The environmental implications of intensified land use in developing countries

The major agricultural intensifications in the developed world over the last half century have produced a range of important environmental problems. These include pollution, damage to wildlife and landscape and other issues, both on- and off-site. These are largely being controlled by scientific investigation and Government regulation. As developing countries increase agricultural production over the next 30 years, this may also cause even more serious environmental damage.

The paper distinguishes between production-related on-site damage, and off-site and more extensive effects. Both may involve soil and water effects, such as soil erosion, salinization, siltation, eutrophication and loss of water quality. The use of more agrochemicals can damage water quality, health, wildlife and biodiversity. Loss of habitat from the extension of farming is particularly damaging to biodiversity. A developing off-site problem is the production of greenhouse gases by farming systems, including the conversion of forests to farmland. In the future the introduction of genetically engineered species of plants, animals or microbes will need secure control.

Work, probably on a catchment basis, is necessary to understand and control these problems. The three main requirements are much better environmental information from the developing world; the selection of environmental indicators to be monitored; and the support of local farmers in protecting the environment. There are encouraging indications of farmer concern and action over obvious on-site damage, but this may not extend to extensive off-site issues. The main danger is that developing food scarcity would cause the environmental issues to be ignored in a race for production.

Allocation of 14 C-carbon in ectomycorrhizal willow

The flow of carbon from plant to fungus in ectomycorrhizal associations has not been well quantified. The objective of this study was to use ¹⁴ C to quantify the increase in fixed carbon translocated below ground in ectomycorrhizal relative to non-mycorrhizal willow (Salix viminalis L, Bowles hybrid). Rooted cuttings were inoculated with Thelephora terrestris (Ehrh). Fr. or left non-mycorrhizal. Non-mycorrhizal plants were grown at the same (4 mg kg^-l bicarbonate-extractable P) (NM-P) or at a higher (21 mg kg^-1 ) soil P concentration (NM +P), one at which the non-mycorrhizal plants were similar in size to the mycorrhizal (M-P) plants. At 41, 51, 76 and 89 days after planting, the shoots were exposed to a pulse of ¹⁴ CO₂ . Plants were harvested after a 202 h chase period. The ¹⁴ Cactivity was quantified in live fractions: shoot tissue, shoot respiration, 'root' tissue (= roots plus fungi), 'root' respiration (= CO₂ released below ground) and soil. Of the total ¹⁴ C detected in these five fractions, M-P plants allocated from 3.9% (harvest 1) to 11.5% (harvest 4) more to the below-ground fractions ('root' tissue, soil and 'root' respiration), than did the NM-P plants. Differences between NM+P and M-P plants were only half of those above (3.1 % and 4.4% at harvests 2 and 3, respectively, compared to 6.4 % and 7.4%, respectively for the difference between NM-P and M-P). Correction for differences in root/shoot ratio between M-P and NM-P plants eliminated the observed differences in carbon distribution only at the first three harvests. There was no evidence for increased 'root' respiration fates or rhizodeposition being responsible for the increased carbon diverted below ground by M-P plants.

Carbon use efficiency in mycorrhizas theory and sample calculations

The benefit to a fungus by a mycorrhizal association is that it gains carbon from its host. A benefit to a host is usually a nutritional one, but any resulting increase in dry weight may be counteracted by the carbon consumed by the fungus. The carbon costs of mycorrhizal fungi have been calculated using ¹⁴ C pulse-chase experiments in the laboratory or by estimating turnover rates in the field. Both of these techniques have their limitations, but estimates have been remarkably consistent amongst most laboratory studies. Carbon demands of the fungus may not reduce theoretical growth enhancement in plants which are sink-limited but would be expected to do so iii source-limited plants. A model of carbon use efficiency is developed based on the economic concepts of cost and benefit. Efficiency is defined in terms of carbon gained via the growth response to infection, and the carbon expended supporting the fungus. Practical considerations of measuring carbon allocation, and calculating carbon use efficiency are discussed. In an experiment on Salix viminalis L., colonized by Thelephora terrestris (Ehrh.) Fr., the carbon use efficiency calculated by this method was on overage 85% based on shoot tissue production, and 95% based on whole plant production.

