Agriculture Reference
In-Depth Information
of N
2
fixed by legumes under field conditions are not available. To some extent, these observations
remain relevant today. Herridge and Danso (1995) reported that for plant breeding and selection
for enhanced N
2
fixation, the following criteria should be taken into consideration: (i) choice of
traits as selection criteria that can be measured precisely and economically and allowing separation
between efficient and inefficient genotypes, (ii) variability in legume germplasm in N
2
fixation, (iii)
identification of genetically diverse parents, incorporating both agronomic and N
2
fixation, choice
of selection units (individual plants or families) that facilitate the precise quantification of traits on
interest and allow the production of progeny from selected plants, and (iv) use of a breeding proce-
dure (i.e., mass selection, family selection) that provides maximum genetic gain for N
2
fixation and
recombination with other desired agronomic traits.
Genetic variability among legume crop species in N fixation has been widely reported (Bliss,
1993; Herridge and Danso, 1995). Nutman (1984) concluded that N
2
-ixing lines of the temperate
forage species, red clover (
Trifolium pretense
), were superior because of an enlarged N
2
fixation
system, rather than because of the increased efficiency of N
2
fixation. Superior plants nodulate ear-
lier, leading to more and larger nodules. Piha and Munns (1987), Graham (1981), and Graham and
Temple (1984) reported significant differences among dry bean genotypes in N
2
fixation. Bliss et al.
(1989) and Bliss (1993) reported significant differences among bean genotypes in N
2
fixation. When
considering future directions for breeding of dry bean, Bliss (1993) suggested that breeding plants
with the capacity to nodulate and fix N
2
in the presence of soil nitrate, that is, nitrate tolerance,
should be a priority. There is also evidence that variation exists for nitrate tolerance both in natural
populations and in mutant lines (Park and Buttery, 1988, 1989). Other selection traits that may have
merit in breeding programs for dry bean are early and late nodulation (Chaverra and Graham, 1992;
Kipe-Nolt et al., 1993; Kipe-Nolt and Giller, 1993).
It may be possible to enhance N
2
fixation in a
Rhizobium-
legume symbiosis by selecting host-
plant phenotypes as well as
Rhizobium
mutants. The number of root nodules is under some genetic
control of the host plant. Two genes controlling the nodule number in peas have been identified, and
the nodule number was correlated with the seed yield (Gelin and Blixt, 1964). Seetin and Barnes
(1977) selected alfalfa genotypes with enhanced acetylene reduction activity in the presence of a
commercial mixture of
Rhizobium
strains and transmitted this character to the progeny. That suc-
cess in the greenhouse was extended to the field, whereas a
15
N technique showed that the elite
alfalfa populations averaged 41% more annual N
2
fixation than a standard variety. Such results are
quite promising from an applied point of view and definitely show that selection pressure on the
symbiotic associations as a whole can be used to enhance N
2
fixation (Phillips, 1980).
Numerous studies have been reported that show the variation in N
2
fixation in soybean geno-
types (Kucey et al., 1988; Herridge and Holland, 1992; Herridge and Danso, 1995). Variation in
nitrate tolerance within a large and diverse germplasm collection of soybean was reported by Betts
and Herridge (1987). Similarly, Serraj et al. (1992) also reported a large population of soybean
genotypes for tolerance to nitrate. Plant mutagenesis was first used to generate peas with greatly
enhanced nodulation and with a degree of nitrate tolerance (Jacobsen and Feenstra, 1984).
Fageria et al. (2014) studied the influence of N fertilization with or without rhizobial inoculation
on growth of tops of 10 principal tropical cover crops. The N × cover crop interaction for shoot dry
weight was significant, indicating that different responses of cover crops exist for varying levels
of applied N and Bradyrhizobial inoculants (Table 7.10). At 0 mg N kg
−1
(N
0
) treatment, the shoot
dry weight varied from 0.98 g plant
−1
produced by crotalaria to 10.89 g plant
−1
produced by lablab
,
with an average value of 5.53 g plant
−1
. When seeds were treated with Bradyrhizobial inoculants
(0 mg N kg
−1
+ inoculant or N
1
), pueraria produced the minimal shoot dry weight and jack bean pro-
duced the maximal shoot dry weight. At N
2
treatment (100 mg N kg
−1
+ Bradyrhizobial inoculants
or N
2
), the differences in responses of these two cover crops were similar. However, when the N rate
was increased to 200 mg kg
−1
(N
3
) treatment, pueraria produced the minimal shoot dry weight but the
maximal shoot dry weight was produced by lablab. Across four N treatments, the maximal shoot dry
weight was produced by the jack bean and the minimal shoot dry weight was produced by pueraria.
