Mycoflora in Exhumed Seeds of Opuntia tomentosa and Its Possible Role in Seed Germination

The funicular cover of the Opuntia tomentosa seed limits imbibition; germination occurs only when the funicle is weakened or the funicular valve is removed. We investigated the role of fungi in funicular weakening and seed germination. Seeds that had been either buried in one of two sites or stored in the laboratory were germinated with and without a valve. Disinfected or nondisinfected seeds and their naked embryos were cultivated on agar or PDA. None of the 11 identified fungal genera grew on the disinfected control seeds or the embryos. The mycoflora present on disinfected and nondisinfected exhumed seeds suggest that the fungal colonization occurred in the soil and differed between the burial sites. Exhumed seeds with and without a valve germinated in high percentages, whereas only the control seeds without a valve germinated. Scanning electron micrographs showed that the hyphae penetrated, cracked, and eroded the funicular envelope of exhumed seeds. 1. Introduction The genus Opuntia has 181 species and numerous varieties [1]. In the subfamily Opuntioideae, a hard funicular envelope completely encloses the seed [2–4]. A study of the seed hardness of 400 Opuntia varieties growing in San Luis Potosi, Mexico, showed that pressures from 171 to 456？kgf were required to break these hard seeds [5]. In species such as Opuntia tomentosa [6–9] the hardness of the funiculus limits water uptake and thus germination of the immature embryo. To achieve germination, the funiculus of Opuntia tomentosa can be weakened in three ways: a valve may form naturally during burial [9], the funiculus may be cracked through exposure to high daytime temperatures [9], or the funiculus may be eroded or cracked by microorganisms [8]. The valve is a region located in the funicular flanks, close to the micropyle; this is the site where the radicle protrudes naturally. This valve may be artificially removed to enhance germination. Many studies relate the presence of microorganisms with deleterious effects on seeds [10]. However, the seed coat’s tannins or other growth-inhibiting substances may serve as a barrier against the penetration of the seed by these. Additionally, the seed coat, formed by testa and tegmen, protects the seed’s embryo from dehydration caused by temperature and humidity fluctuations [11, 12]. Therefore, most of the fungi observed to grow on intact, healthy seeds are saprophytic [13, 14] and even elicit seed germination, as in the impermeable and hard seeds of Albizia julibrissin and the achenes of Rosa corymbifera [15, 16]. It has been reported that in O.