Agriculture Reference
In-Depth Information
variation among genotypes is well documented [126], indicating the po-
tential to develop new crop varieties with increased potential to adapt to
low-P availability.
Breeding for root systems that are more effi cient at phosphorus ac-
quisition could inadvertently lead to the development of varieties with
improved mobilization of organic phosphorus reserves in the soil. Root
apices exude a variety of organic acids, which can infl uence plant nutrition
and provide an easily degradable nutrient source for soil microorganisms
[129]. Of the organic acids exuded by roots under phosphorus defi ciency,
citrate, malate, and oxalate are the most effective at mobilizing soil phos-
phorus [130,131]. These organic acids can release unavailable phosphorus
from bound minerals, allowing for the chelation of Al 3+ , Fe 3+ , and Ca 2+
consequently freeing phosphorus and helping to alleviate P stress. Roots
of white lupin growing under P stress exuded 20-40% more citrate and
malate in comparison to roots provided suffi cient supplies of phosphate
[7,132]. Differences in the exudation of organic acids can be seen between
crops under P-defi ciency or not [133,134], suggesting potential to produce
genotypes with improved ability to mobilize phosphate. Although the ex-
act mechanism linking genetic regulation and the exudation of organic
acids from root tips is largely unknown, gene expression data imply a
complex coordinated induction of genes related to the synthesis, degrada-
tion, and utilization of citrate under P stress [123,135].
In addition to improving access of previously unavailable phosphate
via rhizosphere acidifi cation, exuded carboxylates promote microbial
growth, and could potentially be used to exploit benefi cial microbial rela-
tionships that might correlate with P bioavailability [129]. It has long been
reported that benefi cial relationships between crops and mycorrhizal fungi
can improve availability and uptake of nutrients, in particular phosphorus
[136]. Mycorrhizal fungi can increase phosphorus availability by exudat-
ing various organic acids themselves, freeing phosphates in the same man-
ner as those exuded from plant roots. Colonization by benefi cial fungi can
lead to improved access of phosphorus by extending the crop's root sys-
tem with mycorrhizal hyphae [137], indirectly increasing the root surface
area for nutrient absorption and crop growth. Mycorrhizal hyphae work
to improve nutrient acquisition by increasing their affi nity for phosphorus
ions and decreasing the concentration gradient required for more energy
 
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