Germination and Vigor of Black Bean (Phaseolus vulgaris L.) Cultivars Subjected to Saline Stress Conditions

Natural factors such as salinity act directly on seed germination and vigor and, consequently, affect the establishment and development of seedlings in the field. In view of the above, the objective of this study was to assess the effect of saline stress on the germination and vigor of seeds from black bean cultivars. The study was conducted at the Seed Analysis Laboratory of the State University of Montes Claros, Janaúba-Minas Gerais, Brazil. The experimental design was completely randomized, in a 2 × 5 factorial arrangement (two cultivars × five osmotic potentials), with four repeats of 50 seeds per treatment. Bean seeds from cultivars Campeiro and Esteio were used, with sowing carried out on germitest paper rolls moistened at five osmotic potentials via sodium chloride solutions (NaCl): 0.0 [distilled water]; -0.3; -0.6; -0.9 and -1.2 MPa. Seed rolls were kept in a digital germinator, and the germination and vigor percentages of the seeds were determined by first germination count, germination speed index (GSI), and fresh and dry mass of seedlings. The germination and vigor of seeds from common-bean cultivars, black commercial type, are negatively affected under saline stress caused by NaCl solution with osmotic potential lower than -0.6 MPa. The seeds of the studied cultivars showed a different behavior regarding saline stress conditions, and cultivar Campeiro proved to be more tolerant to saline stress in relation to Esteio during the germination process and seedling formation.


Introduction
Germination tests run in laboratory aim to qualify and quantify the number of living seeds capable of producing normal plants under favorable field conditions (Figliolia et al., 1993). In addition to germination, there are vigor tests, which better predict performance in the field and allow for understanding the actual sowing (Wendt et al., 2017). According to the same author, a simple study relating vigor tests to seedling establishment in the field is usually adopted to ratify the behavior of the potential quantified in laboratory.
An alternative to overcome this unfavorable condition, especially in early stages of development, is the selection of tolerating cultivars, which allows for obtaining a more vigorous plant stand, thus ensuring a successful crop implantation.
For bean seeds (Phaseolus vulgaris L.), it is of paramount importance to conduct researches that indicate the best germination and early development conditions of seedlings under saline stress, as well as the most indicated genotypes, since saline stress can hinder water absorption or make this process irreversible. In light of the foregoing, the objective of the present study was to assess the effect of saline stress on germination and vigor of black bean seeds.

Material and Methods
The investigation was conducted at the Seed Analysis Laboratory of the Department of Agrarian Sciences, State University of Montes Claros-UNIMONTES, Janaúba-Minas Gerais, Brazil. It used seeds of commercial black beans produced at Unimontes' Experimental Farm, Janaúba-MG, 2017 harvest.
The region's climate, as per Koppen classification, is "AW" type-tropical with dry winter. The region's climatic conditions are represented by average temperatures ranging from 21 to 25 o C, daily relative humidity of 60 to 70%, and average annual rainfall of approximately 900 mm.
The experimental design adopted was completely randomized, in a 2 × 5 factorial arrangement (two cultivars × five osmotic potentials), with four repeats of 50 seeds per treatment. The two black bean cultivars (Campeiro and Esteio) and the five osmotic potential levels (0.0 [distilled water];-0.3; -0.6; -0.9 and -1.2 MPad] were sourced from aqueous sodium chloride (NaCl) solutions, prepared according to specifications by Vilela et al. (1991).
For the germination test, the seeds were sown on germitest® paper moistened with the formulated NaCl solutions, in an amount equivalent to 2.5 times the weight of the dry paper, in order to provide the osmotic potentials previously described. Afterwards, rolls were made and placed in digital germinator regulated in advance at a constant temperature of 25 o C. Assessments were carried out on the fifth and ninth days after sowing, and results were expressed as percentages of normal seedlings, in accordance with the Seed Analysis Rules (Brasil, 2009).
The first-germination-count test was run together with the germination test, which provided the number of normal seedlings, determined on the occasion of the fifth-day count after test assembling, with results being expressed as percentages (Brasil, 2009). Along with this, the germination speed index (GSI) was verified, when the number of germinated seeds was daily registered, and, at the end of the test, the GSI was calculated according to formula proposed by Maguire (1962).
Normal seedlings resulting from the germination test were weighed on a 0.0001 precision scale for dry matter weight. Later, the seedlings were placed in paper bags and taken to dry in a forced air circulation oven at 65 o C for 72 hours. After this period, the samples were left cooling in dissectors and then again weighed on precision scale for dry matter weight determination. Results were expressed as g repeat -1 .
Data were subjected to analysis of variance, at a 5% level of probability, and subsequent regression analysis. When significant, the effects of cultivars were studied by Tukey's test at a 5% significance, while the effects of osmotic potentials were studied by regression analysis, choosing the adequate models to represent them as a function of their biological behavior, of the significance of the coefficients and of the determination coefficient value (R 2 ).

Results and Discussion
The results of the analysis of variance revealed that only variables 'germination' and 'germination speed index' (GSI) were influenced by the interaction between factors 'cultivars' × 'osmotic potentials'. There was significant effect as to isolated factors for first germination count, fresh mass and dry mass of seedlings.
For both cultivars, germination results ( Figure 1A) fit into a regression equation of linear descending behavior. As the osmotic potential of the solution reduced, meaning higher NaCl amount and lower water availability, there were proportional decreases in the germination of the bean seeds for both cultivars, although cultivar Esteio has presented higher sensitivity for all potentials ( Figure 1A).   Vol. 12, No. 9;2020