w e r e harvested for yield 137 days after s o w i n g ( D A S ) . A t harvest, pods and shoots w e r e separated and dried to constant

Table 3 Shoot dry weights and yields of pods and seeds of bambara groundnut landraces (experiment 1993) Landrace

Shoot dry weight (g/plant)

Pod yield (g/plant)

Seed yield (g/plant)

Shelling percentage

Harvest index (%)

1AI IB~ 3C~ 8B I b 17A 19A b 21A 24A b 30A b 31A b 49A b 57A b 57B b 66A b 75A 89C SE (mean)

38.2 abcd a 43.3 abc 56.3 a 44.3 ab 45.7 ab 26.4 bcde 46.6 ab 14.4 e 13.6 e 36.7 abcd 22.9 cde 18.4 de 13.7 e 21.2 de 20.8 de 37.2 abcd 4.9

11.0 bc 10.9 bc 9.3 bc 10.4 bc 8.5 bc 19.9 abc 4.5 c 4.1 c 13.0 bc 26.0 ab 20.4 abc 24.0 abc 31.2 a 4.8 c 18.9 abc 11.7 bc 4.2

7.3 ab 7.7 ab 6.4 ab 7.1 ab 5.5 ab 14.9 ab 2.9 b 2.7 b 9.4 ab 17.7 a 12.9 ab 18.5 a 19.0 a 2.7 b 14.0 ab 7.2 ab 2.9

67 abcd 68 abcd 67 abcd 67 abcd 62 cd 74 ab 63 cd 67 bcd 71 abc 67 abcd 63 cd 76 a 75 a 59 d 74 ab 61 cd 2.2

15 bcd 15 bcd 9d 12 cd 10 d 33 ab 5d 15 bcd 33 ab 31 abc 31 abc 42 a 48 a 10 d 35 a 13 bcd 4.5

Within columns, values followed by a common letter do not differ significantly at P = 0.05. b Bunchy type. a

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B.D. Kishinevsky et al. / Field Crops Research 48 (1996) 57-64

Table 4 Seasonal changes of different plant characters as a mean of 20 bambara groundnut landraces (n = 80) inoculated with effective rhizobia (experiment 1994) Time after sowing (days)

Number of nodules/plant

Nodule mass (mg/plant)

Shoot dry weight (g/plant)

N content (%)

Total N (rag/plant)

SNA a (lxmol C 2H 2 g dry weight nodule 1 h-L)

68 105 130 SE (mean)

82.4 a a 94.4 a NT 7.1

445 a 405 a NT 26

16.2 c 27.3 b 37.1 a 1.9

2.28 ab 2.34 a 2.20 b 0.03

368 c 641 b 833 a 49

20.3 b 42.7 a NT 5.4

SNA, specific nitrogenase activity. NT, not tested. a Within columns, values followed by a common letter do not differ significantly at P = 0.05. w e i g h t at 30 and 70°C, respectively. Traits m e a s u r e d w e r e fruit yield ( g / p l a n t ) , shoot ( a b o v e - g r o u n d v e g etative material) dry w e i g h t ( g / p l a n t ) , shelling percentage (ratio o f the w e i g h t o f seeds to the w e i g h t o f pods, e x p r e s s e d as a percentage) and harvest index, calculated as the ratio o f the w e i g h t o f seeds to entire plant w e i g h t and expressed as a percentage. In 1994, two plants w e r e dug up at r a n d o m f r o m each replication 68 and 105 D A S and inoculation to d e t e r m i n e nitrogen fixation traits. T h e plant shoots w e r e dried at 70°C, w e i g h e d , ground and assayed for N - c o n t e n t by the Kjeldahl m e t h o d ( N e l s o n and S o m mers, 1980). A c e t y l e n e reduction (IxM C a l l 4 h - I per plant) was m e a s u r e d for the w h o l e root system, w h i c h had been detached f r o m the plant (Hardy et al., 1968) a f e w minutes after digging, using the m e t h o d o l o g y described for peanut by L o b e l and S c h i f f m a n n (1982). At 130 D A S , all plants o f each row w e r e harvested.

