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same as that of a cortical cell in the same nodule (Sprent et al., 1989). In nodules of tribes
Fabeae and Trifoleae and in
Lupinus
bundles may have vascular transfer cells (Pate
et al., 1969). Inmany woody legumes bundles may be large andmuch branched (Sprent
et al., 1989). Considering the importance of a good transport system to the interchange
of nutrient between nodules and their host plants, it is perhaps surprising that, apart
from two recent papers on lupin (
-
otocka, 2008a, b) gross vascular systems have
received little attention for nearly 20 years, most researchers concentrating on what
goes on in the infected tissue. However, the interface between the cells of the infected
region and the vascular systemwas investigated by Brown et al. (1995), who quantified
symplastic transport in soybean nodules. Walsh (1995) considered apoplastic transport
soybean nodules and also discussed the evidence for (and against) interchange of so-
lutes between nodule and the soil solution at the nodule surface. The latter possibility
has also been ignored in recent studies, which is also surprising since this is a major in-
terface for nodules. If it is significant (e.g. for nitrate andpossibly calciumuptake, Sprent
et al., 1987), then it could also be a route by which toxic agents in the soil could affect
nodules. E. Gross (personal communication) studied nodules of
Centrosema
in iron
mining areas of central Brazil (Plate 5.3) and found their cortices to be impregnated
with iron.
5.7 Functioning nodules: the critical role of oxygen
The supply of carbon to the bacteria and the return supply of combined nitrogen to
the plant are dealt with in detail in Vance (2008). Clearly for the system to function
optimally these have to be closely matched, and how this is done by means of exchange
of critical amino acids in the pea nodule has been demonstrated by Lodwig et al.
(2003). Overall control of functioning as well as nodule formation and structure is
largely under plant control. Possibly the most important way in which this is done
is by means of oxygen supply. The role of haemoglobin in fine-tuning the supply of
oxygen to the bacteroids was mentioned above, but prior to this there has to be a step
down in oxygen concentration from the atmospheric level of 20%, to about 1%. This is
achieved by means of both fixed and variable oxygen diffusion resistances. How these
operate in various types of nodule has been reviewed by Minchin et al. (2008). These
workers also discuss the dilemma of reactive oxygen and antioxidants in nodules.
Increasing the resistance to oxygen diffusion by means of the variable resistance means
that the host plant can reversibly alter the rate of nitrogen fixation, for example under
environmental stresses such as shading, drought, salinity, high and low temperature
and phosphate deficiency (Minchin et al., 2008). It has also been invoked as a way in
which host plants (soybean in this case) may sanction against any nodules formed by
less effective bacteria (Kiers et al., 2008).
5.8 Nitrogen fixation and export of products
Most legumes, whether they are tropical or temperate (or even polar), export amides
from nodules to the rest of the host plant. Determinate nodules from the phaseoloid
