Effects of temperature and substrate on the germination of Hamatocactus setispinus (Cactaceae)

Cacti are widely used as ornamental plants and seed germination is a major method for preserving genetic diversity. Thus, an experiment was carried out to evaluate the effects of different temperatures and substrates on seed germination of Hamatocactus setispinus. Seeds were sown in gerbox boxes containing either germination paper (S1), sand (S2) or vermiculite (S3). After placing the seeds on the substrates, the boxes were kept in germination chambers at 20, 25, 30 and 35° C for 30 days, under a photoperiod of 16 hours. The experiment was in a completely randomized design with four replicates. The speed germination index (SGI), mean germination time (MT) and germination rates (%) were evaluated. The best results were observed at 25o C. The highest SGI (3.94) was observed on seed germination paper, but MT values and germination rates on this substrate did not differ from those obtained on sand. Therefore, both substrates could be used for germination of this cactus species.


INTRODUCTION
Cacti, despite known for multiple and diverse uses, are mostly used for ornamental purposes (ROBBINS, 2003).The barrel cactus (Ferocactus spp.), the prickly pear cactus (Opuntia spp.) and the saguaro cactus (Carnegiea gigantea), are examples that, due to their large and showy appearance, are widely used for landscaping projects.Most miniature cacti are grown from seed to supply the local market, but are also exported for indoor decoration (ROBBINS, 2003).
Hamatocactus setispinus, a miniature barrel cactus species, is originally from southern Texas, USA and Coahuila, Nuevo Leon and Tamaulipas, Mexico.It is a globular plant with yellow flowers with reddish base, very small and shriveled seeds, and the fruits are round, red, indehiscent and fleshy at maturity; characteristics that make this species attractive as ornamental (ANDERSON, 2008).
Seed germination is a major method for the propagation of cacti since it allows the preservation of the genetic diversity of populations (ROJAS-ARÉCHIGA and VÁSQUEZ-YANES, 2000), which might help selecting desirable features, such as biomass yield, fruit quality, tolerance to stress-promoting factors, etc (ALTARE et al., 2006).
Seed germination speed, rate and uniformity can be affected by the environmental temperature, the range of which depends on the plant species (TOLEDO and MARCOS FILHO, 1977;CARVALHO and NAKAGAWA, 2000).According to Rojas-Aréchiga and Vásquez-Yanes (2000), extreme environmental temperatures (below 12° C or above 28° C) do not stimulate cacti seed germination.Moreover, for a wide range of cacti genera, the temperature of 20 ± 2° C leads to good germination rates.
Besides other environmental factors, the substrate is also critical for the germination process (BRASIL, 2009).The physical and chemical properties of the substrate, such as porosity, structure, water holding capacity, pH, among others that may enhance or inhibit seed germination, are of major importance (POPINIGIS, 1985).
According to the Brazilian procedures for seed analyses (BRASIL, 2009), the substrates most frequently used for the germination process in the laboratory are germination paper, cloth, sand and soil.Although the Brazilian procedures for seed analyses contain information about the conditions for conducting germination test for several species, there is no indication about the species in study yet.
The objective of this research was to evaluate the effect of different substrates and temperatures on the seed germination of Hamatocactus setispinus.

Plant material
The seeds were obtained from mature red fruits of Hamatocactus setispinus harvested in October 2009, in Maricá-RJ, Brazil.Immediately after harvesting, the fruits were packed in paper bags and transported to the laboratory where they were kept at a temperature of 25 ± 2° C.
Eleven days after harvesting, time taken to mail fruits to the laboratory, the fruits were opened and their seeds were carefully extracted.After removing their mucilage by scratching them on Kraft paper, the seeds were immersed in 10% bleach solution (2.5% active chlorine) for 20 seconds.After the immersion, the solution was drained out, however seeds were not washed again.

Effects of temperature and substrate
The experiment was in a 4 x 3 factorial scheme consisting of four germination chamber temperatures (20, 25, 30 and 35° C) and three substrates (washed and autoclaved sand, two sheets of seed germination paper and vermiculite) in a completely randomized design with four replicates, each one with 50 seeds.
The clean seeds were placed in transparent plastic germination (gerbox) boxes containing either sand, two sheets of germination paper or vermiculite as substrate.The sand was washed and autoclaved at 121° C and 1.5 atm for 2 hours; the two sheets of germination paper were moistened with deionized water at a ratio of 2.5 times the paper dry weight (BRASIL, 2009).The vermiculite was washed several times, soaked in tap water for 24 hours, and then let to dry out at room temperature.The sand and vermiculite were moistened with deionized water until the substrate capacity, at the beginning of the experiment.All substrates were remoistened when necessary during the experimental period to keep the substrate capacity.Subsequently, the boxes containing the seeds were taken to separate germination chambers, where they were kept for 30 days at constant temperatures of 20, 25, 30 and 35° C, under a photoperiod of 16 hours.Seed germination was considered at radicle emission.
The speed of germination index (SGI) was determined with the equation SGI = G 1 /N 1 + G 2 /N 2 +...+ G n /N n where G 1 , G 2 , ... G n = number of germinated seeds from the first, second to the last germination count; N 1 , N 2 , ... N n = number of days from sowing to first, second to the last germination count, suggested by Maguire (1962); the seed germination rate (%) was obtained by the total percentage of germinated seeds after 30 days; the mean germination time (MT) was estimated as suggested by Edmond and Drapala (1958) by the equation MT = (G 1 T 1 + G 2 T 2 + ... G n T n )/ (G 1 + G 2 + ...Gn) where G 1 , G 2 and G n are the numbers of seeds germinated at T 1 , T 2 and T n time, respectively.
The data were subjected to variance analysis and the means were compared by Tukey test at 5% significance.The germination rate data were transformed to arcsine (x/100) ½ for statistical analysis.

RESULTS AND DISCUSSION
The Hamatocactus setispinus seed highest SGI and lowest MT values were observed at 25° C. The lowest SGI values were observed at 35º C and the highest MT values were obtained at 30 and 35º C for all substrates (Table 1).Different results were observed with seeds of Pilosocereus arrabidae (MARTINS, 2007), which showed the highest SGI and germination rates at 20° C, whereas seed germination was completely inhibited at 35° C.