Traits m e a s u r e d at harvest w e r e fruit yield, shoot dry weight, nitrogen content in shoots and seeds, shelling percentage and harvest index. Data o f both e x p e r i m e n t s w e r e analyzed statistically by analysis of variance f o l l o w e d by the Student-Neuman-Keuls ( S N K ) multiple-range test. B a s e d on data obtained in 1993, landrace 49 A was graded as early maturing and, therefore, it was harvested 10 days before other accessions in 1994.

3. Results

3.1. 1993 experiment Data collected on 16 landraces used are s u m m a r i z e d in Table 3. A t maturity, icant differences w e r e found a m o n g the o f the attributes measured. Landraces

in this study s o m e signiflines in each 3C~, 21A~,

Table 5 Correlations (n = 80) among growth, nodulation, nitrogen fixation traits and yield of bambara groundnuts inoculated with effective rhizobia (experiment 1994) Variable

Variable No. 1

1. No. of nodules ~ 2. Nodule mass (mg) a 3. Shoot dry weight (g) a 4. Shoot nitrogen (mg) a 5. Shoot nitrogen (%) a 6. Specific nitrogenase activity (p.mol 7. Pod yield (g) b 8. Seed yield (g) b

C2H 4

0.89 0.73 0.83 0.46 g 1 dry nodule h i) a -0.49 0.01 0.02

a 105-day-old plants. b 130-day-old plants. * * * Significant at the 0.05 and 0.01 level of probability, respectively.

2

3

4

5

6

7

** * * 0.63 * * * * 0.71 * * 0.96 * * * 0.30 0.06 0.33 * -0.57 * -0.15 -0.25 -0.23 0.34 0.17 -0.14 -0.28 -0.27 0.28 0.22 -0.17 0.06 -0.19 0.97 **

B.D. Kishinevsky et al. / Field Crops Research 48 (1996) 57-64 17A 1, 8B 1, l B I, 1A 1, 8 9 C a n d 31A~ ( m o s t l y s p r e a d ing t y p e s ) p r o d u c e d s i g n i f i c a n t l y m o r e t o p m a s s t h a n 2 4 A 1, 3 0 A , 4 9 A , 5 7 A 1, 6 6 A 1, 7 5 A a n d 5 7 B 1 ( m a i n l y b u n c h y t y p e s ) ( T a b l e 1). L a n d r a c e s 5 7 B 1, 5 7 A 1 , 3 1 A 1, 19A, 7 5 A a n d 4 9 A ( l a r g e l y b u n c h y t y p e s ) y i e l d e d m o r e p o d s a n d s e e d s t h a n the o t h e r l a n d r a c e s tested. T h e d i f f e r e n c e s a m o n g t r e a t m e n t s w e r e n o t a l w a y s statistically s i g n i f i c a n t , h o w e v e r . T h e s e data are c o n s i s t e n t w i t h p r e v i o u s results d e m o n s t r a t i n g yield differences among bambara groundnut cultivars ( D a d s o n et al., 1988). 3.2. 1 9 9 4 e x p e r i m e n t S e a s o n a l c h a n g e s in n o d u l a t i o n , w e i g h t o f a b o v e ground vegetative material, nitrogen accumulation a n d n i t r o g e n a s e a c t i v i t y a v e r a g e d o v e r the total exp e r i m e n t are p r e s e n t e d in T a b l e 4. S p e c i f i c n i t r o g e n a s e a c t i v i t y o f the n o d u l a t e d p l a n t s i n c r e a s e d m a r k e d l y b e t w e e n 68 a n d 105 D A S , a l t h o u g h at the s a m e t i m e the n u m b e r a n d w e i g h t o f n o d u l e s rem a i n e d p r a c t i c a l l y u n c h a n g e d . It s h o u l d b e e m p h a s i z e d t h a t b y 105 D A S t h e r e w e r e n o s i g n i f i c a n t

