High light requirements limit the distribution of several rare plant species endemic to the southern Appalachian region. We studied the influence of light and nitrogen availability on carbon allocation and morphology in one of these species, Houstonia montana Small. Insights into growth and nutrition of H. montana are needed for predicting how it will respond to ongoing changes in its environment associated with atmospheric nitrogen deposition and resulting from succession and (or) management of grassy-bald habitats in which it occurs. We hypothesized that low light constrains belowground allocation, and that elevated N availability reduces limitations to aboveground growth at low light. We tested growth and mycorrhizal colonization of H. montana in response to interactions of light and N availability in a greenhouse experiment. Shade reduced plant biomass, root:shoot ratios, and mycorrhizal colonization, and increased specific leaf area (area/mass). Elevated N reduced root:shoot ratios and mycorrhizal colonization. Under low light, N addition increased specific root length (length/mass) and foliar chlorophyll. We found support for the hypotheses that low light and high N reduce belowground allocation in H. montana. However, we did not find that high N significantly alleviates limitation to plant growth in the shade, despite changes in allocation, morphology, and chemistry that were consistent with more efficient use of C for aboveground growth. Thus, variation in the soil N availability is unlikely to have a marked effect on the ability of H. montana to tolerate shade in its native habitat.
The influence of mineral fertilization on root uptake and arbuscular mycorrhizal fungi-mediated 15 N capture from labeled legume (Medicago polymorpha) residue was examined in winegrapes (Vitis vinifera) in the greenhouse, to evaluate compatibility of fertilization with incorporation of cover-crop residue in winegrape production. Plants grown in marginal vineyard soil were either fertilized with 0.25× Hoagland's solution or not. This low fertilization rate represents the deficit management approach typical of winegrape production. Access to residue in a separate compartment was controlled to allow mycorrhizal roots (roots + hyphae), hyphae (hyphae-intact), or neither (hyphae-rotated) to proliferate in the residue by means of mesh core treatments. Leaves were weekly analyzed for 15 N. On day 42, plants were analyzed for 15 N and biomass; roots were examined for intraradical colonization; and soils were analyzed for 15 N, inorganic N, Olsen-P, X-K, and extraradical colonization. As expected, extraradical colonization of soil outside the cores was unaffected by mesh core treatment, while that inside the cores varied significantly.15 N atom% excess was highest in leaves of roots + hyphae. In comparison, leaf 15 N atom% excess in hyphae-intact was consistently intermediate between roots + hyphae and hyphae-rotated, the latter of which remained unchanged over time. Fertilization stimulated host and fungal growth, based on higher biomass and intraradical colonization of fertilized plants. Fertilization did not affect hyphal or root proliferation in residue but did lower %N derived from residue in leaves and stems by 50%. Our results suggest that even low fertilization rates decrease grapevine N uptake from legume crop residue by both extraradical hyphae and roots.
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