Allometrics is an useful tool to evaluate biomass allocation among different plant organs. It is based in the logarithmic relationships between biomass partitioned to two plant organs (e.g. root to shoot). An increase in the root to shoot ratio is a common plant response to phosphorus deprivation. In this work, we tested the hypothesis that the root to shoot ratio is mainly determined by the total size of the plant rather than by soil phosphorus availability. We performed a field and a greenhouse experiment in which soybean, sunflower and maize were grown under a range of soil phosphorus levels. In successive harvests we measured the root and shoot biomass accumulation and calculated KRMA, which is the slope of the log shoot biomass – log root biomass plot, divided by the correlation coefficient r. Allometric biomass partitioning between root and shoot in the field was very similar with that found in the greenhouse experiment. The root to shoot allometric coefficient did not differ statistically among phosphorus treatments in the three evaluated crops. We consistently observed that the allometric coefficients were not significantly different among the three species, Allometric coefficients KRMA were 0.91, 0.89 and 0.94 for soybean, sunflower and maize, respectively (average of all phosphorus treatments and both experiments). Obtained results allowed us to accept the proposed hypothesis and to conclude that the root to shoot ratio was determined by the total size of the plant and the phosphorus supply level did not modify this ratio.

Contribution of arbuscular mycorrhizal fungi (AMF) to P nutrition is evident in low-P soils, but such benefit may not be expected under non-limiting conditions. A trait to quantify responsiveness to AMF colonization is the mycorrhizal dependency (MD). Our objective was to compare the AMF colonization and MD in soybean, sunflower and maize under contrasting P levels. Synchronized soil and root field sampling allowed the characterization of the relationship between the available soil P and indigenous AMF colonization for the three species on several agricultural soils. A greenhouse experiment including different P levels and AMF inoculation was also conducted. For soybean and maize the AMF colonization was significantly and negatively correlated with available soil P following linear-plateau function, whereas for sunflower we could not find any significant relationship. The high colonization levels observed in the three species suggest that this mechanism is highly relevant for P nutrition of crops grown in the Pampean soils, even in P-rich soils, in which the percentage of AMF colonization did not fell below 30%. Soybean was the specie more favored from AMF colonization and this response was modulated by P availability. Mycorrhized soybean at low and medium P levels enhanced P uptake and total biomass and show the highest MD. Although maize showed low MD values across all P levels, none of them was negative, so there was not carbon drain in mycorrhized maize. Our results show that under contrasting growth conditions soybean had higher AMF colonization and MD than sunflower and maize.