Calluna vulgaris root cells show increased capacity for amino acid uptake when colonized with the mycorrhizal fungus Hymenoscyphus ericae

Ericoid mycorrhizas are believed to improve N nutrition of many ericaceous plant species that typically occur in habitats with impoverished nutrient status, by releasing amino acids from organic N forms. Despite the ubiquity of mycorrhizal formation the mechanisms and regulation of nutrient transport in mycorrhizal associations are poorly understood. We used an electrophysiological approach to study how amino acid transport characteristics of Calluna vulgaris were affected by colonization with the ericoid mycorrhiza fungus Hymenoscyphus ericae. Both the V_max and K_m parameters of amino acid uptake were affected by fungal colonization in a manner consistent with an increased availability of amino acid to the plant. The ecophysiological significance of altered amino acid transport in colonized root cells of C. vulgaris is discussed. © New Phytologist (2002).

Arsenic limits Trace Mineral Nutrition (Selenium, Zinc, and Nickel) in Bangladesh Rice Grain

A reconnaissance of 23 paddy fields, from three Bangladesh districts, encompassing a total of 230 soil and rice plant samples was conducted to identify the extent to which trace element characteristics in soils and irrigation waters are reflected by the harvested rice crop. Field sites were located on two soil physiographic units with distinctly different As soil baseline and groundwater concentrations. For arsenic (As), both straw and grain trends closely fitted patterns observed for the soils and water. Grain concentration characteristics for selenium (Se), zinc (Zn), and nickel (Ni), however, were markedly different. Regressions of shoot and grain As against grain Se, Zn, and Ni were highly significant (P <0.001), exhibiting a pronounced decline in grain trace-nutrient quality with increasing As content. To validate this further, a pot experiment cultivar screening trial, involving commonly cultivated high yielding variety (HYV) rice grown alongside two U.S. rice varieties characterized as being As tolerant and susceptible, was conducted on an As-amended uniform soil. Findings from the trial confirmed that As perturbed grain metal(loid) balances, resulting in severe yield reductions in addition to constraining the levels of Se, Zn, and Ni in the grain.

Relationship between plant phosphorus status and the kinetics of arsenate influx in clones of deschampsia cespitosa (L.) beauv. that differ in their tolerance to arsenate

Uptake kinetics of arsenate were determined in arsenate tolerant and non-tolerant clones of the grass Deschampsia cespitosa under differing root phosphorus status to investigate the mechanism controlling the suppression of arsenate influx observed in tolerant clones. Influx was always lower in tolerants compared to non-tolerants. Short term influx of arsenate by the high affinity uptake system in both tolerant clones was relatively insensitive to root phosphorus status. This was in contrast to the literature where the regulation of the phosphate (arsenate) uptake system is normally much more responsive to plant phosphorus status. The low affinity uptake system in both tolerant and non-tolerant clones, unlike the high affinity uptake system, was more closely regulated by root phosphate status and was repressed to a much greater degree under increasing root phosphorus levels than the high affinity system. © 1994 Kluwer Academic Publishers.

Biomass allocation, phosphorus nutrition and vesicular-arbuscular mycorrhizal infection in clones of Yorkshire Fog, Holcus lanatus L. (Poaceae) that differ in their phosphate uptake kinetics and tolerance to arsenate

Biomass and phosphorus allocation were determined in arsenate tolerant and non-tolerant clones of the grass Holcus lanatus L. in both solution culture and in soil. Arsenate is a phosphate analogue and is taken up by the phosphate uptake system. Tolerance to arsenate in this grass is achieved by suppression of arsenate (and phosphate) influx. When clones differing in their arsenate tolerance were grown in solution culture with a range of phosphate levels, a tolerant clone did not fare as well as a non-tolerant at low levels of phosphate nutrition in that it had reduced shoot biomass production, increased biomass allocation to the roots and lower shoot phosphorus concentration. At a higher level of phosphate nutrition there was little or no difference in these parameters, suggesting that differences at lower levels of phosphate nutrition were due solely to differences in the rates of phosphate accumulation. In experiments in sterile soil (potting compost) the situation was more complicated with tolerant plants having lower growth rates but higher phosphorus concentrations. The gene for arsenate tolerance is polymorphic in arsenate uncontaminated populations. When phosphorus concentration of tolerant phenotypes was determined in one such population, again tolerants had a higher phosphorus status than non-tolerants. Tolerants also had higher rates of vesicular-arbuscular mycorrhizal (VAM) infection. The ecological implications of these results are that it appears that suppression of the high affinity uptake system, is at least in part, compensated by increased mycorrhizal infection. © 1994 Kluwer Academic Publishers.

Phosphorus nutrition of arsenate-tolerant and nontolerant phenotypes of velvetgrass

Velvetgrass (Holcus lanatus L.), also known as Yorkshire fog grass, has evolved tolerance to high levels of arsenate, and this adaptation involves reduced accumulation of arsenate through the suppression of the high affinity phosphate-arsenate uptake system. To determine the role of P nutrition in arsenate tolerance, inhibition kinetics of arsenate influx by phosphate were determined. The concentration of inhibitor required to reduce maximum influx (V(max)) by 50%, K₁, of phosphate inhibition of arsenate influx was 0.02 mol m^-3 in both tolerant and nontolerant clones. This was compared with the concentration where influx is 50% of maximum, a K(m), for arsenate influx of 0.6 mol m^-3 for tolerants and 0.025 mol m^-3 for nontolerants and, therefore, phosphate was much more effective at inhibiting arsenate influx in tolerant genotypes. The high affinity phosphate uptake system is inducible under low plant phosphate status, this increasing plant phosphate status should increase tolerance by decreasing arsenate influx. Root extension in arsenate solutions of tolerant and nontolerant tillers grown under differing phosphate nutritional regimes showed that indeed, increased plant P status increased the tolerance to arsenate of both tolerant and nontolerant clones. That plant P status increased tolerance again argues that P nutrition has a critical role in arsenate tolerance. To determine if short term flux and solution culture studies were relevant to As and P accumulation in soils, soil and plant material from a range of As contaminated sites were analyzed. As predicted from the short-term competition studies, P was accumulated preferentially to As in arsenate tolerant clones growing on mine spoil soils even when acid extractable arsenate in the soils was much greater than acid extractable phosphate. Though phosphate was much more efficient at competing with arsenate for uptake, plants growing on arsenate contaminated land still accumulated considerable amounts of As. Plants from the differing habitats showed large variation in plant phosphate status, pasture plants having much higher P levels than plants growing on the most contaminated mine spoil soils. The selectivity of the phosphate-arsenate uptake system for phosphate compared with arsenate, coupled with the suppression of this uptake system enabled tolerant clones of the grass velvetgrass to grow on soils that were highly contaminated with arsenate and deficient in phosphate.