Correction for non-linear relationships between root size and short term Pi uptake in genotype comparisons

Variation in phosphate (Pi) absorption rates, root length and root mass was investigated in 25 genotypes or inbred lines of Arabidopsis thaliana (L.) Heynh. All variables differed among lines (P< 0.0001), and larger plants with larger roots and shoots had faster Pi uptake rates (influx) (P< 0.0001). Within genotypes, influx was non-linearly related to root size. Increases in influx were not directly proportional to increases in root size; with a doubling in root length or biomass, there was only a 1.30 or 1.27 concomitant increase in influx, respectively. Because inbred lines are genetically homogeneous, this within-genotype pattern was random, and not genetically based. Influx has traditionally been standardized for variation in plant size by dividing by root length or biomass. This assumes a linear relationship between influx and root size. As uptake rate was non-linearly dependent on root size in A. thaliana, this resulted in a spurious negative correlation between influx per cm or mg root and root length or biomass. We present a general method that standardizes influx for variation in root size within genetically homogenous genotypes. This method determines and corrects for random non-linear relationships between influx and root size. In A. thaliana, influx per unit root length or biomass differed among lines (P < 0.0001), but these differences did not correlate with root length or biomass. These data contradict the common interpretation in previous literature that species or cultivars with small roots have evolved greater influx per unit size of root.