Agriculture Reference
In-Depth Information
notable exceptions, such as tree species of
Erythrina
, most of the species in this group
of tribes are small shrubs or herbaceous annuals and perennials. Apart from some
species of
Lupinus
(tribe Genisteae),
Arachis
(peanut, groundnut, tribe Dalbergieae),
Vi -
cia
and
Pisum
(tribe Fabeae), tribe Phaseoleae houses the world's most important grain
legumes, as well as many forage, browse and tuber species These are described in many
topics, including Allen & Allen (1981) and Anon (1979), and some will be discussed fur-
ther in Chapter 5. A few species, such as soybean (
Glycine max
) and dry bean (
Phaseolus
vulgaris
), are industrial crops, although the latter is also extremely important for subsis-
tence farmers in many developing countries, and thus their symbiosis with rhizobia has
been extensively studied. This is particularly true for soybean. In the US, unlike Brazil,
breeders have generally selected potentially high-yielding cultivars on fertilized plots
and thus have unwittingly selected against nitrogen fixation. Denison and co-workers
have been studying ways in which soybean plants might sanction against ineffective
rhizobia and ensure that they only nodulate with effective strains. In their most recent
study (Kiers et al., 2007), they have looked at a range of historic and modern cultivars
and concluded that the latter were less able to sanction against inefficient rhizobia.
The ability to export ureides has been linked to the tropical and sub-tropical habitats
of this group of legumes, for various reasons including their low solubility (Sprent,
1980). It would be interesting to know whether species occurring naturally in cooler
areas - such as in the three genera
Strophostyles
(Phaseoleae),
Pediomelum
and
Rupertia
(Psoraleae) that are found in Southern Canada - export ureides. Because of their de-
terminate nature, desmodioid nodules have a limited life span (usually a few weeks)
and cannot regrow after environmental stress, unlike nodules with an apical meristem,
which may be perennial. This again has been linked to living in warmer regions. The
main environmental threat is drought and this may lead to nodule loss.
As might be expected of such a large group of genera, nodulation can be induced
by a wide variety of bacteria. These include many genera of
-rhizobia, both fast- and
slow-growing. The first reports of nodulation by
-rhizobia are now appearing. In their
study of nodulation in
Cyclopia
(Podalyrieae), Elliot et al. (2007b) found that
Burkholderia
tuberum
could effectively nodulate the promiscuous host plant
Macroptilium purpureum
(Desmodieae). More recently Garau et al. (2009) isolated a new species of
Burkholderia
from
Rhynchosia ferulifolia
, a phaseoloid species endemic to the Cape region of South
Africa. This bacterium did not nodulate other species of
Rhynchosia
tested. Pueppke
and Broughton (1999) tested a vast number of legumes for ability to nodulate with
the wide host range bacterium NGR 234, isolated from Papua New Guinea, including
five species of
Rhynchosia
. Three of these, all from either Africa or Asia, formed effective
nodules, but the other two, from Central and South America, did not nodulate at all
with this strain. Species of this very large genus appear to show quite a high level of
specificity with widely differing bacteria. This contrasts, for example, with
Phaseolus
vulgaris
, which nodulates with a wide range of rhizobia, but as yet none outside the
major
-rhizobia.
1.3.7 The Robinioid clade
In Lewis et al. (2005) the Robinioid clade has three tribes, Loteae, Sesbanieae and
Robinieae (Tables 1.1, 1.16), forming an interesting group from a nodulation point of
