Introduction

At present, more than 74% of soils with agricultural importance show salinity problems worldwide, of which 33% are under production^1,2. Around its problem, 3 arable soil ha are being destroyed every minute^3,4 and approximately 1.5 million ha are lost per year, reducing agricultural productivity by around 11 billion dollars^5,6.

Salinity in soils is caused by the accumulation of mineral salts or waters in the form of cationic and anionic electrolytes⁷, it can be of natural origin or by anthropogenic actions^8-13, and affect soils physic-cochemical properties, which effect adverse in ecological balance¹⁴.

In soil solution, the salts exert on the plant nutrients due to the excessive accumulation of dominant sodium (Na⁺) or chloride (Cl^-) ions^15-18, the impact negative is a limit in agricultural soils and forage production, mainly in arid and semi-arid regions^19-22.

High concentrations of salt cause ionic toxicity in plants, water stress, oxidative stress, nutritional disorder, and disorders at the cell membrane^23-25, in addition, reduction in cell development, alterations in metabolic processes and a decrease in the water availability^26-31.

Tolerance or resistance can be defined as the ability of a plant to withstand salinity in the edaphic solution without manifesting adverse effects on development^32,33. Plants have two mechanisms to resist abiotic factors, such as evasion and tolerance, the first consists in avoiding the accumulation of salts and the second consists in the ability not to lose their productive capacity at a determined salinity level^34-39.

The soils of lower basin of the Lauca River, where is the Uru Chipaya indigenous territory. The soils suffer problems by accumulation sodium (Na) (PSI of 27 to 138% and pH> 8). The salts because cause dispersion of organic matter (OM) and clays affecting plant cover and microbial activity⁴⁰.

Soil salinity is a significant environmental damage that limits the agroecological potential and represents a considerable socio-economic obstacle for the sustainable development of the different forage species. The objective of this study was to evaluate the influence of salinity on the development of forage species between sowing and transplanting techniques.

Materials and methods

Geographic location. The research was carried out 204 km from the city of Oruro-Bolivia, in the lower basin of the Lauca River in the Uru Chipaya territory, located to the South-West of the Oruro region, North of the Salar of Coipasa and the South of the aquatic axis from Titicaca Lake, Desaguadero River and Poopó Lake and at altitude 3640 m above sea level. Geographically located between the coordinates 19°02'17.40" Latitude South and 68°05' 16.05" Longitude West. 10.4 °C annual mean temperature, maximum 27.2 °C and -18 °C minimum. Average annual precipitation 200 to 4000 mm and 70% annual average humidity relative^40-42.

Soil characteristics. The soils samples were analyzed in Spectrolab Soil and Water Laboratory, Faculty Ciencias Agrarias and Naturales, Technical University of Oruro. The analyzes showed sandy-loamy texture with important Na contents, pH> 8.3 and 2.2 to 2.7% OM content.

The electrical conductivity was 7 to 22 dS m^-1 that expresses a high concentration of Na. Nitrogen (N) less than 0.05%, phosphorus (P) between 0.06 to 0.17 meq 100 g^-1, potassium (K) from 0.50 to 0.80 meq 100 g^-1 and cation exchange capacity (CEC) between 20.49 to 30.69. The reports affirm that the lower basin of the Lauca River in Uru Chipaya territory contains high levels of PSI because it is located close to the Salar of Coipasa⁴⁰.

Seed. The seeds used in test, were from obtained of semi-arid zones from the department of Oruro and commercial seed. The six forage species evaluated were: cola of ratón (Hordeum muticum J. Presl), INTA cebadilla (Bromus sp.), native cebadilla (Bromus catharticus Vahl.), festuca alta (Festuca arundinacea Schreber), alkar (Agropyron elongatum (Host) P. Beauv.) and pasto llorón (Eragrostis curvula (Schrad.) Nees).

Soil preparation. 6000 m² area (500 m² for each specie) of soil has prepared (removed) in each Ayllu (Aransaya, Manasaya and Wistrullani) with a disc plow.

