Effects of High Boron on the Nutrients Uptake of Aegilops Genotypes Differing in Their B Tolerance Level

: In this study, we have used 19 Aegilops genotypes differing in their boron (B) toxicity tolerance level along with a B toxicity tolerant cultivar, Bolal and estimated their root-shoot nutrient concentrations under B toxic growth condition. Further, the association between their root-shoot nutrient concentrations and level of B toxicity tolerance was evaluated. The experimental genotypes were grown under three different B growth conditions in hydroponic system including Control (3.1 μM B); toxic (1 mM B), and highly toxic (10 mM B) boron treatment. T he macro and micronutrient concentrations in the roots and shoots of the genotypes showed large variations and were observed to be differentially affected by high B stress.


Introduction
Wheat wild relatives are considered as an important source of tolerance to several abiotic stress conditions [1][2][3]. Their genetic diversity can be effectively utilized to develop breeding lines and modern wheat cultivars with greater stress tolerances. Despite their great potential, Aegilops species have not been completely explored for such tolerances and mechanisms involved in making them tolerant. Boron (B) toxicity is one of the crucial abiotic stress conditions that negatively affect the wheat productivity in arid and semi-arid regions of the world. Other than several symptoms, high B is known to impede the uptake and translocation of macro and micronutrients in plants [4]. Thus, the hypothesis is that the plants showing less effect of B toxic growth conditions on nutrients uptake and translocation can be more tolerant to B toxicity. To test this hypothesis, the elemental analysis of root and shoot samples of 19 Aegilops genotypes ( Figure 1) grown under different B concentrations in hydroponic system was done. The aim of this study was to determine the effect of high B on the macro and micronutrient content in roots and shoots of experimental genotypes. The results may provide an answer to the question that whether the accumulation of different nutrients can be associated with the B toxicity tolerance of the Aegilops genotypes. Further, research can be conducted to see the effect of additional supply of those nutrients towards increasing the tolerance level of less tolerant genotypes.

Materials and Methods
The study comprised of 19 Aegilops genotypes supplied by the Turkish Seed Gene Bank (TSGB), Ankara, and AARI National Gene Bank, Izmir, Turkey and a well-known B toxicity tolerant Turkish hexaploid cultivar Bolal 2973 [5]. The B tolerance level of these genotypes has already been identified in one of our previous studies [3].
The genotypes were grown in a hydroponic system with conditions adjusted to 22 ± 10 °C, 16/8 h light/dark photoperiod, 16,000 Lx/day light intensity and 45-55% humidity. Three replicates of all the genotypes with five plants each were grown in three different B concentrations including Control (1/5th Hoagland solution containing 3.1 µ M B), toxic B (1 mM B), and highly toxic B (10 mM B)]. The plants were treated with different B concentrations for 7 days followed by the harvest of root and shoot samples. After washing with 0.1 N HCl solution and deionized water, the harvested samples were kept in oven at 70 °C for drying. This was followed by the crushing of the dried samples and dissolving 0.15-0.20 g of the powdered sample in 5 mL of 65% HNO3 and 2 mL of 35% H2O2. The mixture was digested in a closed microwave accelerating reaction system (CemMarsxpress, Matthews, NC, USA) and further, ICP-AES (Varian, Vista, Palo Alto, CA, USA) was employed to identify the nutrient concentration in the stock solution [6]. The percentage changes in each of the measured macro and micronutrient under 1 mM and 10 mM B treatment as compared to Control were estimated using MS Excel 2010.

Results
The purpose of this experiment was to determine the variation in 19 Aegilops genotypes in terms of nutrient accumulation in roots and shoots under different B toxic growth environment. Moreover, it was aimed to identify that whether the accumulation of nutrients is associated with the tolerance level of experimental genotypes that was already identified by Khan et al. [3].