Fluxes of carbon and phosphorus between symbionts in willow ectomycorrhizas and their changes with time

One way of viewing a mycorrhizal symbiosis is as a balance between the nutritional 'benefits' and carbon 'costs' to the phytobiont. Phosphorus acquisition efficiency (the amount of phosphorus taken up per unit of carbon allocated belowground) can be used as an indicator of this balance. In this study, phosphorus uptake and belowground carbon allocation were measured using ectomycorrhizal (M) (Thelephora terrestris (Ehrh.) Fr.) and non-mycorrhizal (NM) Salix viminalis L. cv. Bowles Hybrid. Following 50, 60, 85 or 98 d of growth in a gamma-irradiated soil/sand mixture containing 4 mg bicarbunate-extractable P kg^-1 , seven randomly-selected cuttings of each treatment were harvested and their P contents determined. Nine d prior to each harvest, the three median plants from the group of seven were pulse labelled with ¹⁴ C to determine the relative allocation of C aboveground and belowground. Mycorrhizal colonization of willow caused a two-fold increase in growth owing to substantially higher P uptake. Phosphorus inflow rates were almost three times as high for M root systems as for NM root systems over the interval up to the first harvest (3.2 × 10 ^-12 and 1.2 × 10¹² mol m^-1 s-1, respectively). Over the interval from 50 to 98 d, inflows into M plants were 50% higher than into NM plants (1.4 × 10¹² and 0.9 × 10^-13 mol m^-1 s^-1 respectively). The M plants allocated about 25 times as much carbon belowground as the NM plants for both periods. The P acquisition efficiency was higher in M than in NM plants during the first interval (16% and 40% higher using two different calculation methods), whereas during the second interval it was higher in NM than in M plants (33% and 44% higher using the two different methods). Thus, ectomycorrhizas can be very effective in supplying P to their hosts even at an early stage of infection. Furthermore, it is suggested that a temporal separation exists in the maximal fluxes of P and C between the fungus and the host of the mycorrhizal association. The results are discussed in the context of the nutrient requirements and carbon economies of field-grown woody plants.

Regulation of nutrient transfer between host and fungus in vesicular-arbuscular mycorrhizas

Although the overwhelming majority of non-aquatic vascular plants form vesicular-arbuscular (VA) mycorrhizal associations, the extent of colonization of the host root by any given fungal symbiont varies considerably depending on host and environmental factors. Because VA mycorrhizal fungi are obligate biotrophs, transfer of photosynthate from host to fungus may be an important factor in regulating the extent of VA mycorrhizal formation. Host metabolites must cross the plasma membrane before becoming available to the fungus. Several studies on rates of root exudation under various environmental conditions show a strong correlation between rates of root exudation and percent of root length colonized by VA mycorrhizal fungi. However, passive leakage of simple metabolites from roots as the sole means of regulating fungal colonization seems improbable for an obligate biotroph which has not yet been successfully cultured on any artificial medium. So far there has been insufficient investigation of hormone interactions between symbionts, and of the interference by the fungus in host cell wall synthesis, to evaluate the possible role of these factors in controlling growth of VA mycorrhizal fungi. Cytochemical studies of the host-fungus interface suggest modification of host plasma membrane ATPase activity as arbuscules develop, but the function of this altered activity remains unresolved. The presence of a linked P₁ -photosynthate exchange mechanism on the host plasma membrane analogous to the P₁ -photosynthate translocator known to exist in the outer membrane of chloroplasts remains an uninvestigated possible mechanism for balancing photosynthate demand by the fungus with enhanced P uptake.

Phosphorus relationships and production of extrametrical hyphae by two types of willow ectomycorrhizas at different soil phosphorus levels