Sow. Each species was sown with a density tripled by handballing. The seeds were passed with the harrow for bury the seed^43,44. This activity was carried out in December 2018.

Seedling production. In the nursery, the 12 x 18 cm polyethylene bags were filled with salinity-free substrate with a proportion 3:1 soil and manure. Each species was sown with 5 to 8 seeds per bag, then it was watered with tap water for 60 days until the seedlings develop 8 to 10 cm in height.

Transplant. In January 2019, the seedlings were definitively transplanted to saline soils in 500 m² at a density of 20 cm between rows and columns.

Evaluation. The variables evaluated were; emergence, mortality, plant height (PH), number of macollo (NM) and dry matter (DM). For the emergency and mortality evaluation, a 0.50 x 0.50 m⁴⁵ gauge frame was used. Emergence was evaluated from 7 days after sowing to 30 days and mortality up to 90 days in the two test techniques. Four months after the development of the seedlings, 200 g of green biomass were collected and it was introduced to the stove at 60 °C for 72 h to determine the DM. During the development of the seedlings, there were no pests or diseases, they only showed symptoms of stress caused by the salts that the soils contain.

Experimental design. A randomized full block experimental design with four replications was used. The data were analyzed through the analysis of variance (ANOVA), the Tukey test was used with a confidence interval of 95% (p <0.05)⁴⁶.

Results

Emergency and mortality. At sowing, the emergence showed significant differences between the species, A. elongatum (Host) P. Beauv., Is the one that emerged in the highest percentage with 42%, followed by H. muticum J. Presl, B. catharticus Vahl., Bromus sp., and F. arundinacea Schreber with 31 and 30% respectively, the species of E. curvula (Schrad.) Nees was the one that emerged in the lowest proportion with 14% (fig 1A).

Figure 1
Emergence and mortality of the six forage species (AE = A. elongatum, HM = H. muticum, BC = B. catharticus, B = Bromus sp., FA = F. arundinacea and EC = E. curvula) with the sowing technique

Mortality showed high differences, the highest mortality that had was E. curvula (Schrad.) Nees with 37%, the species of F. arundinacea Schreber and B. catharticus Vahl., had intermediate mortalities with 34 and 33% respectively and A. elongatum (Host) P. Beauv., showed low mortality with 14% (fig 1B).

The transplanted seedlings also presented mortalities, but statistically there were no differences, E. curvula (Schrad.) Nees presented 8% mortality that is relatively higher compared to the others, the seedlings of F. arundinacea Schreber, B. catharticus Vahl., H. muticum J. Presl, A. elongatum (Host) P. Beauv. and Bromus sp. showed relatively lower motility (fig 2).

Fig. 2
Mortality of the six forage species (EC = E. Curvula, FA = F. Arundinacea, BC = B. catharticus, HM = H. Muticum, AE = A. Elongatum and B = Bromus sp.) with the soil transplant technique

Height, macollo and DM. The sowing technique shows a lower development in PH and NM compared to transplantation (Fig 3A and 3B). In table 2, B. catharticus Vahl. presents significant but lower differences than transplantation and A. elongatum (Host) P. Beauv. show higher development in PH and NM. E. curvula (Schrad.) Nees they had lower development. DM was similar in five species ranging from 101.33 to 109.33 kg ha^-1, except E. curvula (Schrad.) Nees, showed lower dry biomass in soils of 7 and 16 dS m^-1, but in 22 dS m^-1 none of the species emerged. However, in soils of 7, 16 and 22 dS m^-1 by means of the transplant technique, the seedlings developed much better than in the sowing, presenting highly significant differences in PH, NM and DM.

Fig. 3
Difference between sowing (A) and transplantation (B) in four months of development of B. catharticus Vahl. in saline soils

Between sowing and transplantation there are significant differences in PH. The transplantation technique showed a high difference with respect to the sowing. A. elongatum (Host) P. Beauv. showed remarkable development compared to the sowing technique, likewise H. muticum J. Presl, Bromus sp., F. arundinacea Schreber and E. curvula (Schrad.) Nees presented lower development, but superior to sowing (fig 4A).

