# Phosphorus Dynamics in the Soil-Plant-Environment Relationship in Cropping Systems: A Review
## 4 Phosphorus Dynamics in the Plant  
Phosphorus is an irreplaceable and essential element for life. In plants, it is one of the 16 essential elements for their vital functions, and thus without this element, no high-quality or abundant harvests could be achieved. Nevertheless, the biomass potential in soil is limited to the low P reserves worldwide [71]. As a macronutrient for plants, P is fundamental to the development of many cellular components, such as the nucleic acids, proteins, phospholipids, and ATP [72]. It is essential for the enzymatic regulation and for the interpretation of metabolic signals, and its important mobility within the plant causes the signs of their nutritional deficiency to be reflected first in the old leaves [73; 74], although recent works [75] point out that plants can be heavily affected by a P deficiency for weeks without reflecting any visual symptoms on the leaves. On the other hand, all photosynthetic processes that are influenced by P deficiency seem to be fully reversible and can be restored in less than 60 min by providing orthophosphate to the leaf tissue. To grow under P deficiency, plants have developed several strategies [76] that can be classified into two groups: (i) those improving P absorption from soil, and (ii) those limiting P reallocation in vegetal organs [77]. To increase P uptake from soil, plants may modulate their root system architecture by increasing their surface area and producing different types of special roots called proteoid roots or radioles and basal roots [77; 78; 79]. This leads to the exploration of larger soil volumes, scavenging for P, and to modification of rhizosphere environment to boost P mobilization. Roots may indeed increase the release of exudates, acidifying the rhizogenic area to produce more H\({}_{2}\)PO\({}_{4}\)\({}^{-}\), which is the preferential ion for acquired roots. The exudation includes also the release of organic acids that can compete with P for the same sorption sites, and enzymes, such as phosphatases and phytases that can hydrolyze different organic P forms [80; 81; 82]. The associations of plant roots with mycorrhiza, especially arbuscular mycorrhiza, may further improve the capacity of plants to acquire P via the hypha network that increases the spatial exploration [83; 84]. By contrast, plants limit the use of P as their growth rates decrease, by accumulating sugars and anthocyanins, modifying their glycolytic and respiratory pathways, and by alternating vegetal hormones [85; 86]. Other special strategies that plants have developed to take up P include the capacity to produce different types of special roots called proteoid roots or radioles and basal roots. The second one is the development of associations with mycorrhizas, especially arbuscular mycorrhiza, forming a symbiosis with the plant roots, so the hyphas of these fungi increase the spatial availability of P. The third strategy consists of modifying the environment of the rhizosphere in order to boost P mobilization. This includes the release of exudates by the plant roots, acidifying the rhizospheric area so there is more H\({}_{2}\)PO\({}_{4}\)\({}^{-}\), which is the easier form that plants can absorb (physiological factor) that is found only in acid soils. The exudation of the rhizosphere also boosts the production of phosphatases, as it mineralizes the different forms of organic P in the soil. Plants produce many secondary metabolites that are fundamental for survival under environmental stress. These include flavonoids and especially anthocyanins whose biosynthesis is modulated not only by a number of intracellular signals but also by environmental stimuli [87,88]. The problems regarding P deficiency in crops cause anthocyanins to accumulate in plants. These adaptive responses are due to protein production, arising from the differential expression of many stress-sensitive genes [89].