Evaluation of soil EDTA applications on crop performance and uptake of macro- and micronutrients by agricultural crops

EDTA Applikation zum Boden beeinflusst Pflanzenwachstum und Aufnahme von Haupt- und Spurenelementen

2 Material and methods

The different plant species showed strong differences in their sensitivity towards EDTA: sunflower was the most sensitive crop and reacted already to a dose of 0.5 g pot^–1 with a biomass reduction of more than 50% and necrotic lesions on the leaves (Table 1, Fig. 1). At 1.7 g pot^–1 EDTA an elevated number of necrotic lesions on leaves was counted and at the highest dose of 3.3 g pot^–1 EDTA plants almost died off (Fig. 2). At the highest EDTA dose the biomass of sunflower was reduced by 90% (Fig. 1).

Fig. 1. Relative biomass development (control plants = 100%) of shoots and roots of maize, oilseed rape and sunflowers in relation to EDTA rates.

Fig. 2. Influence of soil EDTA application on growth of maize, oilseed rape and sunflower.

Necrotic lesions could be detected also on older leaves of oilseed rape when 1.7 and 3.3 g pot^–1 EDTA were applied. Shoot biomass of oilseed rape was significantly reduced by 43% at 1.7 g pot^–1 EDTA (Table 1) in comparison to the control (Fig. 1). Maize was least sensitive against EDTA and no visible symptoms were observed (Fig. 2), but shoot biomass was still reduced at the highest EDTA dose by up to 32% and root biomass by 57% compared to the control (Fig. 1).

Visible necrotic lesions were only observed in the dico­tyledonous crops oilseed rape and sunflower. Lesions first appeared on older leaves but also stems showed toxicity symptoms after EDTA application (Fig. 3). Oilseed rape plants regularly lose older leaves during main growth. Plants that were affected by EDTA lost more leaves when compared to the control plants.

Fig. 3. Symptoms of EDTA to­xicity on leaves of oil­seed rape (A) and sunflower (B), and stems of sunflower (C).

Root growth of all crops was stronger reduced by EDTA than that of shoots and the effect was dose dependent (Fig. 1). Root biomass was reduced by 57%, 76%, and 82% in maize, oilseed rape and sunflower when compared to the control by the highest EDTA rate (Fig. 1).

EDTA negatively affected shoot and root growth of oilseed rape, maize and sunflower. Microelements such as Fe and Mn form EDTA complexes and by this their availability and plant uptake is significantly increased (Vassil et al., 1998). In the presented experiment the supply with essential plant nutrients was sufficiently high for maximum growth. Expectedly EDTA influenced the uptake of minerals. The effect of graded doses of EDTA on the uptake of macro- and micronutrients is summarized in Table 2 for maize and oilseed rape and in Table 3 for sunflower. The data in Table 2 and 3 reveal that all crops were sufficiently supplied with macro- and micronutrients (Reuter and Robinson, 1997).

The plant species were affected differently by EDTA appli­cation. EDTA application increased the uptake of all macronutrients in oilseed rape. In case of maize EDTA yielded a higher K, P and S uptake, but reduced that of Ca. With a view to sunflowers EDTA increase the K uptake in leaves, while that of Ca and Mg decreased. In stems a higher K, P and S content was found and in roots a higher Ca, P and S concentration (Table 2). All of these effects proved to be significant.

Not only macro-, but also patterns of micronutrient concentrations varied between the tested crop plants. EDTA increased the micronutrient concentration in shoots and roots of maize. A similar effect was observed in shoots of oilseed rape, but in roots EDTA caused a significant decrease of the Fe, Mn and Mo content (Table 2). With reference to sunflower the Mn content increased significantly in roots, stems and leaves, the Cu and Zn in leaves and the Fe content in stems (Table 3). EDTA significantly enhanced the Mn and Zn uptake of all three plant species.

EDTA application increased the uptake of several macro- and micronutrients (Table 2, 3), but it negatively influenced biomass production (Table 1). The off-take of macro- and micronutrients with harvest products reflects changes in nutrient uptake and crop growth in relation to EDTA rates. In Table 4 values for the mean nutrient off-take per pot by shoots are shown for all three crop plants in relation to EDTA rates.

The nutrient off-take of most macro- and microelements decreased with EDTA application because of its negative effect on growth. Only in case of maize where EDTA had only minor effects on plant growth the Fe, Mn and B off-take increased together with the EDTA dose. The concentration of these nutrients increased over-proportionally and strongest in relation to the EDTA rate compared to Cu, Zn and Mo (Table 2). Striking is that the B and Mn off-take of all crops exceeded consistently the supply with the nutrient solution irrespective of the EDTA treatment. For both elements the nutrient concentration is in the range of the upper limit for an optimum supply. This suggests that plants mobilized B and Mn from impurities of the sand that were not washed off by water and EDTA apparently reinforces this effect.

4 Discussions

Oilseed rape was less sensitive than sunflower but considerably more susceptible than maize. Oilseed rape showed necrotic lesions on older leaves when ≥ 1.7 g pot^–1 EDTA was applied to the soil. In case of maize the highest dose of 3.3 g pot^–1 EDTA reduced shoot and root biomass significantly but no visual toxicity symptoms appeared. Shen et al. (2002) determined a different response of plant species (cabbage, mung bean, and wheat) to EDTA application in a pot trial on a soil contaminated with Pb. Cabbage reacted most sensitive with a biomass reduction of 38% in comparison to 11% of mung beans and 27% of wheat. Chen et al. (2004) determined differences in the sensitivity of dicotyledonous and monocotyledonous species to a treatment with 5 mmol EDTA kg^–1 soil. On average the shoot dry weight of the monocotyledonous species was less affected by the EDTA application. Rengel (1999) found adverse effects on the growth of wheat at an H-EDTA concentration of 50 μM in hydroponics while no toxicity symptoms were observed at concentrations ≤ 25 μM. He assumed that toxic effects are induced by direct toxicity of the chelators caused by interferences with molecular structures and functions at higher H-EDTA concentrations.

