Nutrient stress uptake

Overall the results reported in this review indicate that water scarcity might increase metal exposure (due to low dilution), metal uptake (due to higher retention under low flow), and metal toxicity and/or accumulation (depending on the dose and time of exposure), but also might cause opposite effects depending on the source of pollution. In addition, water scarcity will influence nutrient loads and will also modulate the fate and effects of metals. Thus, future studies addressing the role of environmental stress on the effects of toxicants at community scale are key to predict the impact of toxicants in the aquatic ecosystems. 

Shone, M.G.T. Flood, A.V. (1983). Effects of periods of localised water stress on subsequent nutrient uptake by barley roots and their adaptation by osmotic adjustment. New Phytologist, 94, 561-72. 

It should be stressed that PROFILE needs the nutrient uptake limited to PROFlLE-acceptable layer (0.5-1 m depth) for simplicity, whereas the real nutrient uptake takes place down to the 3-5-7-10 m depth corresponding to the distribution of tree roots. So, the nutrient uptake in a PROFlLE-acceptable layer is always less than the whole nutrient uptake. This might be a source of uncertainty in critical load calculations. 

Prolonged exposure to acid rain causes forest soils to lose valuable nutrients. It also increases the concentration of aluminum in the soil, which interferes with the uptake of nutrients by the trees. Lack of nutrients causes trees to grow more slowly or to stop growing altogether. More visible damage, such as defoliation, may show up later. Trees exposed to acid rain may also have more difficulty withstanding other stresses, such as drought, disease, insect pests and cold weather. 

Several roles of endophytic fungi for the host plant have been postulated. These include acting to increase access to mineral nutrients (a mycorrhizal function), to increase access to organic soil N, P and C, to increase drought and stress tolerance, to improve water uptake, protection from herbivory (mammals, insects), and for protection from plant pathogenic fungi, bacteria, nematodes, and other parasites. We should not be surprised that endophytic fungi are such common plant symbioses. 

Both salinity and sodicity impair plant growth and reduce agricultural yields. In severe cases, they can cause complete crop failure (Qadir et al. 2000). Plants on saline areas suffer from lack of water, because salts bind water in the soil and thus make it inaccessible to plants and microorganisms. Beside this osmotic stress situation, an excess of specific ions, such as sodium, is toxic to plants or results in an ion imbalance, which complicates the uptake of specific nutrients (Marschner 1995). Some crops have developed tolerance to salty conditions and can thus be used to a certain extent on soils affected by salinity. 

Further, compartmentalization allows cells to control the import of nutrients and the export of products that manipulate the environment, or excrete waste. These include siderophores that enhance uptake of metal ions, quorum-sensing molecules that inform cells of the presence of others of their kind, molecules that form biofilms that allow attachment to surfaces and protection from mechanical and chemical stress, and toxins that inhibit the growth of competitors. 

Iron uptake by iron-inefficient soybeans was not increased when they were placed in nutrient solutions that contained reductant (14). This may mean that reductants in the external solution indicate a leaky root resulting from the release of hydrogen into the nutrient solution. More important may be the adaptive production of reductants inside the root or at the root surface that keeps iron in the more available Fe2+ form (13). We have concluded that iron absorption and transport is controlled inside the root, and iron uptake is greatest while the iron-stress—response mechanism is functioning. 

Soil anaerobiosis also affects plant nutrient uptake in wetland environments. Anaerobiosis in the rhizosphere, a dominant factor in wetland areas, causes physiological stresses that can limit active nptake of essential elements snch as nitrogen. The nitrogen status, in turn, can affect photosynthetic activity in plants. In addition to effects on plant growth, both intensity and capacity of reduction... 

AM fungi contribute to plant growth and plant nutrition as their extraradical hyphae increase the volume of the explored soil. These extraradical hyphae can extend up to more than 10 cm from the roots (Harley and Smith, 1983) and make up a hyphal density of 1-30 m g soil (Smith and Read, 1997). Therefore, AM roots can more easily access soil nutrients localized outside the root depletion zone, especially the nonmobile elements such as P and Zn. Mycorrhizas also improve plant water uptake, stress tolerance, and affect the microbial community structure (loner et al, 2001). In polluted soils, mycorrhizal plants also benefit from the presence of the AM fungi. 

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