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An understanding of how environmental factors can alter accumulation of nutrients by plants is needed to anticipate and prevent mineral deficiencies in both plants and grazing animals. To determine the effects of root zone temperature (RZT) and Ca level on mineral concentrations of winter wheat (Triticum aestivum L.), 'Centurk' seedlings were grown under three RZT regimes (constant 8°C, constant 16°C, and transferral from 8°C after 23 d to 16°C) and four Ca levels (0.2, 0.6, 2.0, and 5.0 mM) in nutrient solution. Plants grown at 8°C RZT had significantly lower shoot and root concentrations and unit absorption rates of P, S, Cu, Zn, and Mn than did those grown at 16°C. Within 2 wk after transfer from 8°C to 16°C RZT, concentrations of P. S, Cu, Zn, and Mn in shoots and roots increased significantly. Increasing Ca levels in solution significantly increased shoot and root concentrations and unit absorption rates of P and Cu. In contrast, increasing Ca levels significantly decreased concentrations and unit absorption rates of Mn and Zn. Thus, a suhoptimal RZT could depress accumulation of P, S, Cu, Zn, and Mn by winter wheat forage, and possihly induce mineral deficiencies in both plants and grazing animals. A high solution Ca level could partially ameliorate this adverse effect on P and Cu accumulation by wheat, but it also could exacerbate the problem of low Mn and Zn concentrations at a cool RZT.

Leaf composition of the bronze brz) mutant Pea [Pisum sativum L. cv. Sparkle E107 (brz brz)] and its Parent genotype (cv-Sparkle) have been compared when grown in nutrient culture, but not when grown in soil where pH has been adjusted to reduce readily available Fe and Mn. Both genotypes were grown in a growth chamber in five soils that differed in pH and DTPA-extractable Fe and Mn. The brz genotype accumulated very high, and often toxic, concentrations of Fe and Mn when grown in acidic soils, while Sparkle did not. Liming reduced leaf-Fe and leaf-Mn concentrations in the brz mutant, but Fe toxicity symptoms were still evident in older leaves. In the acidic soils, leaves of the brz plants also accumulated significantly more Zn, Cu, Ca, Mg, K, Na, Al, and P than did leaves of Sparkle plants. Roots of the brz genotype depressed soil pH by several tenths of a pH unit more than did roots of Sparkle. In contrast to result obtained in the acidic and near-neutral soils, the brz mutant and its parent grew similarly in a calcareous soil (pH 8.2), which contained low concentrations of DTPA-extractable Fe and Mn. In conclusion, the brz mutant Pea consistently accumulates high levels of Fe and Mn when these metal ions are present in readily available forms. Liming an acid soil to near-neutral pH did not prohibit the brz mutant from accumulating potentially toxic concentrations of Fe, but growing the mutant in a calcareous soil did

We report on the identification of a voltage-dependent Ca2+ transport system that mediates Ca2+ influx across the plasma membrane (PM) of wheat (Triticum aestivum) root cells. The experimental approach involved the imposition of transmembrane electrical potentials (via K+ diffusion potentials) in populations of purified, right-side-out PM vesicles isolated from wheat roots. Using 45Ca2+ to quantify Ca2+ influx into the PM vesicles, the voltage-dependent characteristics of Ca2+ transport were found to be similar to those exhibited by L-type voltage-gated Ca2+ channels in animal cells. The putative PM Ca2+ channel opened upon depolarization of the membrane potential, and Ca2+ flux increased to a maximum upon further depolarization and then decreased back to zero upon further successive depolarizations. This channel was found to be selective for Ca2+ over Mg2+, Sr2+, K+, and Na+; was blocked by very low concentrations of La3+; was unaffected by high concentrations of the K+ channel blocker tetraethylammonium; and exhibited Michaelis-Menten-type transport kinetics. Based on these transport properties, we argue that this transport system is a PM Ca2+ channel. We suggest that the use of radiotracer flux analysis of voltage-clamped PM vesicles derived from plant roots is a straightforward approach for the characterization of certain voltage-gated ion channels functioning in cellular membranes of higher plant cells.

The effects of Al on Ca transport could influence Ca accumulation in plants. Low forage Ca concentrations may cause hypocalcemia, a Ca deficiency syndrome of ruminants. Solution culture experiments were conducted in a growth chamber to examine the effects of Al (5-50 μM) on uptake and translocation of Ca ( 45 Ca) in winter wheat (Triticum aestivum L.) cultivars (Al-tolerant Atlas 66 and Al-sensitive Scout 66). Three-day-old seedlings, grown either in a CaCl 2 solution (0.1 or 1.0 mM) or in a complete-nutrient solution, were used. For plants grown in CaCl 2 solution, addition of 5 μM Al significantly decreased Ca uptake and translocation in Scout 66, but not in Atlas 66. Higher levels of Al (20 and 50 μM decreased Ca uptake and translocation much more in Scout 66 than in Atlas 66 [...]

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.

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.

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.

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

A method for determining organic acid levels in biological materials by silica gel partition column chromatography (pcc) is described. It uses tetrabromophenolphthalein ethyl ester, which has E. high molar absorbency, for quantitating the acids in the column effluent. The concentrations of several organic acids were determined in two species of crested wheatgrass, Nordan (Agropyron ciesertorum) and Fairway (A. cristatum), grown. under different conditions of K fertilization. For comparison, aconitic, citric, and malic acids were also determined by polarography, spectrophotometry, and fluorometry, respectively. Results obtained by these methods agreed with the data obtained by pcc. The concentration of trans-aconitic acid averaged about 96 and 21 mequiv/kg of dry matter in Nordan and Fairway, respectively, when grown in the greenhouse without K. K fertilization (312 kg/ha) approximately doubled the trans-aconitic acid concentration in both species. Fairway contained 363 mequiv/kg of malic acid, while Nordan contained 280 mequiv/kg. K fertilization increased these concentrations to 611 and 446 mequiv/kg, respectively. Citric acid was increased by K fertilization, but there was no significant difference between species and in no case was the citric acid concentration greater than 80 mequiv/kg of dry matter.