RESULTS AND DISCUSSION 1. Screening experiment

tr n

Poor N-fixer

it it tt

Local variety

mutant variety approved variety

Samplings were made at early pod-fill for dry matter yield, nodule weight, nodule number, total N and ¹⁵N determinations; seed-yield determinations were made at maturity. The harvest areas were 1.0 m2 and 4.0 m2 for 1SN analysis and seed yield, respectively. Samples were taken from the 2nd and 3rd rows, leaving the 1st and 4th as borders. Plant samples were weighed, chopped and quartered, and oven-dried at 70°C for two days until constant weight was obtained. Ground samples were sent to Seibersdorf for total N and ^1SN determinations as above.

In the following season, maize was grown on the same plots without additional N fertilization.

Planting distance was 25 cm between hills, and 50 cm between rows, with one plant per hill. With standard cultural practices of irrigation, weed and pest control, plant samples (fifteen plants/plot) were collected from the ¹⁵N plots at 45 days after planting. Sub-samples were obtained, weighed and dried for several days at 70°C until constant weight was obtained. After grinding, 5-g sub-samples were sent to Joint FAO/IAEA Soil Science Unit, Seibersdorf, Austria, for total N and determination of ^I5N

enrichment. Kernel yields were determined at maturity.

Statistical comparisons of means were made as described above.

3. RESULTS AND DISCUSSION 3.1. Screening experiment

Vegetative dry-matter yields ranged from 1.99 to 3.05 (Table HI) and 0.92 to 2.35 t/ha (Table IV) for the late- and early-dry seasons, respectively. Vegetative-N yields also were generally higher

TABLE E. CHEMICAL CHARACTERISTICS OF SOILS AT PNRI AND UPLB

O.M. Total Avail. Exchangeable bases Micronutrients

N P Ca Mg Na K Zn Cu Mn Fe pH (%) (%) (mg/kg) (meq/lOOg) (mg/kg)

PNRI 2.4 0.12 6.1 18.7 10.0 0.26 0.51 7.4 9.8 149 67 6.7 UPLB 4.0 0.20 21 8.23 3.83 0.84 2.48 2.0 8.1 79 130 6.9

during the late-dry season, by an average of 16%. This pattern of better early growth and accumulation of N can be explained in terms of greater availability of mineral N, from soil and fertilizer, during the late-dry season. It is noteworthy that, in contrast with vegetative growth and N accumation by early pod-fill, seed yields were generally better in the early-dry season. The reason for this incongruity is unclear, but it is possible that conditions for N² fixation during grain fill were superior during the early-dry season. In conditions of abundant N, some species invest relatively more resources in vegetative than in reproductive growth, however, with only 37% of the total N originating from soil and fertilizer (Table III) this explanation seems unlikely.

The three reference crops did not vary significantly in % atom-excess values (data not shown), therefore %Ndfa values were estimated using mean reference values. These data indicate that, at least in some conditions, plants of different rooting pattern may be used as controls for isotope-dilution determinations of N fixed by legumes [9-12].

Genetic variability for N fixed was not well pronounced. In the late-dry season screening, 15 genotypes did not fix significantly less N than did Pagasa 5, which had the highest value of 52 kg N/ha, and 14 genotypes did not fix significantly more than did Accession 2041, the lowest at 33 kg N/ha. In the early-dry season, 14 genotypes did not fix significantly less than did PAEC 3 at 70 kg N/ha; although Accession 379 alone did not fix more N than did Accession 2041 at 26 kg N/ha, twelve genotypes did not fix significantly more than did 379. No single genotype emerged as superior in both seasons; fifteen of the seventeen genotypes fixed amounts of N that were not significantly different from PAEC 3 or from Accession 687 in one or other of the trials, including Accession 58, 71, and 379, which had been previously designated as poor-fixing checks (Table I, [3]). Similarly, the estimates for %Ndfa were remarkably uniform, with no statistically significant differences. It may be inferred that N was not the chief factor limiting vegetative growth, and consistent with this deduction is the fact that seed yields were low, averaging only 680 and 730 kg/ha (Tables III and IV).

With better growth conditions and improved potential for expression of symbiotic N2 fixation, then superior N2-fixing genotypes may have emerged more clearly.

TABLE III. DRY WEIGHT, N ACCUMULATION AND N2-FIXATION CHARACTERISTICS AT 45 DAYS AFTER PLANTING, AND SEED YIELD, OF 17 MUNG-BEAN GENOTYPES DURING THE LATE-DRY SEASON OF 1991-92

Cultivar

VI PAEC3 V2 PAEC 10 V3 Pagasa 3 V4 Pagasa 5 V5 Pagasa 7 V6 Twn. Grn.

V7 Ace 658 V8 Ace 687 V9 Ace 638 V10 Ace 211 VI 1 Ace 174 V12 Ace 188 V13 Ace 2041 V14 Ace 379 V15 Ace 58 V16 Ace 71

V17 PAEC pro.

Grand Mean

*ti derived from fertilizer. ^bN derived from soil. °N derived from fixation.

^Numbers in columns followed by the same letter are not significantly different (P = 0.05).

3.2. Residual-effects experiment 3.2.1. Mung bean

The patterns of vegetative growth and seed yield differed between the PNRI and UPLB sites.

The highest dry-weight (Table V) and total-N (Table VI) values were obtained with NIAB 54 at PNRI and with Taiwan Green. Total N accumulation at early pod-fill did not correlate well with seed yield;

for example, Taiwan Green at UPLB had 130 kg N/ha and yielded 860 kg/ha of grain, whereas Accession 638 at PNRI accumulated only 64 kg N/ha but yielded 1.58 t/ha of grain. The low %N value of Accession 638 (2.12%, Table VI) was consistent with its low nodule weight (Table V) indicating poor compatibility with the inoculant and with indigenous rhizobia.

