Functional aspects of root architecture and mycorrhizal inoculation with respect to nutrient uptake capacity

ACESSO via B-on: http://dx.doi.org/10.1007/s00572-003-0254-5The aim of this research was to investigate theeffect of arbuscular mycorrhizal (AM) colonisation onroot morphology and nitrogen uptake capacity of carob(Ceratonia siliqua L.) under high and low nutrientconditions. The experimental design was a factorialarrangement of presence/absence of mycorrhizal fungusinoculation (Glomus intraradices) and high/low nutrientstatus. Percent AM colonisation, nitrate and ammoniumuptake capacity, and nitrogen and phosphorus contentswere determined in 3-month-old seedlings. Grayscale andcolour images were used to study root morphology andtopology, and to assess the relation between rootpigmentation and physiological activities. AM colonisationlead to a higher allocation of biomass to white andyellow parts of the root. Inorganic nitrogen uptakecapacity per unit root length and nitrogen content weregreatest in AM colonised plants grown under low nutrientconditions. A better match was found between plantnitrogen content and biomass accumulation, than betweenplant phosphorus content and biomass accumulation. It issuggested that the increase in nutrient uptake capacity ofAM colonised roots is dependent both on changes in rootmorphology and physiological uptake potential. Thisstudy contributes to an understanding of the role of AMfungi and root morphology in plant nutrient uptake andshows that AM colonisation improves the nitrogennutrition of plants, mainly when growing at low levelsof nutrients

Rapid Uptake of Aluminum into Cells of Intact Soybean Root Tips (A Microanalytical Study Using Secondary Ion Mass Spectrometry)

A wide range of physiological disorders has been reported within the first few hours of exposing intact plant roots to moderate levels of Al3+. Past microanalytic studies, largely limited to electron probe x-ray microanalysis, have been unable to detect intracellular Al in this time frame. This has led to the suggestion that Al exerts its effect solely from extracellular or remote tissue sites. Here, freeze-dried cryosections (10 [mu]m thick) collected from the soybean (Glycine max) primary root tip (0.3-0.8 mm from the apex) were analyzed using secondary ion mass spectrometry (SIMS). The high sensitivity of SIMS for Al permitted the first direct evidence of early entry of Al into root cells. Al was found in cells of the root tip after a 30-min exposure of intact roots to 38 [mu]M Al3+. The accumulation of Al was greatest in the first 30 [mu]m, i.e. two to three cell layers, but elevated Al levels extended at least 150 [mu]m inward from the root edge. Intracellular Al concentrations at the root periphery were estimated to be about 70 nmol g-1 fresh weight. After 18 h of exposure, Al was evident throughout the root cross-section, although the rate of accumulation had slowed considerably from that during the initial 30 min. These results are consistent with the hypothesis that early effects of Al toxicity at the root apex, such as those on cell division, cell extension, or nutrient transport, involve the direct intervention of Al on cell function

Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 μmol g (root FW)−1 h−1], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only ∼50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional ‘leak-pump’ model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes

The rhizotoxicity of metal cations is related to their strength of binding to hard ligands

Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognized as rhizotoxic for approximately 100 yr, but there is no consensus on its mode of action. The authors contend that the primary mechanism of rhizotoxicity of many metal cations is nonspecific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands, especially carboxylate ligands of the cell-wall pectic matrix. Specifically, the authors propose that metal cations have a common toxic mechanism through inhibiting the controlled relaxation of the cell wall as required for elongation. Metal cations such as Al3+ and Hg2+, which bind strongly to hard ligands, are toxic at relatively low concentrations because they bind strongly to the walls of cells in the rhizodermis and outer cortex of the root elongation zone with little movement into the inner tissues. In contrast, metal cations such as Ca2+, Na+, Mn2+, and Zn2+, which bind weakly to hard ligands, bind only weakly to the cell wall and move farther into the root cylinder. Only at high concentrations is their weak binding sufficient to inhibit the relaxation of the cell wall. Finally, different mechanisms would explain why certain metal cations (for example, Tl+, Ag+, Cs+, and Cu2+) are sometimes more toxic than expected through binding to hard ligands. The data presented in the present study demonstrate the importance of strength of binding to hard ligands in influencing a range of important physiological processes within roots through nonspecific mechanisms