Department of Plant Sciences

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.