The Role of Intercropping in Plant Biofortification 11organic acids, which reduce the pH of the rhizosphere and release phosphorus
from unavailable compounds. Beans and soybeans mainly secrete citrates
(Shen et al., 2002, Nwoke et al., 2008 ), while lupins, field peas and faba beans
mainly secrete malate (Nuruzzaman et al., 2005 ). The effectiveness of
carboxylates depends on the number of carboxyl groups and the molecular
structure. Tricarboxylates (citrate) are generally more effective than
dicarboxylates (e.g., malate, malonate) due to stronger ligand binding
(Veneklaas et al., 2003 ). Like other species, pulses can release phosphatase
enzymes, which decompose phosphorus organic compounds, into the soil. Inal
et al., ( 2007 ) also observed an increase in phosphorus uptake in peanut/maize
intercropping in a greenhouse experiment. This may be due mainly to a
decrease in rhizosphere pH, and, to a greater extent, to increased phosphatase
activity in the soil and roots in intercropping, and thus increased phosphorus
concentration in the rhizosphere (Table 2). In another study (Inal and Guns
2008 ), the P-concentration of peanut decreased more than 1.5 times in the
intercropping system, while it increased 5.0 and 2.2 times in maize and barley
respectively (Figure 2). Interspecific root interactions between mixed cropped
peanut/maize and peanut/barley may have important roles in phosphorus
acquisition by the mixed system. Higher total root dry weights from the
intercropping, as compared to the root dry weights of monocropped plants
might be responsible for the increased acid phosphatase activity in the
rhizosphere soil (Table 2). The possible higher root exudates from the higher
root density might be important for the availability and acquisition of plant
nutrients, especially P. Further, when maize or barley and peanut are grown
together, peanut may mobilize and absorb some organic P by releasing
phosphate into soil, and also leave some inorganic P for barley and maize (Inal
and Guns, 2008).
According to Zhang et al., (2016), in maize/faba bean intercropping
neighbouring species significantly modified root interactions between maize
and faba bean, influencing P uptake and biomass production. Enhanced root
exudation of citrate and acid phosphatase by faba bean could facilitate soil
phosphorus availability to benefit maize growth in the maize/faba bean
mixture. According to Veneklaas et al., (2003) the large amounts of
carboxylates exuded by chickpea and white lupin, and the ability of these
compounds to solubilise P in soils, suggests that these species are of great
potential value as crops in soils with relatively large amounts of sorbed P.
Therefore, utilisation of root exudate-producing crops to enhance soil P use
will require the identification of those physiological or soil chemical factors
that stimulate or inhibit the exudation of organic anions.