Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

In most soils, inorganic phosphorus occurs at fairly low concentrations in the soil solution whilst a large proportion of it is more or less strongly held by diverse soil minerals. Phosphate ions can indeed be adsorbed onto positively charged minerals such as Fe and Al oxides. Phosphate (P) ions can also form a range of minerals in combination with metals such as Ca, Fe and Al. These adsorption/desorption and precipitation/dissolution equilibria control the concentration of P in the soil solution and, thereby, both its chemical mobility and bioavailability. Apart from the concentration of P ions, the major factors that determine those equilibria as well as the speciation of soil P are (i) the pH, (ii) the concentrations of anions that compete with P ions for ligand exchange reactions and (iii) the concentrations of metals (Ca, Fe and Al) that can coprecipitate with P ions. The chemical conditions of the rhizosphere are known to considerably differ from those of the bulk soil, as a consequence of a range of processes that are induced either directly by the activity of plant roots or by the activity of rhizosphere microflora. The aim of this paper is to give an overview of those chemical processes that are directly induced by plant roots and which can affect the concentration of P in the soil solution and, ultimately, the bioavailability of soil inorganic P to plants. Amongst these, the uptake activity of plant roots should be taken into account in the first place. A second group of activities which is of major concern with respect to P bioavailability are those processes that can affect soil pH, such as proton/bicarbonate release (anion/cation balance) and gaseous (O2/CO2) exchanges. Thirdly, the release of root exudates such as organic ligands is another activity of the root that can alter the concentration of P in the soil solution. These various processes and their relative contributions to the changes in the bioavailability of soil inorganic P that can occur in the rhizosphere can considerably vary with (i) plant species, (ii) plant nutritional status and (iii) ambient soil conditions, as will be stressed in this paper. Their possible implications for the understanding and management of P nutrition of plants will be briefly addressed and discussed.

Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: A review

Contents

 Summary

I Introduction
II Variability of N : P ratios in response to nutrient  supply
III Critical N : P ratios as indicators of nutrient  limitation
IV Interspecific variation in N : P ratios
V Vegetation properties in relation to N : P ratios
VI Implications of N : P ratios for human impacts  on ecosystems
VII Conclusions

 Acknowledgements


 References



Summary
Nitrogen (N) and phosphorus (P) availability limit plant growth in most terrestrial ecosystems. This review examines how variation in the relative availability of N and P, as reflected by N : P ratios of plant biomass, influences vegetation composition and functioning. Plastic responses of plants to N and P supply cause up to 50-fold variation in biomass N : P ratios, associated with differences in root allocation, nutrient uptake, biomass turnover and reproductive output. Optimal N : P ratios – those of plants whose growth is equally limited by N and P – depend on species, growth rate, plant age and plant parts. At vegetation level, N : P ratios 20 often (not always) correspond to N- and P-limited biomass production, as shown by short-term fertilization experiments; however long-term effects of fertilization or effects on individual species can be different. N : P ratios are on average higher in graminoids than in forbs, and in stress-tolerant species compared with ruderals; they correlate negatively with the maximal relative growth rates of species and with their N-indicator values. At vegetation level, N : P ratios often correlate negatively with biomass production; high N : P ratios promote graminoids and stress tolerators relative to other species, whereas relationships with species richness are not consistent. N : P ratios are influenced by global change, increased atmospheric N deposition, and conservation managment.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2004.01192.x

