VIGOR AND TOLERANCE OF COWPEA (Vigna unguiculata) GENOTYPES UNDER SALT STRESS VIGOR E TOLERÂNCIA DE GENÓTIPOS DE FEIJÃO-CAUPI (Vigna unguiculata) SOB ESTRESSE SALINO

Saline stress is a frequent phenomenon in the arid and semi-arid regions of the globe, affecting the agricultural production of these regions, and it is necessary to use strategies that minimize the impacts of saline stress under agriculture. This requires the incorporation of species, variety and genotypes tolerant to increase agricultural production in those regions. This study aimed to evaluate germination and initial growth of cowpea genotypes under salt stress. The experimental design was completely randomized in 19 x 3 factorial scheme, composed of nineteen cowpea cultivars and three osmotic potentials, with four replicates of 50 seeds each. The germination test lasted for eight days, when the seeds were evaluated for germination percentage, germination speed index, length of shoot and root, accumulation of dry mass of shoot and root. The increase in salinity affected germination and initial growth of the cowpea genotypes. The genotypes 6 MNCO2-689F-2-8, 10 MNCO2-675F-4-10, 12 MNCO3-737F-5-9, 16 MNCO2-677F-2, 18 BRS-Pajeú and 19 – Paulistinha exhibited higher tolerance to salt stress in the stage of germination and initial growth. The genotypes 11 MNCO2-675F-9-5, 13 BRS-Tumucumaque, 15 MNCO3-736F-7 and 17 BR17-Gurgueia were more susceptible to the effects of salt stress in the stage of germination and initial growth.


INTRODUCTION
Cowpea (Vigna unguiculata (L.) Walp.) has great socioeconomic importance in the northeast region of Brazil, besides being one of the basic components of the diet of the northeastern population (LIMA et al., 2011). With the expansion of cowpea cultivation, new markets and commercialization perspectives open for this crop, which requires the use of new production technologies, such as irrigation, one of the agricultural technologies that have contributed the most to the increase in food production (MURTAZA et al., 2006;LIMA et al., 2011). However, its inadequate use accelerates soil salinization, especially under semi-arid conditions (ALMEIDA et al., 2012;COELHO et al., 2014).
Although sensitive to salinity, cowpea is widely spread in semi-arid regions of Northeast Brazil, where salinity problems are frequent. Hence, one alternative to improve cowpea yield would be the study of different genetic materials subjected to conditions of high saline concentration in the soil solution, to select the most adapted ones (SANTOS et al., 2009).
The osmotic effect and toxicity of ions are the main agents of salt stress on plants, but the osmotic effect is the most damaging to germination and vigor of the plantlets, because it reduces germination speed and its uniformity, initial size and the adequate establishment of the stand (SANTOS et al., 2009;SCHEEREN et al., 2010;ALMEIDA et al., 2012). The reduction in seed vigor caused by salt stress retards, therefore, the establishment of the plantlets at the field due to the decrease in the mobilization of reserves and induction of disorders in cell membranes, caused by the increase in the osmotic pressure in the soil solution, reducing the availability of water to the seeds (BARRETO et al., 2010;ALMEIDA et al., 2012;COELHO et al., 2014).
Given the above, this study aimed to evaluate germination, initial growth and tolerance of cowpea genotypes under salt stress.

MATERIAL AND METHODS
The experiment was conducted in the Laboratory of Physiology of the Federal University of Campina Grande (UFCG), Campina Grande-PB, Brazil, from July to August 2016, using seeds of cowpea genotypes.
The solutions used in irrigation were prepared through the addition of salts to distilled water, using sodium chloride (NaCl), which accounts for 70% of the salt ions in water sources used for irrigation in small properties of Northeast Brazil (MEDEIROS et al., 2003).
The seeds were put in rolls of Germitest ® paper, moistened until 2.5 times their dry weight, according to the treatment, and placed to germinate in a Biochemical Oxygen Demand (B.O.D.) chamber, at 25 ºC and with photoperiod of 8 hours of light and 16 hours of darkness. The germination test lasted for eight days (BRASIL, 2009).
During the experiment, the germination of cowpea seeds was monitored through daily counts of the number of germinated seeds, i.e., with production of the first radicle, without discarding them, thus obtaining a cumulative value. Hence, the number of emerged plantlets referring to each count was obtained by subtracting the reading of the previous day from the value of the current day. Then, the number of germinated seeds referring to each count was used to calculate the germination speed index (GSI), based on the equation described by Maguire (1962): Where: GSI = germination speed index; G = Number of germinated seeds in each count; N = number of days from sowing to each count.
After the germination test, at eight days after sowing, the percentage of normal plantlets (PNP) (%) was determined based on the relationship between the number of germinated seeds and the number of seed sown. Also at the end of the germination test, the primary root and the shoots of normal plantlets of each replicate were measured using a ruler graduated in millimeter and the results were expressed in cm plantlet-1. Shoot dry matter (SDM) and radicle dry matter (RDM) were determined by cutting the plantlets and storing the different portions in Kraft paper bags, which were dried in a forced-air oven at 65 ºC until constant weight and weighed on an analytical scale (0.0001 g), with results expressed in g plantlet-1 (NAKAGAWA, 1999).
The obtained data were subjected to analysis of variance by F test. In case of significance, the Scott-Knott means grouping test was applied for the factor genotype and the Tukey test was applied for the factor salinity, both at 0.05 significance level, using the statistical software SISVAR® (FERREIRA, 2011).

RESULTS AND DISCUSSION
There was significant interaction (p < 0.05) between the salinity levels and studied genotypes for the variables: germination percentage, germination speed index, shoot length, radicle length, shoot dry matter and radicle dry matter (Tables 1, 2 and 3).
The genotypes 1 -MNCO3-736F-2, 13 -BRS-Tumucumaque, 15 -MNCO3-736F-7 and 17 -BR17-Gurgueia were not influenced by the increase in salinity, but they showed the lowest accumulations of dry matter, even in the control treatment (0.0 MPa). This indicated lower development and probably consumption of seed reserves for plantlet growth, which is consistent with the results for shoot dry matter (Table 3).

CONCLUSIONS
The increase in salinity affects germination and initial growth of the cowpea genotypes.