In transplantation, the species of A. elongatum (Host) P. Beauv. showed greater formation of NM against all the species of the sowing (fig. 4B). On the other hand, B. catharticus Vahl. and A. elongatum (Host) P. Beauv. in transplantation they presented higher DM compared to the others, but the species E. curvula (Schrad.) Nees both in sowing and in transplantation presented lower DM (fig 4C).

Fig. 4
Difference of PH, NM and DM between sowing (SEC = E. curvula, SFA = F. Arundinacea, SBC = B. catharticus, SHM = H. muticum, SAE = A. elongatum and SB = Bromus sp.) and transplantation (TEC = E. curvula, TFA = F. arundinacea, TBC = B. catharticus, THM = H. muticum, TAE = A. elongatum and TB = Bromus sp.) on four months of development

Discussion

All the species planted in saline soils were reduced in their emergence. No species managed to emerge more than 42%, emergence has been limited reaching even 14% in E. curvula (Schrad.) Nees (fig 1A), it is confirmed that each species has genetic variability to the tolerance of salinity, and that the most vulnerable stage to adapt is during germination and early development. The results are similar to the statements where they indicate that each seed requires a certain percentage of water for its activation^47-49

Tolerance of the species to salinity during the germination and emergence stage depends on their capacities to support the water potential of the surrounding environment. With toxic ions can inhibit the absorption of water by the roots^50-53. Species that were subjected to different saline concentrations, showed greater sensitivity than others at the time of the beginning of their development^54-59. In a flat area with outcrops of salts at more than 3600 altitude with extreme climatic factors, it has been observed that salinity negatively affects the epidermal cell division of the root and the elongation rate, these effects affected the area part of the species causing a decrease in their yields

The sowing technique, six species were seriously affected by salinity and had heterogeneous behaviors in the emergence of 58 to 86%, it should be clarified that in 22 dS m^-1 they did not manage to emerge (table 2), these results reveal that the high concentrations of salts increase the potential forces of the water in the soil solution and are consistent with the announcements that the presence of solutes produces the decrease of the osmotic potential of the soil solution^12,60-62, it is also mainly related to the low hydric potential of the solution surrounding the seed^63-66, which suggests that there is a combination of osmotic and toxic effects of the salinity. Report that there are species that are more sensitive to salinity in the emergence stage and at the beginning of seedling development than in germination^67,68. The effect of osmotic stress on plants depends both tolerance of each species as well as on the state of its development.

Three days after emerging, the seedlings suffered mortality due to the effect of soil salinity, the species with the highest sensitivity to salinity was E. curvula (Schrad.) Nees, which reached 37% mortality, while A. elongatum (Host) P. Beauv., was the one that behaved more tolerant to soil salts (fig 1B). The transplanted seedlings also suffered mortality due to the effect of salt, but it was lower than those of the sowing, only the species that in sowing showed higher mortality reached 7.6%, Bromus sp., Presented 3.6% mortality (fig. 2). Therefore, it is validated that the best option to implement forage species in saline soils is through the seedling transplant technique to reduce mortality (fig. 3). The results are similar to other studies where they indicate if the roots of the plants are exposed to high concentrations of salts, cause osmotic and ionic stress^57,65,69-71, to survive in adverse conditions of stress, the plants have developed physiological mechanisms, morphological, biochemical and genetic that allows them to resist without drastically affectting their metabolism^5,57,72-75. High accumulations of Na⁺ or Cl^- ions cause toxicity damage, negatively affecting physiological processes, mainly the absorption of water and nutrients, as well as photosynthesis^36,73,76-79. Furthermore, it leads to the low absorption of K⁺ and stimulates its release, reducing enzymatic reactions and osmotic adjustments within cells^36,73,80-82. Salinity, by hindering the absorption of water (physiological drought), causes cellular damage through the leaf perspiration inhibiting their growth.