61

d i f f e r e n c e s a m o n g l i n e s in the n u m b e r a n d w e i g h t o f n o d u l e s , p r o b a b l y b e c a u s e o f s o m e v a r i a t i o n in p h e n o t y p e w i t h i n l a n d r a c e s ( d i v e r s i t y in p l a n t a n d s e e d t y p e s ) that b e c a m e a p p a r e n t at t h a t time. T h e r e w a s also g r e a t v a r i a b i l i t y in the d r y w e i g h t a n d total a m o u n t o f n i t r o g e n a m o n g t h e l a n d r a c e s t e s t e d but, as c a l c u l a t e d o v e r the total e x p e r i m e n t , s h o o t d r y m a t t e r a n d the a m o u n t o f total n i t r o g e n p e r p l a n t ([N] × s h o o t d r y m a s s ) i n c r e a s e d s t e a d i l y u p to the last h a r v e s t ( T a b l e 4). It is i m p o r t a n t to n o t e t h a t b e t w e e n 105 a n d 130 D A S t h e r e was a d e c r e a s e in N c o n t e n t in v e g e t a t i v e p a r t s o f l a n d r a c e s , p r e d o m i n a n t l y in h i g h - y i e l d i n g lines, 4 9 A , 5 7 B 1 a n d 57A~. T h i s w a s p r o b a b l y d u e to t r a n s l o c a t i o n o f N to r e p r o d u c t i v e p a r t s a n d to loss o f leaves. T h e l i n e a r c o r r e l a t i o n s for n o d u l a t i o n , g r o w t h a n d n i t r o g e n - f i x i n g p a r a m e t e r s d e t e r m i n e d in this experim e n t at 105 D A S are p r e s e n t e d in T a b l e 5. B o t h n o d u l e n u m b e r a n d n o d u l e m a s s w e r e h i g h l y correlated w i t h s h o o t dry m a t t e r a n d s h o o t n i t r o g e n . Specific n i t r o g e n a s e a c t i v i t y w a s n e g a t i v e l y c o r r e l a t e d with number of nodules and nodule mass and non-

Table 6 Yield of pods and seeds, shelling percentage, harvest index, pod/shoot ratio and efficiency of N utilization for seed production of bambara groundnuts at final harvest (1994 experiment) Landrace

lA lB 3C 1 8B 1 b

11A 3 b 11B b 16B2 b 17A l 19A~ b 26D 30A i b 31A b 47A b 49A h 57A1 u 57Bl u 75A 79C b 88A b 89C SE (mean)

Pod (g/plant)

Seed (g/plant)

Shelling percentage

Harvest index (%)

Pod/shoot ratio

mg/plant

Nitrogen in seeds % of total N in the yield

9.7 bc a 7.4 bc 2.5 c 7.4 bc 13.3 abc 10.1 bc 7.2 bc 5.0 bc 17.0 ab 2.4 c 9.2 bc 8.7 bc 13.7 abc 21.9 a 14.1 abc 14.8 abc 10.1 bc 4.4 C 3.9 C 7.0 bc 2.46

6.6 abcd 5.1 abcd 1.3 d 5.1 abcd 9.4 abcd 6.9 abcd 5.0 abcd 3.1 bcd 12.0 a 1.5 d 6.9 abcd 5.7 abcd 10.0 abcd 12.0 a 10.4 abc 11.1 ab 7.2 abcd 2.6 bcd 2.3 cd 4.1 abcd 1.71

68 abcd 69 abcd 63 cde 68 abcd 71 abc 68 abcd 69 abcd 61 def 71 abc 64 bcde 76 a 65 bcde 72 ab 55 f 74 a 74 a 69 abcd 58 ef 60 59 ef 1.8

14.6 bcde 10.3 cde 2.4 e 15.1 bcde 16.3 bcde 12.6 bcde 8.5 cde 7.2 cde 26.4 ab 4.6 de 22.1 abc 14.0 bcde 27.4 ab 20.4 bc 25.1 ab 34.6 a 19.8 bcd 8.3 cde 7.4 cde 7.3 cde 3.18

0.29 0.16 0.04 0.29 0.30 0.24 0.13 0.14 0.60 0.08 0.53 0.29 0.76 0.60 0.48 0.89 0.34 0.18 0.12 0.14 0.10

161 bcd 113 bcd 20 d 113 bcd 251 abc 181 abcd 128 bcd 77 cd 343 ab 32 d 226 abcd 131 bcd 305 abc 401 a 307 abc 297 abc 248 abcd 65 cd 63 cd 125 bcd 49

17 ll 2 15 18 13 10 9 31 4 40 16 37 34 35 46 29 9 9 10 4.0

a Within columns, values followed by a common letter do not differ significantly at P = 0.05. b Bunchy type.

