Agriculture Reference
In-Depth Information
Both drought and salinity adversely affect root hair development (Sprent & Zahran,
1988). Thus it might be expected that species with a non-hair infection might be more
resistant to these stresses. Certainly some, such as
Arachis hypogea
, are quite drought
resistant, but another groundnut crop,
Vigna subterranea
, which has a hair infection,
is even more drought tolerant (Basu et al., 2007a). Perhaps the most salinity-tolerant
legumes are several species of the mimosoid genus
Prosopis
(Felker, 2008), which also
has a hair infection (Rasanen et al., 2001). Another saline-tolerant legume, currently
being developed for its potential in biodiesel production, is
Millettia pinnata
, being
marketed under its synonym
Pongamia pinnata
and known colloquially as pongamia
(Scott et al., 2008). This will be considered in more detail under new crops, but is
mentioned here because of the possibility that is has a non-hair infection, like its close
relative
Lonchocarpus
(Cordeiro et al.,1996). In its native habitat, pongamia has Na
+
and Cl
-
contents similar to those of adjacent mangroves (Dagar et al., 1993). Why
hair-infected legumes should be drought and salinity tolerant is unclear, but there
are two possibilities. First, the studies cited in Sprent and Zahran (1988) are all on
crop plants such as
Vi c ia faba
, from non-stressed environments, so it is possible that
the osmotic potential in hairs of tolerant legumes is higher than that of non-tolerant
legumes. Second, infection of roots may take place at times when the stress conditions
are absent. This is more likely for drought than salinity tolerant species, as even in arid
areas, there are periods of rain.
Most rhizobia are tolerant of low levels of salinity - indeed they are adapted to live
within the confines of a plant cell whose osmolarity is usually in the range of 300 to
400 mOsm.kg
−
1
. However, selection from saline soils can yield strains that will grow
in sea water (P.M. Gresshoff, personal communication). Salt tolerance, together with
antibiotic resistance, is often used as a preliminary screening tool for rhizobial classifi-
cation. The problems are more in the plant than the bacterial side of the symbiosis.
5.11.2 Temperature
Both high and low temperatures can be a problem for crop species, especially when
agronomists are attempting to extend their range. For many crops, such as soybeans,
breeding programmes have produced material suitable for a wide range of tempera-
tures. There are many nodulated legumes that occur naturally in the arctic polar region
(Sprent, 2001), and their rhizobia are cold tolerant, sometimes having a higher level of
cold shock proteins (Prevost et al., 2004) and/or unusual 'decorations' on their Nod
factors (Fig. 4.1; Poinsot et al., 2001). The fact that all legumes from cooler regions
appear to depend on nodulation (Section 5.2) is interesting in view of the conclusion
of Houlton et al. (2008) that nitrogen fixation globally is constrained by low temper-
atures. Low temperatures are also found in mountainous regions where plants have,
in addition, to cope with low atmospheric pressures. However, in both the Andes and
in the Qinghai-Tibetan plateau, where the average altitude is over 4500 m (Hou et al.,
2008), legumes seem to grow and fix nitrogen happily when the atmospheric pressure
is much less than half that at sea level (Plates 2.12 and 2.13). Considering that nodule
functioning is closely related to the partial pressures of the various gases involved, this
poses interesting physiological questions.
