Plasticity in root shoot partitioning: Optimal, ontogenetic, or both?

1. We tested whether plants increase root,shoot ratios to compensate for limitations of below-ground resources in a manner consistent with optimal partitioning theory or whether the relative production of roots and shoots is controlled by species-specific developmental patterns. Individuals of two annual plant species, Abutilon theophrasti and Chenopodium album, were grown from seed in controlled greenhouse conditions under high- or low-nutrient regimes. Mid-way through the experiment, a sub-set of low-nutrient-grown plants were given high nutrient availability and a sub-set of high-nutrient-grown plants were transferred to a low nutrient environment. 2. Under continuous nutrient regimes: (1) high-nutrient-grown plants of both species grew faster and had a lower root:shoot ratio than low-nutrient-grown plants, consistent with optimal partitioning theory; (2) both species exhibited a substantial amount of ontogenetic drift as root:shoot ratios decreased through ontogeny (subsequent to an initial increase in R/S shortly after germination); (3) allometric analyses revealed that increased allocation to roots occurred very early in ontogeny for both species, after which the relative growth of shoots exceeded that of roots in low-nutrient-grown plants compared to their high nutrient-grown counterparts - a result inconsistent with optimal partitioning theory. 3. Under temporally varying nutrient regimes: (1) growth substantially increased in low-nutrient-grown plants that were switched to a high-nutrient environment without a change in root:shoot partitioning; (2) there was no change in growth or partitioning when plants were switched from a high- to a low-nutrient regime. 4. We conclude that, for these annual species, root/shoot partitioning is partially consistent with optimal partitioning theory but that is also highly ontogenetically constrained. This constraint is evident both in substantive ontogenetic drift in partitioning and in the period during development that plasticity in partitioning can be expressed.