Nutrient solutions, plant-induced

Abstract Cadmium is an important poiiutant in the environment, toxic to most organisms and a potential threat to human heaith Crops and other plants take up Cd from the soil or water and may enrich it in their roots and shoots. In this review, we suimnarize natural and anthropogenic reasons for the occurrence of Cd toxicity, and evaluate the observed phytotoxic effects of plants growing in Cd-supplemented sou or nutrient solution. Cd-induced effects include oxidative stress, genotoxicity, inhibition of the photosynthetic apparatus, and inhibition of root metabolism. We explain proposed and possible interactimis between these modes of toxicity. WhUe discussing recent and older studies, we further emphasize the environmental relevance of the experiments and the physiological response of the plant. 

Figure 2. Plant-induced pH changes of nutrient solutions caused by differential iron-stress response of 25-day old T3238fer...

Figure 3. Plant-induced pH changes of nutrient solutions (top) and release of reductant Fe3+ to Fe2+ (bottom) caused by differential iron stress response of 25-day old iron-stressed T3238fer and T3238FER tomato plants placed in nuirient solutions lacking iron and containing NH -N and N03-N...

Iron-efficient and iron-inefficient plants can have several hundred /xgFe/g of root, but the iron-inefficient plant may die from lack of iron in its tops. In contrast, iron-efficient plants respond to iron stress, and the root makes iron available for transport and use in tops. In a similar way, iron may remain in the nutrient solution as Fe3+ chelate or Fe3+ phosphate and not be transported to the plant top until it is made available for transport through chemical reactions induced by iron stress. These observations stress the importance of a plant being able to respond to iron stress. Iron is usually used in plant tops once it is made available for transport by the roots. 

Iodine has not been shown to be essential to plants, and stimulatory effects on plant growth at low levels have not been reported. Mengel and Kirkby (1978) reported that a stimulatory effect of iodine was observed at 100 pg I L in nutrient solutions, whereas toxic effects in plants occurred at an iodine concentration of 500-1000 pg L b As the toxic concentration is higher than the soluble iodine content of soils, iodine toxicity is rare in plants under natural field conditions. However, a physiological disease of rice plants named Akagare has been reported (Yuita 1979), induced by excessive absorption of iodine from soil enriched with easily soluble iodine when land was converted for submerged paddy fields (Kabata-Pendias and Pendias 1992). 

After 5 weeks of growth at 16 h/8 h, 30°C/18°C, polyethylene glycol (PEG) and NaCl were tested for induction of CAM in M. crystallinum. As shown in Fig. 1, ABA added to the nutrient solution was also very effective in inducing increased activities of PEPC and NADP-ME. Six days after treatment, the diurnal fluctuations in titratable acidity and malate content were similar in the abscisic acid treated and the salt or PEG treated plants. SDS-PAGE of the soluble protein from leaf extracts showed the appearance of a major polypeptide band at approximately 100 kD with ABA, salt and PEG treatment. This band corresponds to PEPC and is consistent with the large increase in activity observed after 6 days. 

Plants can also balance their uptake of different nutrients through their production of enzymes and other compounds that help to make specific nutrients more available. Nitrate reductase is required to assimilate NO3 into plant biomass, and its production is triggered by the presence of NO3 in soil solution. Phosphorus limitation induces production of root phosphatase enzymes that cleave organically bound PO4, or side-rophores, which solubihze mineral phosphorus by chelating with other minerals that bind to PO4, such as iron. 

Figures 1 and 2 show relationships among concentrations of U and selected major and trace elements in spinach leaves and petioles, respectively. It is noteworthy that concentrations of U in spinach were significantly positively correlated (p<0.01) with concentrations of Fe and A1 in both leaves and petioles. These relationships suggested that the absorption and transport processes of U in spinach could be related to those of Fe and Al, as was also suggested by Kametani et al. who showed that plants with higher Fe concentrations tended to absorb more U. Less U was extracted by 1 mol L ammonium acetate solution from soil (Table 2), meaning that U in soil was less available to plants. Spinach favours neutral-to-weak alkaline conditions and has the ability to acquire insoluble mineral nutrients such as Fe under neutral-to-alkaline conditions. Helal et al. compared spinach and beans with respect to the ability of the root to uptake Fe and found that spinach root absorbed Fe more efficiently. The differences in Cu, Zn, and Cd uptake by two spinach cultivars were attributed to different abilities to exude oxalate, citrate, and malate from root l The application of organic acids to soil facilitated the phytoextraction of U by hyperaccumulator plants thus, those root exudates could induce U dissolution from soil. Since part of U is associated with Fe and Al minerals in the soil it was likely that the absorption of U was accompanied by Fe and Al absorption, possibly triggered by the secretion of protons or organic acids to solubilise Fe and Al from soil.

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