Severe Phosphorus Stress Affects Sunflower and Maize but Not Soybean Root to Shoot Allometry

The predictions of two models on biomass allocation were compared on P‐stressed and non‐stressed crop plants. Allometric coefficients were calculated from paired measurements of root and shoot biomass obtained from field and greenhouse experiments with soybean [ Glycine max (L.) Merr.], sunflower ( Helianthus annuus L.), and maize ( Zea mays L.) plants. Soybean consistently followed the allometric model, with the allocation pattern governed by the plant size (common slope K of 0.96 and 0.82 in the field and greenhouse, respectively). Sunflower and maize showed allometric trajectories in the field but optimal partitioning trajectories in the greenhouse. Field data for sunflower and maize adjusted to a unique line ( K = 0.92 and 1.05, respectively) indicating that the biomass allocation is explained by allometric trajectories irrespective of the P level. In contrast, greenhouse data adjusted to two parallel lines (i.e., different elevation coefficient but similar slope: 0.91 for sunflower, 0.96 for maize). Only under severe P stress sunflower and maize plants modified their allocation pattern. Since the severity of the P stress needed to induce a shift in the allocation pattern would be large, we conclude that the three species follow unique root to shoot allometric trajectories under P levels usually found in the field. Most studies analyze the slope of the allometric relationships irrespective of the intercept. Here, we show that only the joint analysis of the slope and the elevation helps understand the effect of P availability on the biomass allocation pattern of relevant crop species.