1. Cuscuta species are rootless, leafless and branchless stems without meaningful photosynthesis. Hence, sink-free and source-free mechanistic growth models are possible yet remain unexplored. Furthermore, phytohormone expression has not yet been studied in the absence of hosts. 2. We use mass conservation in second-order differential equations to build mechanistic models for Cuscuta growth dynamics and UPLC-MS/MS to identify and mathematically score phytohormone expression. 3. We identified four sequential stages of growth – exponential, linear, parabolic deceleration and terminal stages and, in Cuscuta chinensis, the phases are discernable by eye. 4. Analytical solutions to the differential equations fit the growth data well and the model also predicts faster growth in Cuscuta species with smaller seeds, in agreement with the fact that Cuscuta chinensis attains terminal stage faster than Cuscuta japonica. 5. We found evidence for stage-specific phytohormone expression and for the existence of stem-wide phytohormone gradients, especially for the Auxin components MEIAA and ICAId in Cuscuta japonica. 6. We have built models for the roles of phytohormones in Cuscuta growth dynamics, and performed ad hoc calculations which suggest a continuously-increasing Cytokinin/Auxin ratio in growing Cuscuta seedlings, thereby implying a maximum value beyond-which growth slow-down begins. 7. Synthesis: We have created the first-ever source-free and sink-free plant growth models using mass conservation in second-order differential equations and in so-doing, uncovered four growth stages in Cuscuta, observable by eye in Cuscuta chinensis. We found stage-specific phytohormone expression and a likelihood of stem-wide phytohormone expression gradients. We then brought it all together by building models for the roles of phytohormones in Cuscuta growth dynamics and by performing ad hoc calculations which have suggested that the Cytokinin to Auxin ratio increases continuously in growing Cuscuta seedlings, to perhaps attain a maximum value beyond-which slow-down in growth begins.

Parasitic plants have been shown to affect soil-organic-matter (SOM) decomposition, but the mechanism is unknown. As arbuscular mycorrhizal fungi (AMF) can affect decomposition and compete with parasitic plants for carbon, we hypothesized that parasitic plants can indirectly regulate decomposition by suppressing AMF. We conducted two experiments in which Bidens pilosa was inoculated with AMF or not, and parasitized with Cuscuta australis or not. In hyphae-in-growth bags, we provided SOM in one experiment as 13C-/15N-labelled maize leaves and in the other as phytate P. In the presence of AMF, Parasitization increased the 13C but decreased the organic P remaining in the bags. In the absence of the parasite, AMF decreased the 13C and increased the organic P remaining in the bags. Our results demonstrate that parasitic plants can indirectly regulate the decomposition of organic materials by suppressing the effect of the extraradical AMF hyphae on decomposition.