B.D. Kishinevsky et al./ Field Crops Research 48 (1996) 57-64

62

,3

/

12 11

~ y - 0.97883 + 0.35717x r'2 - 0.80520

plant shoots (1460 mg plant -1 compared to 3101260 mg plant -~ for all other landraces) produced the smallest amounts of pods and seeds and had the poorest rating in seed and N harvest indices. This demonstrated the need to take into account seed N as well as the total N accumulated in the plants. It should be indicated that 3C~ was characterized by a long-term flowering down to the last harvest. Because non-inoculated plants (landrace 3C 1) remained free of nodules during the whole growth period, N-fixation could be estimated by deducting the quantity of nitrogen in non-inoculated plants from that in inoculated ones (Virtanen and Saubertvon-Hausen, 1952) and assuming that amounts of N absorbed from the soil by inoculated and non-inoculated plants were similar. As shown in Table 7, even by 68 DAS, inoculation with effective rhizobia increased the shoot dry weight by 188% and the amount of total N in the plant shoots by 325% over the non-inoculated control. The amount of fixed N as measured in the bambara groundnut shoots of 130day-old plants was 1170 mg per plant, forming 80% of the total N accumulated in the plants. A similar rate of N 2 fixation in bambara groundnut was recently demonstrated by Kumaga et al., 1994), using the ~SN isotope dilution and A-value methods.

y - 2.29889 + 0.47248x r'2 - 0,65310

/ /

~

to

,

/

~7~

o/

20

~

e ~,0 e ~s 4

10 /

2

t ~ 10

i

i

20

30

' 0

10

20

30

40

Harvest index (%)

Harvest index (~1

Fig. 1. Relationship between harvest index per single plant and pod and seed yields of the plant.

significantly correlated with all other parameters tested. Also, percentage shoot nitrogen was non-significantly correlated with all the parameters tested except number of nodules ( r = 0.46, P < 0.05). It is interesting to note that none of the above-mentioned parameters was significantly correlated with yield of pods and seeds of the 130-day-old plants (Table 5), but a distinct regression was recorded between harvest index per single plant and pod and seed yields of the plant (Fig. 1). Differences in yield of pods and seeds, seed N, shelling percentage, harvest index of seed or N and the pod/shoot ratio are revealed in Table 6. The highest pod and kernel yields were produced by landrace 49A, followed by 19A, 57B] and 57A~; all of these have a bunchy growth habit. Other lines produced smaller yields of pods and seeds, although not always significantly different from the highyielding treatments. Landrace 3C 1, which at 130 days had the largest amount of N accumulated in

4. Discussion These results confirm those of Stanton et al. (1966) showing that it is beneficial to inoculate bambara groundnut when it is grown on soil free of V. subterranea-nodulating rhizobia. For the 20 bambara groundnut landraces evaluated in 1994, inoculation with two effective Bradyrhizobium strains, 280A and 100M, resulted in abundant nodulation and enhanced nitrogen-fixing activity, as indicated by large amounts of N accumulated in the plants and by

Table 7 Amount of symbiotic nitrogen accumulated in shoots of bambara groundnut (landrace 3C l) plants inoculated with Bradyrhizobium Non-inoculated

Inoculated

plant (days)

shoot dry weight (g/plant)

N content (%)

total N (mg/plant)

shoot dry weight (g/plant)

N content (%)

total N (rag/plant)

N 2 fixed (rag/plant)

68 105 130

5.7 11.9 16.5

1.54 1.94 1.83

86 231 302

16.4 29.2 64.0

2.23 2.27 2.30

366 663 1470

280 432 1170

B.D. Kishinevsky et al. / Field Crops Research 48 (1996) 57-64

acetylene reduction rates of nodules that were typical for a closed acetylene reduction system using detached roots (Hunt and Layzell, 1993). In this regard, it seems significant that the same strain(s) of rhizobia can be used for inoculation of different bambara groundnut lines. The differences among landraces in the number and weight of nodules, however, even though not always statistically significant (Table 4), indicate that it may be important to consider both the host plant genotype and the B r a d y r h i z o b i u m strain in selection programs for higher N 2 fixation rates in V. subterranea.