In the presented experiment it was most likely this direct toxicity of EDTA that caused the strong negative impact on plant performance as no critical heavy metal concentrations existed in the growth medium. EDTA was applied in its free acidic form and it was shown by Vassil et al. (1998) that free H-EDTA is more phytotoxic than metal-bound EDTA, and that it caused stronger growth reductions. The plant available heavy metal fraction is often low in agricultural soils due to the strong association with organic matter, Fe-Mn-oxides, clay minerals or precipitations as carbonates, hydroxides and phosphates (McBride, 1994). EDTA can enhance the mobility of such heavy metals and thus cause its accumulation in shoots. Toxicity symptoms like the formation of necrotic lesions is accompanied by a significant loss of water from shoot tissue (Vassil et al., 1998) and may be attributed to the presence of free protonated EDTA which can bind to various essential divalent cations, disrupting the biochemistry of the cells and ultimately causing cell death. It was speculated by Vassil et al. (1998) that EDTA-metal complexes were taken up by plants when a threshold concentration of EDTA is reached above which the physiological barrier(s) in roots are destroyed that usually function to control uptake and translocation of solutes. The sensitivity of different plant species against EDTA can be explained probably by differences in their physiological barriers: in case of the most sensitive crop of this research, sunflower, biomass development of roots and vegetative parts were similarly affected by EDTA while in case of more resistant crops the root biomass was stronger affected than that of the vegetative parts (Fig. 1) indicating more efficient barriers.

It was shown in different studies that application of chelates to the soil did not only increase the total dissolved metal concentration but did also change the primary route of metal uptake from the symplastic to the apoplastic pathway as damage of physiological barriers in plant roots can cause a rapid uptake from soil solution via the xylem into the shoots (Chen et al., 2007; Luo et al., 2006; Nowack et al., 2006; Wenger et al., 2005). Wenger et al. (2005) supposed that a high-affinity transport system exists together with a low-affinity system for the uptake of metals and the proportions of these transport pathways can be changed by the availability of ligands. As chelatation and sequestration in vacuoles are part of the natural plant defense mechanisms and metals are not bioavailable in this form, it is possible that plants do not perceive metals bound in this form and will not activate feed-back inhibition to reduce further uptake. Collins et al. (2002) concluded from their studies that for Zn-EDTA species specific uptake routes exist: while Hordeum vulgare and Solanum tuberosum took up Zn-EDTA via the apoplastic pathway, Brassica juncea was only able to take up Zn-EDTA after physiological damage. In another study with two different genotypes of sunflower no effect of EDTA in the nutrient solution (0.1 mm EDTA) on metal uptake (Pb) or even plant growth was detected (Doncheva et al., 2013).

All previously mentioned studies demonstrate that besides metal and EDTA concentration there are species-dependent differences in metal uptake in relation to EDTA. A comparison of the existing studies suggests that experimental conditions seem to influence kind and strength of observed effects.

In the present pot experiment a significant increase of the Mn uptake took place in all three plant species which resulted in tissue concentrations that can be regarded as high (Humphries et al., 2007). Obviously Mn was mobilized from the substrate by EDTA application and accumulated in the plant tissue. This effect was most pronounced in case of sunflower when 1.7 g pot^–1 EDTA was applied. Then the Mn concentration increased in roots by factor 9 and in vegetative parts by factor 1.6–4.5 in comparison to the control. For the same treatment the Mn content increased by factor 4 in roots and by factor 1.9 in shoots of maize. Only in oilseed rape an opposite effect was determined. Here, the highest Mn content was determined in the roots of the control plants. In the shoots of oilseed rape the Mn concentration increased twofold.

Based on the analyzed data a translocation concentration factor (TCF) can be calculated which equals the shoot to root concentration of an element (De La Rosa et al., 2007). The TCF delivers information about the translocation of minerals from roots to shoots. Values < 1 indicate a higher accumulation of an element in the roots, while values > 1 reflect an accumulation in shoots. In case of oilseed rape the TCF for Mn increased from 0.8 (control) to 2.1 (highest EDTA dose). In contrast, in maize and sunflower the TCF proved to be consistently < 1 irrespective of the EDTA dose for Mn. These data show that the response of plants to graded doses of EDTA is species-specific.

Mn and Zn were the only metals that showed a significant response to EDTA in all crop plants. The Cu concentration boosted with graded EDTA rates in leaves and stems of sunflowers while this effect was not observed for maize and oilseed rape. A similar effect was found for the Mo concentration in the shoots of maize. A heterogeneous effect was also established for Fe: the concentration was elevated in shoots of maize and leaves and stems of sunflower, but reduced in roots of oilseed rape.

The presented experiment revealed that graded doses of soil-applied EDTA increased the micronutrient concentration in maize, sunflower and oilseed rape. At the same time the off-take of micronutrients declined because of the negative impact on plant growth (Table 4). This strong growth effect is restricted to pot experiments, in particular those in hydroponics and sand culture because of an unnaturally high root density and lack of buffer capacity of natural soils. Accordingly, it was shown in a field experi­ment that EDTA application rates similar to that in the presented pot trial had only a slight negative impact on plant growth because of the interaction of EDTA with the soil matrix and the translocation of EDTA within the soil profile (Bloem et al., 2016). The most distinctive effect found in the current study is that EDTA significantly increased Mn and Zn concentrations of shoots in all three crops. Hereby mobilization of Mn and Zn seems to be fortified strongly by EDTA (see above).