Calcium (Ca)
The percentage change in shoot Ca (SCa) accumulation under highly toxic B treatment as compared to Control ranged from -145% to 38%. While At3 and At4 showed 145% and 71% decrement in the SCa content under a highly toxic B condition as compared to Control respectively, Ac4 and Bolal revealed the highest increase (38%) in SCa content as compared to Control (Table 1).
In root Ca (RCa) accumulation, the percentage change in 10 mM B treatment ranged from -287% to 60%. A maximum reduction (287%) was observed in Ac2 followed by Ac1 (251%). However, the Aegilops accession, At1 (60%), and Al1 (56%), showed a maximum increase in RCa content under 10 mM B treatment as compared to Control.

Phosphorus (P)
The percentage change in shoot P (SP) accumulation under highly toxic B treatment as compared to Control ranged from −181% to 50%. While At3 and As2 showed 181% and 170% decrement in the SP content under a highly toxic B condition as compared to Control respectively, Ac4 and Au2 revealed the highest increase of 50% and 35% respectively in SP content as compared to Control (Table 1).
In root P (RP) accumulation, the percentage change in 10 mM B treatment ranged from −457% to 100%. A maximum reduction (457%) was observed in As2 followed by Ac2 (313%). However, the Aegilops accession, Ab2 (100%), and Ab3 (48%), showed a maximum increase in RP content under 10 mM B treatment as compared to Control.

Sodium (Na)
The percentage change in shoot Na (SNa) accumulation under highly toxic B treatment as compared to Control ranged from −47% to 76%. While As2 showed 47% decrement in the SNa content under a highly toxic B condition as compared to Control, Au2 and Au3 revealed the highest increase of 76% and 71% respectively in SNa content as compared to Control (Table 1).
In root Na (RNa) accumulation, the percentage change in 10 mM B treatment ranged from −236% to 78%. A maximum reduction (236%) was observed in As2 followed by Ac1 (212%). However, the Aegilops accession, Ab3 (78%), and Ac4 (58%), showed a maximum increase in RNa content under 10 mM B treatment as compared to Control.

Magnesium (Mg)
The percentage change in shoot Mg (SMg) accumulation under highly toxic B treatment as compared to Control ranged from −146% to 26%. While At3 and As2 showed 146% and 117% decrement in the SMg content under a highly toxic B condition as compared to Control respectively, Ac4 and Bolal 2973 revealed the highest increase of 26% and 24% respectively in SMg content as compared to Control (Table 1).

Manganese (Mn)
The percentage change in shoot Mn (SMn) accumulation under highly toxic B treatment as compared to Control ranged from −350% to 40%. While At3 and As2 showed 350% and 213% decrement in the SMn content under a highly toxic B condition as com-pared to Control respectively, Ac4 and Bolal 2973 revealed the highest increase of 40% and 5% respectively in SMn content as compared to Control (Table 1).

Copper (Cu)
The percentage change in shoot Cu (SCu) accumulation under highly toxic B treatment as compared to Control ranged from −229% to 37%. While Ac3 and Ab3 showed 229% and 192% decrement in the SCu content under a highly toxic B condition as compared to Control respectively, Au2 and At2 revealed the highest increase of 37% and 31% respectively in SCu content as compared to Control (Table 1).

Discussion
A large variability was observed in the root-shoot nutrient content of the studied Aegilops genotypes under B toxic stress condition. The two Aegilops genotypes, Ab2 and Ac4 that were found to be tolerant in our previous study [3] did not show maximum decrement in the uptake and translocation of any of the studied nutrients. In fact, among the two genotypes, Ac4 showed maximum concentration of several nutrients in shoots including Ca, P, Mg and Mn. However, no clear association has been found between the tolerance level of the genotypes and uptake or accumulation of any specific nutrient. The large folds of increment and decrement in the nutrient uptake of different genotypes showed the greater influence of high B stress on the ion equilibrium of plants. The results were in accordance with the results of several previous studies where abiotic stress is known to be affecting nutrient content in roots and shoots [7,8].