Roger T. Koide

EFFECTS OF SOIL RESOURCES ON PLANT INVASION AND COMMUNITY STRUCTURE IN CALIFORNIAN SERPENTINE GRASSLAND

Non-native annual grasses dominate most Californian mediterranean-cli- mate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine-derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non-native species in serpentine grassland. Factorial combinations of nitrogen, phos- phorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non-native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders sup- pressed some native forbs. Increased macronutrient availability can increase production on serpentine-derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy-metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant com- munities may be directly influenced by nutrient availability, independent of physical dis- turbance.

Plant water status, hydraulic resistance and capacitance

Many excellent reviews have been written about plant water status and its measurement (e.g. Slatyer, 1967; Barrs, 1968; Boyer, 1969; Brown and van Haveren, 1972; Slavik, 1974; Turner, 1981). The reader is referred to these sources for a more complete review, particularly of the older literature. In this chapter, our major goals are to introduce the reader to the concept and measurement of plant water potential and its components, and then to discuss the consequences of gradients in these components within the plant. First, we describe the most commonly used techniques for measuring the water potential of higher plants growing under field conditions, specifically the psychrometric and pressure chamber techniques. Second, we describe methods for measuring the components of water potential, particularly turgor pressure and osmotic potential, and water content. Since the transpirational path of the plant can be regarded as a hydraulic resistor, transpirational fluxes occur only when gradients in the various components of water potential exist within the plant. Thus, the third concept that we introduce is hydraulic resistance. Techniques for its calculation are described for steady-state transpiration, when root water absorption equals shoot evaporation. Nonsteady-state flux occurs when there is a net movement of water between the transpirational path and tissues adjacent to it. Such tissues are regarded as capacitors, the final topic of our discussion.

A history of research on arbuscular mycorrhiza

This is not a review paper in the traditional sense, of which there are many. Three of the most influential reviews that summarized well some of the “older” literature include those by Nicolson (1967), Gerdemann (1968) and Mosse (1973). Instead, in this brief and incomplete work, we attempt to show the historical development of research on arbuscular mycorrhizas. We owe much to those who have written other historical accounts, including Rayner (1926–1927), Trappe and Berch (1985), Mosse (1985), Schenck (1985), Harley (1991) and Allen (1996), but the contents of this work naturally reflect our own ignorance, interests and biases. It was often difficult to distinguish between the historical and the contemporary, and we did not use any specific cutoff date in making this distinction. The degree to which we include “contemporary” literature was determined by our own assessment of its connectedness to older literature. In any case, we hope this will be of some interest to those of you who study the arbuscular mycorrhiza, and that it will serve the purpose of providing what we consider to be an important historical context for current researchers. We wish you good fortune in your research.

INFLUENCES OF ESTABLISHED TREES ON MYCORRHIZAS, NUTRITION, AND GROWTH OF QUERCUS RUBRA SEEDLINGS

Established plants may facilitate the regeneration of closely related seedlings if they increase populations of mutualistic symbionts that would otherwise be limiting. In this study we examined the influence of ectomycorrhizal and vesicular-arbuscular mycor- rhizal (VAM) trees on Quercus rubra seedlings to determine how trees influence mycorrhizal infection, nutrient uptake, and growth of seedlings. In two related experiments, we planted Q. rubra acorns adjacent to stump sprouts of Q. montana (=Near-Quercus) and Acer rubrum (=Near-Acer), and, in the second experiment, near Quercus spp. stumps that had not re- sprouted (=Near-Dead-Quercus). Congeneric Quercus were used to prevent root grafting; using stump sprouts minimized aboveground differences between treatments by limiting canopy size. In both experiments, Near-Quercus seedlings were infected by ectomycorrhizal fungi to a significantly greater extent than Near-Acer or Near-Dead-Quercus seedlings. Near-Quercus seedlings were also infected by a different and more diverse community of ectomycorrhizal fungi. Cenococcum geophilum, the only ectomycorrhizal fungus more abundant on Near-Acer seedlings than on Near-Quercus seedlings, appears to be relatively ineffective at increasing seedling nutrient uptake. Near-Quercus seedlings had greater con- centrations and contents of N and P than other seedlings in both experiments. In the first experiment, Near-Quercus seedlings had greater growth than Near-Acer seedlings, although it was not clear if this represented beneficial influences of Q. montana, or an undetermined negative influence of A. rubrum. No significant growth responses were found in the second experiment; severe drought may have prevented the expression of growth potential. The results demonstrate indirect facilitation of seedlings by established congeneric trees through increased seedling ectomycorrhizal infection.

Nutrient transport in mycorrhizas: structure, physiology and consequences for efficiency of the symbiosis

Nutrient transport in mycorrhizas occurs across specialized interfaces which are the result of corrdinated development of the organisms. The structural modifications give rise to large areas of either inter- or intra-cellular interface in which wall synthesis is frequently modified and in which altered distribution of membrane bound ATPases is important, particularly with respect to mechanisms that may be involved in bidirectional transfer of nutrients. Except in orchid mycorrhizas, net movement of organic carbon from plant to fungus occurs, complemented by mineral nutrient movement in the opposite direction. The general consensus is that sustained transfer at rates that will maintain the growth and development of the organisms requires increases in the rates at which nutrients are lost from the organisms; possible mechanisms for this are discussed. The transfer processes are essential in determining both plant and fungal productivity and an approach to calculating the efficiency of the symbiosis in terms of the expenditure of carbon (or of phosphorus) is discussed.

