N partitioning in tomato: vascular constraints versus tissue demand.

Allocation of root-derived resources is influenced by tissue demand; however, vascular pathways mediate resource flow from roots to shoots. In vascularly constrained plants (i.e. sectored plants), effects of vascular connections likely limit homogenous resource delivery, especially when environmental resource distribution is patchy. Here, we quantify relative roles of vascular connections, demands by different leaves (i.e. by leaf age and size), and molecule size of transported N compounds (effective sectoriality: nitrate v. ammonium) on allocation of N in the sectored tomato (Solanum lycopersicum L.). Vascular connections were the strongest predictor of both accumulation (amount per leaf; P<0.0001) and δ (estimate of concentration; P<0.0001) N values in mature leaves, but young expanding leaves did not show such dramatically sectored uptake (accumulation: P=0.0685; δ: P=0.0455), suggesting that sectoriality is less strong in young expanding tissue, especially in the youngest leaf. In patchy environments sectoriality, then, should have large consequences for the ability of a plant to allocate resources in mature tissue; however, young leaves do not appear to experience such strong vascular constraints when building new tissue.