Introduction

Brachiaria brizantha (Hochst. Ex A. Rich.) Stapf, is a perennial plant native to tropical Africa belonging to the gramineae family, introduced to Mexico under the name of Insurgente, being widely used by farmers in tropical areas. The physiological quality of the seed is related to its ability to emerge under field conditions (^{Bhering et al., 2003}). In this regard, the germination test is the most widely used and accepted procedure; however, it is not the most adequate to guarantee seed establishment in the field (^{Copeland and McDonald, 2001}), because it is carried out under controlled conditions of humidity, temperature and substrate (^{Stanisavljevic et al., 2013}).

In a test of vigor, the objective is to differentiate those lots that despite having a similar viability, show different capacity to germinate and to emerge. In this test, the seeds are exposed to high temperature conditions (41 to 45 ºC) and 100% relative humidity for a specific period (^{Carambula, 1984}). Therefore, in the present paper, different levels of temperature and accelerated aging were used to evaluate the vigor of Insurgente grass seeds. The research was carried out in the Laboratory of Seed Analysis of the Institute of Genetic Resources and Productivity (IREGEP) of the Colegio de Postgraduados, Montecillo Campus, Estado de Mexico.

Two seed lots of B. brizantha cv. Insurgente, harvested on September 28^th (lot 1, autumn) and December 21^th (lot 2, winter) of 2007, in the Experimental Field of the University of Papaloapan, in Loma Bonita, Oaxaca. The obtained seeds were packed in Kraft paper bags and stored under laboratory ambient conditions for six months. At the beginning of the experiment, the values of physical purity, weight of 1 000 seeds, viability by the tetrazolium test and percentage of germination were obtained (Table 1).

Table 1 Initial physical and physiological quality of two seeds lots of Brachiaria brizantha cv Insurgente, used in accelerated aging treatments. 

In both seeds lots, six temperatures (40, 41, 42, 43, 44 and 45 °C) and seven exposure periods (24, 36, 48, 60, 72, 84 and 96 h) were evaluated. Accelerated aging was performed using the “Gerbox” method described by Perry (1980), placing 200 seeds per treatment on a stainless steel mesh inside a plastic box, to which 100 ml of distilled water were added, covered and placed inside a drying oven at the temperatures and aging periods previously indicated. After the aging period, the germination percentage was evaluated through a standard twinning test, where the seeds were placed in sandwich boxes of 14*14*5.5 cm, provided with a lid and absorbent paper inside a germinating chamber at constant temperature of 25 °C and relative humidity of 100%, counting the number of normal plants for 21 days (ISTA, 2005). A completely randomized experimental design was used in a 6*7 factorial arrangement (six temperatures and seven periods of aging). The comparison of means of the treatments was carried out by the Tukey test, with a level of significance of 5% (SAS, 1998).

Table 1 shows that the physical quality in terms of pure seed, weight of 1 000 seeds, viability and germination was higher in lot 2 (winter harvest) compared to lot 1 (fall harvest). The data in Table 1 were not analyzed statistically, serving only for the characterization of the seed lots quality.

In both lots, the germination percentage varied between the evaluated temperatures (p< 0.001; Table 2). Seeds harvested in autumn (lot 1) showed the highest germination value (28.9%) with a temperature of 40 ºC, similar (p> 0.05) to that obtained with 41 ºC (24.7%), being different and superior to the other temperature levels (p< 0.05). A similar behavior was observed for seeds harvested in winter (lot 2), where the highest germination values (50.1 and 46.9%) were obtained with the levels of 40 and 41 °C, respectively.

Table 2 Germination percentage of two seed lots of Brachiaria brizantha cv Insurgente, at different temperature levels. 

abcd= literales diferentes en cada columna, indican diferencia significativa (p< 0.05).

Temperatures of 40 and 41 ºC were the best estimates of seed vigor behavior. The decrease of germination as temperature increases is attributed to the acceleration and affect of the physiological processes of the seed, increasing its deterioration and reducing the processes of respiration and biosynthesis (^{Stanisavljevic et al., 2013}). It is also observed that lot 2 showed, on average, 74% higher germination compared to that harvested in autumn (lot 1), due to the low percentage of pure seed, viability and germination of lot 1 because it was harvested at a lower physiological maturity. As regards the periods of accelerated aging, a significant effect (p< 0.001; Table 3) was found in germination percentages. In lot 1, it is observed that at 48 h, 40.6% of germination is obtained, a similar value (p> 0.05) at 36 h (34.7%), but different (p< 0.05) and higher than the values obtained in other periods. In lot 2, the highest germination was obtained at 60 h, with a value of 63%.

Table 3 Percentage of germination of two seed lots of Brachiaria brizantha cv Insurgente, to different periods of accelerated aging. 

abcde= literales diferentes en cada columna, indican diferencia significativa (p< 0.05).

The results indicate that the accelerated aging periods of 36 and 48 h are adequate to differentiate low quality seed lots; while for high quality lots, 60 h are enough to evaluate seed vigor. Lot 2 showed on average 58% higher germination compared to lot 1.

Finally, in terms of interactions between temperatures and aging periods, significant differences were found (p< 0.001, Table 4). In lot 1 (Autumn harvest), the highest germination values were at 40 ºC/48 h and 40 ºC/60 h, with accelerated aging, with values of 46 and 47%, respectively. In lot 2 (Winter harvest), it was found that at all temperature levels, the highest germination values were recorded at 60 h of aging; being the highest value (69.2%) at 40 ºC/60 h of accelerated aging. In lot 1 (Autumn harvest), the highest germination values were shown at 40 ºC/48 h and 40 ºC/60 h of accelerated aging exposure, with germination values of 46 and 47%, respectively.

Table 4 Percentage of germination of two seed lots of Brachiaria brizantha cv Insurgente at different temperature levels and periods of accelerated aging. 

Literales diferentes por columna, indican diferencia (p< 0.05).

In lot 2 (winter harvest), it was found that at all temperature levels, the highest germination values were recorded at 60 h of aging; however, the highest value (69.2%) was obtained at 40 °C/60 h of accelerated aging exposure.

The results indicate that in order to evaluate the vigor of seed lots of low quality, it is possible to use 40 ºC/48 h of aging; while for high quality batches 40 °C/60 h accelerated aging is required. Other studies have reported that temperature of 43 °C for 48 h, with 100% relative humidity is the best accelerated aging process to differentiate seed lots (^{Fernandes et al., 2004}). Also in B. brizantha^{Meschede et al. (2004)}, recommended 43 ºC/48 h of accelerated aging for medium high quality batches and 43 ºC/12 to 36 h of aging for low quality batches, concluding that the temperature and the period of seed exposure vary between species and lots of seeds.

Conclusions

The accelerated aging test detected differences between the seeds lots evaluated. The seeds harvested in winter were superior in physiological quality and vigor compared to Lot 1, harvested in autumn. The best combinations of temperature and the accelerated aging period to evaluate the seeds vigor of B. brizantha cv. Insurgent was 40-41 °C/48 h for low quality batches and 40-41 °C/60 h accelerated aging for superior quality batches.