N : P ratios in terrestrial plants: variation and functional significance

Zinc deficiency is a well-documented problem in food crops, causing decreased crop yields and nutritional quality. Generally, the regions in the world with Zn-deficient soils are also characterized by widespread Zn deficiency in humans. Recent estimates indicate that nearly half of world population suffers from Zn deficiency. Cereal crops play an important role in satisfying daily calorie intake in developing world, but they are inherently very low in Zn concentrations in grain, particularly when grown on Zn-deficient soils. The reliance on cereal-based diets may induce Zn deficiency-related health problems in humans, such as impairments in physical development, immune system and brain function. Among the strategies being discussed as major solution to Zn deficiency, plant breeding strategy (e.g., genetic biofortification) appears to be a most sustainable and cost-effective approach useful in improving Zn concentrations in grain. The breeding approach is, however, a long-term process requiring a substantial effort and resources. A successful breeding program for biofortifying food crops with Zn is very much dependent on the size of plant-available Zn pools in soil. In most parts of the cereal-growing areas, soils have, however, a variety of chemical and physical problems that significantly reduce availability of Zn to plant roots. Hence, the genetic capacity of the newly developed (biofortified) cultivars to absorb sufficient amount of Zn from soil and accumulate it in the grain may not be expressed to the full extent. It is, therefore, essential to have a short-term approach to improve Zn concentration in cereal grains. Application of Zn fertilizers or Zn-enriched NPK fertilizers (e.g., agronomic biofortification) offers a rapid solution to the problem, and represents useful complementary approach to on-going breeding programs. There is increasing evidence showing that foliar or combined soil+foliar application of Zn fertilizers under field conditions are highly effective and very practical way to maximize uptake and accumulation of Zn in whole wheat grain, raising concentration up to 60 mg Zn kg−1. Zinc-enriched grains are also of great importance for crop productivity resulting in better seedling vigor, denser stands and higher stress tolerance on potentially Zn-deficient soils. Agronomic biofortification strategy appears to be essential in keeping sufficient amount of available Zn in soil solution and maintaining adequate Zn transport to the seeds during reproductive growth stage. Finally, agronomic biofortification is required for optimizing and ensuring the success of genetic biofortification of cereal grains with Zn. In case of greater bioavailability of the grain Zn derived from foliar applications than from soil, agronomic biofortification would be a very attractive and useful strategy in solving Zn deficiency-related health problems globally and effectively.

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Enrichment of cereal grains with zinc: Agronomic or genetic biofortification?

Cellular Mechanisms of Aluminum Toxicity and Resistance in Plants

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeno and South American 5) maize genotypes. In a growth solution containing 6 μM Al3+, Tuxpeno and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al3+ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

The relationship of zinc (Zn) deficiency to phosphorus (P) toxicity was examined in 'Emerald' okra [Abelmoschus esculentus (L.) Moench] grown for 21 days (D21) in complete nutrient solution and then transferred to treatments with two levels of P (P1, P2 = 250, 2,000 P) and four levels of Zn (Zn1, Zn2, Zn3, Za4 = 0, 0.25, 1, 2 added Zn). No symptoms of "little leaf" or "resetting" developed in any plants. "Mottle leaf" symptoms developed as chlorotic and necrotic patches in the interveinal areas of the older leaves of Zn, plants at P1 and in Zn1, Zn2, and Zn3 plants at P2. Thus, increasing the level of P induced symptoms which were eliminated by adding Zn. Treatments had little effect on dry matter at D45 and D63: at D111, low Zn depressed dry weight of P1 tops and even more markedly depressed dry weight of P2 tops, roots, and fruits. Increasing solution P increased the concentration and amount of P in all plant parts at all harvests. At D45 it also depressed Zn concentrations in all parts of Zn1 plants but in no other Zn treatment and in no treatment at later harvests. At Zn1, leaves had relatively low concentrations of Zn and high concentrations of P. Increasing levels of Zn enhanced their Zn concentrations and depressed their P concentrations; at D45 and D63 they also depressed the concentrations and total amounts of P in tops and whole plants but increased them in roots. Thus Zn deficiency markedly enhanced P absorption by roots, transport to tops, and accumulation in leaves. Concentrations of magnesium in plant organs responded to treatments in the same way as did P but to a smaller degree, particularly in the leaves. No other element measured (Ca, K, Na, Fe, Cu, Mn) responded in the same way. The development and intensity of symptoms in old leaves correlated closely with P concentrations. Leaves with faint symptoms had 0.8% P; those with marked symptoms had > 1.5% P; and dead leaves had nearly 5% P. The intensification of symptoms is attributed to accumulation of P to toxic levels in leaves. It is suggested that Zn deficiency interferes with P metabolism enhancing the amounts of P absorbed by roots and transported to tops: under conditions of high P supply, P accumulates to toxic levels in leaves inducing or accentuating symptoms resembling Zn deficiency. This effect of Zn on P metabolism in roots seems to explain previously puzzling observations in which P treatments enhanced symptoms of Zn deficiency without any reduction in Zn contents of plant tops: there is no need to invoke any effect of P in inactivating Zn in leaves. Additional Index Words: P-Zn interactions, Zn deficiency, Zn requirements, Abelmoschus esculentus (L.) Moench. View complete article To view this complete article, insert Disc 4 then click button8

