Intermediate Nutrient Levels Research Articles

Interactions between infection by endophytic fungi and nutrient limitation in the grasses Lolium perenne and Festuca arundinacea

SUMMARYAlthough grasses infected by fungal endophytes often exhibit increased growth relative to uninfected plants, the results of infection may be manifested as decreased growth in plants growing under stressful conditions. A preliminary experiment compared perennial ryegrass (Lolium perenne L.) infected by Acremonium lolii Latch, Christensen & Samuels with uninfected plants grown at three soil moistures and three nutrient concentrations. Total biomass, tiller number, and relative tillering rate were significantly affected by nutrient concentration and moisture. Although major differences between infected and uninfected plants were not apparent, relative growth rates of infected plants were significantly greater than uninfected plants in three of nine treatment combinations. Interactions between infection and nutrient limitation were examined in seedlings and adults of perennial ryegrass and tall fescue (Festuca arundinacea Schreb., infected by A. coenophialum Morgan‐Jones and Gams). A significant nutrient × infection interaction for tall fescue seedlings indicated that, compared to uninfected seedlings, infected seedlings had greater biomass at the high nutrient level but a significantly lower biomass at the low nutrient level, suggesting a metabolic cost to the host due to competition with the fungus for nutrients or photosynthate. Infected perennial ryegrass seedlings had significantly more biomass at high and intermediate nutrient levels, but not at low nutrient levels. Although infection did not significantly affect total biomass in perennial ryegrass adults, in tall fescue adults the benefits of endophyte infection to host plants (recorded as greater biomass) became increasingly greater with increasing nutrient availability.

The Mathematical Structure of an Aquatic Ecosystem Model

As ecological models become more complex, the ques tion of whether they are stable becomes more diffi cult to answer. Some ecologists believe that a com plex system is more stable than a simple system because it is less dependent on any single path through it. Mathematicians, on the other hand, have shown that systems of equations are more likely to become unstable as the number of equations increases. Neither tendency may apply to ecological systems that include time-varying components and in which the number of significant interactions does not de pend strongly on the number of components. In the case of the aquatic ecosystem model MS. CLEANER, solutions of the governing differential equations are stable even though the model contains up to 40 state variables and simulates many biologi cal, biochemical, microbial, and chemical processes. The stability is the result of feedback in the system and the sparseness of the interaction matrix. A qualitative analysis of the model shows that the functions used to represent predation have signifi cantly different forms at high, low, and intermediate nutrient levels. The different forms of this func tion cause unique, bounded solutions for high and low nutrient conditions and two stable, periodic solutions for intermediate conditions. The predic tion of a unique solution independent of the initial conditions in one case and more than one stable periodic solution in another case agrees with the published results of simulation.

Growth of phytoplankton and vascular aquatic plants at different nutrient levels

SummaryGreenhouse experiments to determine optimal levels of inorganic nitrogen and phosphorous for growth of aquatic plants were conducted as a basis for establishing eutrophic conditions in twenty experimental ponds. Various formulations, levels, and ratios of inorganic N and P were applied to cultures of aquatic vascular plants (Myriophyllum spicatum var. exalbescens, Potamogeton crispus and Elodea canadensis) and freshwater phytoplankton populations (mostly Chlorophyceae) established in battery jars containing natural soil and water. Nutrient levels applied ranged from 0.015–50 mg/l N and 0.005-5.0 mg/l P. The yields of the plants after five weeks growth under experimental conditions were used as indices of response.The highest nutrient levels produced the best growth of phytoplankton while the lower levels resulted in higher yields of vascular plants. Filamentous algae grew best at the intermediate nutrient levels. High yields of vascular plants never coincided with large phytoplankton populations. At the highest nutrient levels applied, no vascular plant growth occurred. Of two inorganic phosphorous compounds evaluated, CaH4 (PO4)2 produced the greater yields. Smaller differences were apparent amoung three inorganic nitrogen compounds.This laboratory data suggest that future uncontrolled enrichment of natural bodies of water will result in the growth of larger populations of phytoplankton at the expense of benthic aquatic macrophytes. These conclusions are being examined under natural conditions in experimental ponds.