For most of the lines tested, there was no increase in the number and weight of nodules after day 68 from sowing. This is in contrast, for example, to groundnut, which is characterized by development of nodules over a longer period of time (Ratner et al., 1979; Kishinevsky et al., 1984). On the other hand, the specific nitrogenase activity ( C 2 H 4 activity g-~ dry weight of nodules, Table 4) of most of the landraces increased markedly between 68 and 105 DAS (pod-filling stage), indicating that N 2 fixation probably can provide nitrogen during the period of pod and seed formation. It should be indicated, however, that specific nitrogenase activity measured at 105 days after planting was nonsignificantly correlated with other traits measured (Table 5). The present data are in agreement with those of Somasegaran et al., 1990 showing that specific nitrogenase activity is a nonreliable indicator of nitrogen fixation capacity in bambara groundnut. It has been shown for soybean that fruiting can stimulate nitrogen fixation, possibly because of a sink effect (Peat et al., 1981), and it should be emphasized that such phenomena have been observed in other annual legumes with a long reproductive stage, such as peanut (Hardy and Havelka, 1976). This in contrast to the situation in many other legumes in which N 2 fixation usually ceases after the beginning of flowering (Minchin and Pate, 1973). N partitioning to fruit is fundamental, particularly for seed legumes. It was recently reported (Linnemann, 1991) that photoperiod controls phenological development of bambara groundnut and that there is a delayed or inhibited fruit set in some bambara groundnut selections under long photoperiod (14 h). This was probably the case with some landraces (3C, 21A, 24A 26D and 66A), which produced extremely

63

low yields of pods and seeds in our experiments. The Smithsonian Meteorological Tables (List, 1951) indicate that bambara groundnut in Israel (31°N) is exposed to slightly longer daylengths than in Malawi (15°N) where the landraces were collected. It is assumed that in soils of low N status, N 2 fixation may be slightly overestimated, because of restricted root growth of non-nodulating plants (Herridge, 1982). On the other hand, it has been known that plants that are entirely dependent on soil N are most likely to have larger root systems than nodulated ones. In the present experiments, roots of nodulated and non-nodulated bambara groundnut plants (landrace 3C~) were approximately equal in their extension. This probably indicates that bambara groundnut lines, even those producing small quantities of seeds, but with a relatively large amount of vegetative tissue, may be an effective sink for N. The large N 2 fixation in the landrace 3C 1 (about 80% of total N) obtained in the present study is comparable with data reported by Dakora et al. (1992) and Kumaga et al. (1994). Considering that the seed rate of bambara groundnut is near 100 kg ha -I (Anonymous, 1992) and the seed mass of landrace 3C l is 253 mg (Table 1), the amount of N 2 fixed by 3C 1 turns out to be about 450 kg N ha J Care should be used, however, in relating these data to different soil systems and cultivars and to plants grown at high density. To the best of our knowledge, this is the first report indicating a genetic effect on amounts of nitrogen translocated to the reproductive parts of bambara groundnut plants. Several of the lines (57B 1, 57A 1, 19A and 49A) were more effective in both years than others in promoting pod and seed yields. These differences, however, were not always statistically significant. Although the factors influencing intracultivar variation, observed in both experiments, are still not clearly understood, this probably, can be explained by some irregularity in initial size of seeds used for sowing. Further field evaluation tests would be of considerable help in resolving this question.

Acknowledgements This work was supported by the German-Israel Agricultural Research Agreement for the benefit of the Third World (GIARA), project No. 91-9. Contri-

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B.D. Kishinevsky et al./ Field Crops Research 48 (1996) 57-64

bution the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 1604-E, 1995 series.

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