There is much circumstantial evidence for a role of increased P uptake in the growth response of plants to ectomycorrhizas. Full response curves with and without mycorrhizal infection along a P gradient in soil are, however, required to test this hypothesis fully. In this experiment, rooted cuttings of Salix viminalis L. cv. Bowles Hybrid were grown in a 1:2 mixture by volume of gamma-irradiated soil and sterile sand, with bicarbonate-extractable P concentrations of 4, 6, 10, 21, 60 or 90 mg P kg^-1 . The cuttings were inoculated by mixing peat/vermiculite spawn of Laccaria proxima (Boud.) Pat., or Thelephora terrestris (Ehrh.) Fr., or autoclaved spawn 1: 5 by volume with the soil: sand mixture. The plants showed a positive growth response to mycorrhizal infection by either fungus at the two lower P levels, and to L. proxima only at 10 mg P kg^-1 . At 21 mg kg^-1 and above, infection was reduced and neither mycorrhizal inoculation nor further P additions caused significant growth increases. These results imply that the growth responses to ectomycorrhizas in this experiment were solely due to increases in P uptake. Cuttings infected with L. proxima tended to be larger than those infected by T. terrestris. Estimates of percent mycorrhizal infection did not differ between the fungi at the lower P levels. However, overall production of extramatrical hyphae per gram of soil was highest in soil inoculated with L. proxima. At 10 mg P kg^-1 the length of L. proxima hyphae per unit length of mycorrhizal root, P uptake per unit root weight, and total P content in plants infected with L. proxima were significantly higher than for T. terrestris. This study confirms that increased P uptake can be an important component of growth stimulation by ectomycorrhizas. It also presents the first quantification of extramatrical ectomycorrhizal hyphae in soil and suggests a role for them in the growth response.

The development of endomycorrhizal root systems: VII. A detailed study of effects of soil phosphorus on colonization

Leek plants (Allium porrum L.) were grown in a controlled environment on a mixture (2: 1 w/w) γ-irradiated (1.0 Mrad) sandy loam and sand, at six concentrations of bicarbonate-soluble phosphorus (P) ranging from 22 to 344 mg P kg^-1 (soil basis). Inoculum of the vesicular-arbuscular (VA) mycorrhizal fungus Glomus mosseae (Nicolson & Gerdemann) Gerdemann and Trappe was placed (M) or not (NM) in a layer 3 cm below the soil surface. At intervals of 10 d, lengths of main axes of roots and their lateral branches, and of the segments of infection within them, were measured. From these data we calculated the mean (harvest interval method} rates of linear extension of root tips and of infection fronts for each member of root, averaged over the whole root system. The mean delay, d, between a root encountering the layer of inoculum and the subsequent formation of internal infection, was also derived. Addition of P to soil did not affect rates of extension of roots, but increased the initiation of main axes and laterals. Infection segments extended twice as fast in laterals as in main axes. At low rates of addition, P did not affect fungal behaviour but increased the length of root available for colonization. When bicarbonate-soluble P exceeded 140 mg kg^-1 , the rates of extension of infection fronts in both main axes and laterals were approximately halved, and d was considerably increased. The density (the area ratio of fungal to host tissue in a longitudinal squash) of the hyphae and arbuscules respectively, and the number of entry points per unit length of root, were greatly reduced by added P. However, the ratios of numbers of entry points/hyphal density and of arbuscule density/hyphal density were unaltered, and the morphology of the fungus was not noticeably affected. It is probable that formation of entry points was the rate-limiting step for colonization, and that this rate was reduced by added P.

Levels, distribution and chemical forms of trace elements in food plants

The content of trace elements in plants can vary widely, depending upon the composition of the soil in which they grow, other environmental factors, and the species or cultivar of the plant. A high growth rate of the plant may cause internal 'dilution' of trace elements. Complex formation with soil organic colloids and compounds, cell wall material and ligands in and inside the cell membranes are of critical importance in uptake, though most evidence shows that it is the free metal ion in the external solution that is absorbed; the detailed mechanisms are still unknown. Other processes such as excretion of organic compounds, reductants and hydrogen ions from the root greatly alter availability of trace metals, and iron has to be reduced to the ferrous form before uptake. The mean composition of plant shoots is affected by age and season; element mobility in the xylem and phloem determines translocation, and hence concentrations in individual parts of the plant. The rate of retranslocation can be strongly affected by the abundance of the element. Symptoms of deficiency or excess are well documented, but are often not dependable. The essentiality of the trace metals depends upon their function as part of enzymes, and these are briefly reviewed, with stress on processes in plants. Only a small fraction of the total amount of an element is bound in the enzyme; of the remainder, some is present as the free metal ion (Mn) or as complexes of small molecular mass (Cu, Zn, Ni, Fe), the rest being bound to cell wall material. Certain species or genotypes have resistance against high levels of some elements in the soil. Several mechanisms may be involved, one being very strong binding to root cell walls. There are also large genetic differences in susceptibility to trace element deficiencies.