Introduction

In the Ciénega de Chapala región, Michoacán portion most forage crops are the main support of traditional livestock systems (^{Moreno et al., 2012}). However, increases in arable farming areas have led these systems to move to less productive, marginal, problem-prone areas, including: environmental factors such as water scarcity and increased salinity and sodicity due to the recurrent use of poor quality groundwater (^{Silva et al., 2006}). These environmental factors affect the establishment and development of species oriented to agricultural production (^{García et al., 2005}; ^{Colunga et al., 2009}). This situation is reflected directly in the economy of producers.

According to ^{Villa et al. (2006)}, sodium (Na⁺) is one of the dominant ions in saline environments and throughout the phenological cycle of plants (especially glycophytes), inducing malfunction of physiological processes as it can be considered equivalent to drought, because it retains water but is unavailable to seeds or seedlings; suppressing the net absorption of nutrients, affecting the membrane integrity, causing problems in its growth and development (^{Tester and Davenport, 2003}; ^{Munns and Tester, 2008}).

However, the salinity tolerance of each species depend on ambient conditions and the ability it possess to control the absorption and transport of Na⁺ to photosynthetic tissue (^{Laynez et al., 2008}; ^{Reyes et al., 2013}), mainly during the germination phase, where changes and adaptations occur that may affect not only the germination process itself, but also the future growth and development of the plants, being the first crucial stage in the life cycle of many species (^{Ruiz and Terenti, 2012}). This has led to inquire about the effects of salts on germination and emergence in various crops, such as beans (Caupi Vigna cinencis) (^{Paliwal and Maliwal, 1973}), rice (Oryza sativa) (^{Pearson et al., 1966}), wheat (Triticum aestivum) (^{Hampson and Simpson, 1990}), and soybean (Glycine max L.) (^{Hosseini et al., 2002}).

To achieve productivity improvements of the aforementioned livestock systems it would be imperative to increase information about the physiological and biochemical processes occurring in forage species, when immersed in stressful environments due to the effect of NaCl, as several researchers have used these two criteria as useful indicators to assess tolerance to salinity between species (^{Ulfat et al., 2007}; ^{Aghaei et al., 2008}; ^{Sankar et al., 2011}). For this reason, the objective of this research is to evaluate the ability of germination and emergence seven forage species, including ryegrass (Lolium perenne L.), star grass (Cynodon nlemfuensis), barley (Hordeum vulgare L.), janamargo (Vicia sativa L.), the chickpea (Cicer arietinum L.), alfalfa (Medicago sativa L.) and oats (Avena sativa L.) under in vitro conditions, when exposed to different concentrations of sodium chloride (NaCl).

Materials and methods

Results and discussion

Table 1 shows the mean values and standard deviations obtained in the inbibition of the seeds of the forage species for each saline concentration evaluated. C. arietinum, A. sativa and C. nlemfuensis seeds were significantly different (p≤ 0.05) from the 50 and 100 mM treatments compared to its controls. In the case of H. vulgare, M. sativa, L. perenne and V. sativa species, significant differences were observed in the treatments with higher saline concentration; i. e. at 200 and 400 mM.

Letras iguales no difieren significativamente entre sí según prueba de Tukey (p≤ 0.05).

Table 1 Imbibition of the seven forage seeds.  

According to ^{Murillo et al. (2001)}, high concentrations of NaCl cause the mobility of water to decrease and thus the speed of seeds imbibition, which in turn, affects the synthesis of biopolymers, proteins, nucleic acids and the amount of regulatory hormones of the plant cell; aspects that altogether limit the intensity of the growth processes as they develop in the subsequent germination stage, also called the glumella breaking phase, where some physiological initiation mechanisms related to the first cycles of cell division and differentiation are raised in the embryo, regardless of the hydrolysis products of the reserve substances of the seeds. According to ^{Hernández et al. (2015)}, this differentiated response between species could be due to the fact that each genotype requires a critical percentage of water for its germination, derived from the dependence of the chemical nature of its reserves and structural compounds. This was validated when the different varieties showed a reduction in the percentage of germination through the different concentrations to which they were submitted (Table 2).

Letras iguales no difieren significativamente entre sí según prueba de Tukey (p≤ 0.05).

Table 2 Effects of the NaCl salinity gradient on germination, root length and hypocotyl length of seven forage species at 21 days of evaluation. 

The results show a higher than 70% germination percentage of for all species in concentrations of 0, 50 and 100 mM NaCl, except for C. nlemfuensis whose germination rate was reduced to 61.45% at 100 mM. However, it was found that the H. vulgare species, was the one with the best germination rate (> 95%) for these three levels of salt concentration.