The species that were sown in soils with 7 and 16 dS m^-1, showed differences in development of PH, NM and DM, A. elongatum (Host) P. Beauv., was superior in PH reaching 9.00 and 6.80 cm and with 7.67 and 6.33 NM respectively, while the species of E. curvula (Schrad.) Nees presented a lower development in PH, which only reached up to 4.33 and 2.67 cm and with 4.00 and 2.33 NM respectively (table 2). These findings are similar to the versions that mention that salinity affects the development of the roots and affects development and reduces yields due to the low extraction of nutrients from the soil^83-87. The tolerance to salinity of a species involves a gradual acclimatization to this phenomenon and not direct exposure to a high salt concentration.

The DM, the B. catharticus Vahl. species was superior of all, which reached 109.33 and 107.63 kg ha^-1 respectively, the species that had the lowest development in both PH and NM, was the one that presented the lowest DM with 41.00 and 37.00 kg ha^-1 respectively (table 2). When the content of the salts in the soil solution is higher than the water content of the plant cells, the roots cannot absorb the water from the soil, which is consistent with the claims that salts cause alterations in various physiological and metabolic processes due to ionic imbalance and osmotic stress, these effects reduce the development and production of biomass^88-97. The salts considerably reduce the amount of adsorbent hairs, because of this reduction the absorption of water and nutrients from the soil solution is affectted, which affects the biomass.

The seedlings that were transplanted in soils of 7, 16 and 22 dS m^-1 presented statistical differences in PH, NM and DM, the species A. elongatum (Host) P. Beauv. was higher in the three salinity concentrations with 15.50, 12.00 and 8.00 cm of PH and with 19.67, 18.00 and 11.00 NM respectively, the E. curvula (Schrad.) Nees showed less development with 6.83, 4.00 and 3.83 cm of PH and with 7.00, 6.33 and 5.66 NM respectively. The results found have a similar coincidence with reports that indicate that plants in high salt concentrations have difficulties in the extraction of water through the roots, this inhibits cell growth and elongation, as well as stomatal closure^{15,16,29, 91,98-102}, as well as Ca²⁺ and K⁺ undergoes homeostasis^103,104 so that survival to complete the vegetative cycle will depend on the ability to maintain low internal osmotic potential^105-109. Furthermore, Na⁺ may inhibit the function of enzymes in plant metabolism^110,111. Tolerance to salinity can be maintained or decreased depending on the species and the time of exposure to saline stress. However, the problem of salinity is complex and requires proper soil management, as well as the use of salt-tolerant species.

In DM biomass, the species of B. catharticus Vahl. and A. elongatum (Host) P. Beauv., they presented superiority compared to the others, while E. curvula (Schrad.) Nees showed low performance in the three salinity levels (table 2). The proper management of saline soil for plant growth depends on the mixture of the different factors, as well as the amount of salts present. The results are similar to reports from other trials, where they indicate that excessive concentrations of Na⁺ or Cl^- in plant tissues prevent the uptake and absorption of K⁺, Ca²⁺ and NO₃^-. Many factors influence the limitations to the production of forage species due to salinities that have an impact on the decrease in productivity^{81,80,112-117}. To maintain a positive turgor pressure, plants need to adjust osmotically to remain in saline soils and not be adversely affected by biomass production.

Between the sowing and transplanting technique of seedlings in saline soils they showed significant differences in PH, NM and DM. The transplanted species showed superiority compared to the sowing, the A. elongatum (Host) P. Beauv. showed greater development in PH with 11.83 cm, but at sowing it only reached 7.90 cm; followed by B. catharticus Vahl. that grew 7.45 cm, the same species in sowing reached only 5.67 cm. The species E. curvula (Schrad.) Nees both in transplantation and in sowing had a lower development with 4.89 and 3.50 cm respectively (fig. 4A). It has been observed that, by means of the sowing technique, the species did not manage to develop normally because they suffered stress from the moment of germination and emergence. The salts were concentrated enough to adversely affect the initial development of the seedlings. These results found are similar to the claims that plant cells lose water and reduce cell elongation for osmotic adjustment^72,78,98,106 and the accumulation of NaCl in plant cells affects their functions^118,119, which decreases epidermal cell division roots ^120-122. The accumulation of salts in the organelles of the seedlings, can cause a delay or an inhibition in the development, so that they can tolerate saline soils, the root development is essential.