TABLE IV. DRY WEIGHT, N ACCUMULATION AND N2-FIXATION CHARACTERISTICS AT 45 DAYS AFTER PLANTING, AND SEED YIELD, OF 17 MUNG-BEAN GENOTYPES DURING THE EARLY-DRY SEASON, 1992

Cultivar

VI PAEC 3 V2 PAEC 10 V3 Pagasa 3 V4 Pagasa 5 V5 Pagasa 7 V6 Twn. Grn.

V7 Ace 658 V8 Ace 687 V9 Ace 638 V10 Ace 211 VI 1 Ace 174 V12 Ace 188 V13 Ace 2041 V 14 Ace 379 V15 Ace 58 V16 Ace 71 V17 PAEC pro.

Grand Mean

derived from fertilizer. bN derived from soil. 'N derived from fixation.

Numbers in columns followed by the same letter are not significantly different (P = 0.05).

The lowest values for vegetative dry weight, total N, amount of N fixed, %Ndfa, and seed yield were obtained with PAEC 3 at UPLB. In contrast, these plants had the highest average value for %N, showing that in this case the limitations in growth and seed yield were not caused by incompatibility with the available rhizobia - some other nutritional or environmental factor(s) caused PAEC 3 to fail to achieve its potential at UPLB.

In contrast with the screening experiment, there was clear genotypic variability for amount of N fixed and for %Ndfa. These data were generated using cotton alone as the non-fixing reference crop; when maize was used, differences among genotypes were insignificant owing to high experimental error (coefficient of variation 82%), therefore we disregarded maize as a reference crop.

Values for %Ndfa were lower at UPLB than at PNRI, consistent with the higher soil %N at the former location (Table II).

On the assumption that the highest benefit to a subsequent cereal will accrue from a legume crop that fixes a large amount of N but has low grain yield and thus contributes fixed N to the soil in the form of vegetative residues [13], then the best candidates for residual benefit to a following maize crop would be expected at UPLB from Taiwan Green, Accession 638, and NIAB 54.

3.2.2. Maize

Positive effects of mung bean on subsequent maize dry-matter yield were clear, and particularly well pronounced at UPLB where growth was better (Table VII). There was a similar trend in terms of maize seed yield, although there was statistical signficance only in the difference between the seed yield after cotton (0.63 t/ha) and after Ace. 638 (1.05). Highest maize dry-matter yields were obtained after Ace. 638 at both locations despite the fact that it was not superior in N² fixation (Table VI).

Very little of the fertilizer N applied to the mung was available to the maize, as revealed by the low values for %NdfF (Table VIII). It would be interesting to have applied ¹⁵N-labelled mung residues to adjacent plots to determine how much of the organic N therein was available to subsequent maize.

3.3. General considerations

These data justify further work on mung bean, on its capacity to fix N² and its ability to improve subsequent cereal yields. There clearly exists genotypic variability in mung bean for N² fixation (Tables III, IV, VI) and in ability to utilize that fixed N to make grain yield (Tables III, IV).

TABLE V. SHOOT DRY WEIGHT, NODULE WEIGHT AND NODULE NUMBER AT EARLY POD-FILL, AND SEED YIELD OF FIVE MUNG-BEAN GENOTYPES AT PNRI AND UPLB

Genotype

Shoot dry wt. Seed yield PNRI IPB PNRI IPB

(g/plant) (t/ha)

Nodule d. wt. Nodule no.

PNRI IPB PNRI IPB (mg/plant) (per plant)

PAEC 3 8.03b 4.84d 1.87ab 0.19c Twn. Grn. 8.22b 14.0a 1.41c 0.86a Ace. 638 10.2ab ll.Oab 1.58bc 0.59b NIAB 54 11.Oa 10.3ab 1.99a 0.57b NIAB 92 8.84ab 7.53cd 1.96ab 0.55b

70.0ab 39.5c 41.3a 21.0ab 46.5bc 68.3b 39.6a 24.4a 22.8c 46.3bc 28. Ib 24.9a 81.8a 98.8a 26.5b 18.6ab 29.8c 27.5c 4.2c 12.3b

C. V. (%) 18 22 13 26 39 29 24 35

Differences in compatibility with the chosen inoculant strain and/or the indigenous rhizobia were also apparent (Table V), and must be accommodated in the future; if the indigenous flora are not suitable for a superior genotype then it becomes critically important that the appropriate strain be applied as inoculant.

TABLE VI. N CONCENTRATION, TOTAL N, PERCENT N DERIVED FROM FIXATION AND AMOUNT OF N FIXED BY FIVE MUNG-BEAN GENOTYPES AT EARLY POD-FILL, AT PNRI AND UPLB

Genotype

N cone. Total N Ndfa N fixed PNRI UPLB PNRI UPLB PNRI UPLB PNRI UPLB

(kg/ha) (kg/ha)

PAEC 3 3.19a 3.79a 76.9b Twn. Grn. 3.10a 3.10bc 76.3b Ace. 638 2.12b 3.37b 64.4b NIAB54 3.20a 2.97cd 105a NIAB 92 3.06a 2.66d 80.9b

55.0c 78.5ab 36.6c 59.8bc 21.3b 130a 70.8bc 66.0ab 53.9bc 84.9a 11 lab 64.4c 59.9ab 43.Ic 66.4a 91.5b 80.8ab 71.7a 85.2a 65.3a 60.0c 87.0a 52.9b 70.3ab 32.0b

C. V. (%) 2.9 3.2 17 23 9.9 18 20 23

TABLE VII. TOTAL DRY MATTER AND SEED YIELDS OF MAIZE GROWN AFTER MUNG BEAN AT PNRI AND UPLB

Previous treatment

PAEC 3