On the other hand, the germination rate of all the species at 200 and 400 mM was drastically reduced, at these levels of salinity the germination percentage oscillated between 40 and 0% respectively; except for L. perenne and H. vulgare species because, in the first, the germination percentage was reduced to only 47.3% at 200 mM, while, for the second, the reduction was 33.33 and 76% at 200 and 400 mM respectively (Table 3). Although in the latter species, the reduction was more dramatic, H. vulgare was the only one able to germinate at 400 mM NaCl. This result coincides with those reported by ^{Munns and Tester (2008)}; ^{Royo and Aragües (1991)}; ^{Martínez-Cob et al. (1987)}, who noted that at this level of salt concentration H. vulgare shows a suitable germinability-emergence. The general trend of germination reduction with increasing salt concentration is a frequently observed response in several species (^{Ruiz and Terenti, 2012)}.

Letras iguales no difieren significativamente entre sí según prueba de Tukey (p≤ 0.05).

Table 3 Content of Na⁺, K⁺, Ca²⁺, Mg²⁺ and K⁺/Na⁺ ratio. 

^{Campos et al. (2011)} found that germination in different cultivars of Phaseohus vulgaris L. were affected only from 100 mM NaCl. Meanwhile, ^{Madueño et al. (2006)}, found that from 150 mM NaCl, the rate and percentage of germination of Rhynchosia minima L. a herbaceous species of particular interest for its use as forage, is reduced. These results are also consistent with those obtained by ^{Lombardo and Saladino (1997)} who after evaluating the effect of salinity on germination in various vegetables (Cichorium endivia L., Chicorium intybus L., Daucus carota L. y Petroselinum crispum L.), and forage (Trifolium alexandrinum L., Vicia sativa L., Medicago sativa L., Hedysarum coronarium L. y Lens culinaris L.), they found that as the electrical conductivity increases, the germination decreases.

According to ^{Flowers et al. (2010)}, germination reduction with increasing concentrations of NaCl is the result of a decrease or delay of water absorption in the seeds by toxic effects exerted by ions on them, since the functions of the membrane are affected and in the embryo cell wall; product of a reduction in the permeability of plasma membranes, increased influx of external ions and efflux of cytosolic solutes. On the other hand, ^{Mahdavi and Modarres (2007)} indicate that the reduction of germination in salinity conditions is also due to the fact that the seeds increase their latency and dormancy, two mechanisms that help the seeds to germinate at the most appropriate times so that the new plants would have the maximum possibilities of survival.

Despite these positions, it is evident that most authors agree that the inhibitory effect of salts on germination is both ionic and osmotic (^{Khan et al., 2006}), and in nature it would act by inducing dormancy to synchronize germinative events with environmental conditions (^{Redbo, 1994)}.

However, researchers like ^{Mansour and Salama (2004)} found that differences in the salt tolerance varies with the permeabilities of each genotype, which is why both the composition and lipid structure as well as cytoplasmic viscosity of each species are presented as key factors in preserving the integrity of the plasma membrane. Signaling that would explain why H. vulgare and L. perenne achieved a high rate of germination at higher salinity concentrations. Table 2 also shows that the size of the radicle (LR) and the length of the hypocotyl (LH) of the seven forage species were influenced by the increases in the concentrations of NaCl; as well as, the highest growth values in all species were found at 0, 50 and 100 mM.

H. vulgare, M. sativa, L. perenne and V. sativa species exhibited a lower degree of involvement lower than 50% in growth, both in the root and the hypocotyl under these same levels of salt concentration compared to their own controls. C. nlemfuensis was the species with slower growth at 100 mM in relation to the rest of the evaluated forage. In the 200 mM salt concentration there was a greater than 90% affectation in root growth and hypocotyl of all species, except for H. vulgare and L. perenne, whose growth shrank between 43.35 and 69.85% for the root And in 30.02 and 100% for the hypocotyl respectively. ^{Jamil et al. (2007)}; ^{Llanes et al. (2005)} noted that the length of radicle and hypocotyls are reduced by increasing concentrations of NaCl from 50 mM.

Such a decrease is the result of cell turgor loss caused by decreased osmotic potential in the growth medium, which is essential to the weakening of the endosperm and the expansión of the embryo; since it is a growth process impulsed by water absorption (^{Nawaz et al., 2013}). Table 3 shows the sodium content (Na⁺), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺) and the K⁺/ Na⁺ ratio of the seven forages at 21 days of the test.

Conclusions

From the seven forage species evaluated, H. vulgare was the most tolerant to different levels of salt concentration, followed by L. perenne L. and V. Sativa. While C. nlemfuensis was the species with the greatest involvement from 100 mM which was reflected in the radicle size and hypocotyl length of these same species. A 200 mM, all species showed the highest content of Na⁺ and further reductions in the K⁺, Ca²⁺ y Mg²⁺ content. However, H. vulgare and L. perenne species showed a lower K⁺/Na⁺ ratio, thus validating to be more tolerant to this level of salt concentration.