Likewise, the species A. elongatum (Host) P. Beauv. manages to get out with 16.22 in the development of NM in transplantation, but in the sowing it has only 7.00 macollos, followed by B. catharticus Vahl. which formed 12.33 NM in transplantation and 6.34 in sowing. The same species that had less development in PH both transplantation and sowing have low formation of NM with 6.33 in transplantation and 3.17 in sowing (fig. 4B). During the development of NM, it has been observed that high concentrations of salinity have caused alterations in cell membranes. These results are similar to other statements where they indicate that Na⁺ causes depolarization of the electrical potential of the cell membrane, causing the entry of K⁺ and causing serious physiological disorders^123-126. Plants in their evolution have developed several mechanisms to be able to adapt to salinity, for example, some halophytes can accumulate large amounts of inorganic salts in their organelles (vacuole) of cells, this response is common in the adaptation process of plants.

In DM, the species B. catharticus Vahl. and A. elongatum (Host) P. Beauv. transplanted plants showed higher yields of 159.55 and 143.00 kg ha^-1 respectively, but at sowing it was lower with 108.50 and 102.82 kg ha^-1 respectively. The E. curvula (Schrad.) Nees both in transplantation and in sowing showed lower yields with 49.67 and 39.00 kg ha^-1 respectively (fig. 4C). The salts present in the soil solution cause the plants to be smaller and can even cause death before completing their vegetative cycle, which has a direct impact on the decrease in yields. The best option to implement forage species in saline soils is through the transplant technique, since the seedlings have a better chance of adapting and surviving. The results of this study are similar to the reports where they mention that the different species in saline conditions are harmed in development and biomass due to osmotic effect-ts^{24,87,127-132}. The understanding of plants to an abiotic or biotic stimulus is complex, since in the face of this external stimulus, the plant triggers the activation of multiple signal pathways mediated by plant hormones and that have complex interactions with each other. The energy cost generated by the plant to cope with stress will depend on having a greater or lesser impact on its development.

The six forage species sown at 7 and 16 dS m^-1 had an emergency decrease of more than 50% due to the high concentrations of salts in the soil, A. elongatum (Host) P. Beauv. and B. catharticus Vahl. are the ones that they adapted better to these two concentrations of salinity, and the one that showed the most susceptibility was E. curvula (Schrad.) Nees, but at 22 dS m^-1 neither species were successful. All suffered mortality, the most affected was E. curvula (Schrad.) Nees with 37% and the one that performed the best was A. elongatum (Host) P. Beauv. which was only reduced by 14.3%.

In the transplant, the six species were adapted to saline concentrations of 7, 16 and 22 dS m^-1, but they also showed mortality in a lower proportion than in sowing, the species E. curvula had 7.6%, while Bromus sp. 3.6%.

There was a great difference between sowing and transplantation, it was observed both in the development of PH, NM and DM, those of trans-plantation had superiority compared to sowing. Therefore, the best option to implement forage species in saline soils is by transplanting seedlings with capacities that can tolerate salinity.

The seedlings showed a reduction in the absorption of water and nutrients due to the effects of Na⁺ ions that are dissolved in the soil solution. On the other hand, the ions transported into the intercellular spaces of the leaves dehydrated the cells and inhibited the enzymatic reactions that are generated by photosynthesis.

The species that were subjected to different concentrations of salinity by the transplantation technique, have survived until completing their vegetative development, this reflects that their roots have the ability to exclude a large amount of Na⁺ so that it cannot enter the interior of their tissues and accumulate especially in the aerial parts.

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