Phosphorus Accumulation and Toxicity in Leaves in Relation to Zinc Supply1

The role of Ca2+ transport in the mechanism of Al toxicity was investigated, using a Ca2+-selective microelectrode system to study Al effects on root apical Ca2+ fluxes in two wheat (Triticum aestivum L.) cultivars: Al-tolerant Atlas 66 and Al-sensitive Scout 66. Intact 3-day-old low-salt-grown (100 micromolar CaCl2, pH 4.5) wheat seedlings were used, and it was found that both cultivars maintained similar rates of net Ca2+ uptake in the absence of Al. Addition of Al concentrations that were toxic to Scout (5-20 micromolar AlCl3) immediately and dramatically inhibited Ca2+ uptake in Scout, whereas Ca2+ transport in Atlas was relatively unaffected. The Al-induced inhibition of Ca2+ uptake in Scout 66 was rapidly reversed following removal of Al from the solution bathing the roots. Similar studies with morphologically intact root cell wall preparations indicated that the Al effects did not involve Al-Ca interactions in the cell wall. These results suggest that Al inhibits Ca2+ influx across the root plasmalemma, possibly via blockage of calcium channels. The differential effect of Al on Ca2+ transport in Al-sensitive Scout and Al-tolerant Atlas suggests that Al blockage of Ca2+ channels could play a role in the cellular mechanism of Al toxicity in higher plants.

https://www.jstor.org/stable/info/4274076

Aluminum effects on calcium fluxes at the root apex of aluminum-tolerant and aluminum-sensitive wheat cultivars.

The effects of aluminum on the concentration-dependent kinetics of Ca(2+) uptake were studied in two winter wheat (Triticum aestivum L.) cultivars, Al-tolerant Atlas 66 and Al-sensitive Scout 66. Seedlings were grown in 100 μM CaCl2 solution (pH 4.5) for 3 d. Subsequently, net Ca(2+) fluxes in intact roots were measured using a highly sensitive technique, employing a vibrating Ca(2+)-selective microelectrode. The kinetics of Ca(2+) uptake into cells of the root apex, for external Ca(2+) concentrations from 20 to 300 μM, were found to be quite similar for both cultivars in the absence of external Al; Ca(2+) transport could be described by Michaelis-Menten kinetics. When roots were exposed to solutions containing levels of Al that were toxic to Al-sensitive Scout 66 but not to Atlas 66 (5 to 20 μM total Al), a strong correlation was observed between Al toxicity and Al-induced inhibition of Ca(2+) absorption by root apices. For Scout 66, exposure to Al immediately and dramatically inhibited Ca(2+) uptake over the entire Ca(2+) concentration range used for these experiments. Kinetic analyses of the Al-Ca interactions in Scout 66 roots were consistent with competitive inhibition of Ca(2+) uptake by Al. For example, exposure of Scout 66 roots to increasing Al levels (from 0 to 10 μM) caused the K m for Ca(2+) uptake to increase with each rise in Al concentration, from approx. 100 μM in the absence of Al to approx. 300 μM in the presence of 10 μM Al, while having no effect on the V max. The same Al exposures had little effect on the kinetics of Ca(2+) uptake into roots of Atlas 66. The results of this study indicate that Al disruption of Ca(2+) transport at the root apex may play an important role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars, and that differential Al tolerance may be associated with the ability of Ca(2+)-transport systems in cells of the root apex to resist disruption by potentially toxic levels of Al in the soil solution.

Aluminum effects on the kinetics of calcium uptake into cells of the wheat root apex : Quantification of calcium fluxes using a calcium-selective vibrating microelectrode.

David L. Grunes

Papers

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

Phosphorus Accumulation and Toxicity in Leaves in Relation to Zinc Supply1

Aluminum effects on calcium fluxes at the root apex of aluminum-tolerant and aluminum-sensitive wheat cultivars.

Aluminum effects on the kinetics of calcium uptake into cells of the wheat root apex : Quantification of calcium fluxes using a calcium-selective vibrating microelectrode.

Effect of Zinc Deficiency on the Accumulation of Boron and Other Mineral Nutrients in Barley1