Role of mycorrhizal infection in the growth and reproduction of wild vs. cultivated plants. I: Wild vs. cultivated oats

SummaryWe tested the hypothesis that mycorrhizal infection benefits wild plants to a lesser extent than cultivated plants. This hypothesis stems from two observations: (1) mycorrhizal infection improves plant growth primarily by increasing nutrient uptake, and (2) wild plants often possess special adaptations to soil infertility which are less pronounced in modern cultivated plants. In the first experiment, wild (Avena fatua L.) and cultivated (A. sativa L.) oats were grown hydroponically at four different phosphorus levels. Wild oat was less responsive (in shoot dry weight) to increasing phosphorus availability than cultivated oat. In addition, the root: shoot ratio was much more plastic in wild oat (varying from 0.90 in the low phosphorus solution to 0.25 in the high phosphorus solution) than in cultivated oat (varying from 0.44 to 0.17). In the second experiment, mycorrhizal and non-mycorrhizal wild and cultivated oats were grown in a phosphorus-deficient soil. Mycorrhizal infection generally improved the vegetative growth of both wild and cultivated oats. However, infection significantly increased plant lifespan, number of panicles per plant, shoot phosphorus concentration, shoot phosphorus content, duration of flowering, and the mean weight of individual seeds in cultivated oat, while it had a significantly reduced effect, no effect, or a negative effect on these characters for wild oat. Poor positive responsiveness of wild oat in these characters was thus associated with what might be considered to be inherent adaptations to nutrient deficiency: high root: shoot ratio and inherently low growth rate. Infection also increased seed phosphorus content and reproductive allocation.

The effect of dandelion or a cover crop on mycorrhiza inoculum potential, soil aggregation and yield of maize

A field experiment was conducted to observe the influence of a cover crop (winter wheat, Triticum aestivum L.), and a perennial weed (dandelion, Taraxacum officinale Weber ex Wigg.), on vesicular-arbuscular mycorrhiza (VAM) inoculum potential, soil aggregation, and maize yield after one season. Mycorrhizal colonization of maize roots was higher following the autumn planting of either winter wheat or dandelion compared with fallow. In the dandelion plots maize had significantly higher P content, shoot dry weight and yield relative to the winter wheat plots. Winter wheat and dandelion both improved soil aggregation. ©2000 Elsevier Science B.V. All rights reserved.

Winter wheat cover cropping, VA mycorrhizal fungi and maize growth and yield

The relationships among winter cover cropping, inoculum potential of vesicular-arbuscular mycorrhizal (VAM) fungi, and the growth and yield of a subsequent maize crop were investigated. In the first experiment, an autumn-sown winter wheat cover crop increased VAM fungal inoculum potential of a field soil as measured by an in situ maize bioassay during the following growing season. Infective extra-radical hyphal densities were significantly increased by cover cropping as interpreted from the effect of soil disturbance on infection of the maize bioassay plants. In a second experiment the following year, the winter wheat cover crop again increased VAM fungal inoculum potential as assessed by an in situ maize bioassay during the following growing season. Moreover, the degree of mycorrhizal infection of maize was correlated with maize growth and yield. This study suggests that the management of mycorrhizal fungi by cover cropping may be a useful practice in sustainable agriculture.

Effects of mycorrhizal fungi on plant populations

We discuss four potentially important interactions between mycorrhizal fungi and populations of plants. First, vesicular-arbuscular mycorrhizal colonization has been shown to increase reproduction (via both male and female functions) and offspring survival, and thus it can increase population size, at least in the short term. This is undoubtedly important to wild plant species and especially to those whose success depends on high rates of reproduction such as early successional annuals. Second, the positive response in growth and reproduction to vesicular-arbuscular mycorrhizal colonization may be inversely related to plant population density. All else being equal, this would tend to stabilize the density of natural plant populations over time. It may also explain why positive responses to mycorrhizal inoculation of dense crops are rare. Third, vesicular-arbuscular mycorrhizal fungi can increase inequality in size and reproduction among plants within a population. Mycorrhizal fungi may thus exaggerate the genetic overrepresentation in the next generation of the most robust individuals in the current generation. Fourth, established mycorrhizal plants may serve as important sources of inoculum for initially nonmycorrhizal, conspecific seedlings. This may affect regeneration, and could contribute to patchy distributions of species within the community.

Mycorrhizal symbiosis increases growth, reproduction and recruitment of Abutilon theophrasti Medic. in the field

We examined in the field the effect of the vesicular-arbuscular (VA) mycorhizal symbiosis on the reproductive success of Abutilon theophrasti Medic., an early successional annual member of the Malvaceae. Mycorrhizal infection greatly enhanced vegetative growth, and flower, fruit and seed production, resulting in significantly greater recruitment the following year. In addition, the seeds produced by mycorrhizal plants were significantly larger and contained significantly more phosphorus than seeds from non-mycorrhizal plants, an effect which may improve offspring vigor. Infection by mycorrhizal fungi may thus contribute to the overall fitness of a host plant and strongly influence long-term plant population dynamics.