Respuestas Morfogenéticas In Vitro De Vasconcellea Chilensis Planch. Ex A. DC (Caricaceae)

ABSTRACT

Multiple shoot formation was induced in vitro on nodal sections excised from adult "palo gordo" plants of Vasconcellea chilensis ex Carica chilensis, in the presence of relatively high levels of thidiazuron (TDZ), indole-3- acetic acid (IAA) and organic addenda included in the WPM formulation including casein hydrolysate, adenine sulfate and cystein. Multiple shoots appeared simultaneously in strands or clusters on the surface of the explants; usually two or more clusters with new shoots were initiated, all showing synchronous growth. In subculture, in the presence of indole-3-butyric acid (IBA) as single hormone, root formation did not take place; the shoots turned green instead, evidenced elongation and development of new shoots at the base of the explants. Other explants, such as petioles and leaf sections exhibited callus initiation upon the vascular system after 4-5 weeks; however, this tissue turned brown later on and died.

Key words: papaya silvestre, palo gordo, monte gordo, endemic, endangered, in vitro, multiple shoots.

Introduction

The endemic to Chile Carica chilensis (Planch. ex A.B.C) Solms-Laub, at present Vasconcellea chilensis (Badillo, 2000, 2001) "palo gordo", (Fig.1a) has been classified as an endangered species (Benoit, 1989; Squeo et al., 2001). V. chilensis grows restricted to coastal semiarid rocky locations in low-density natural stands within South Latitude 30°-33°. In its habitat, the plant shows very slow growth, poor biomass production, small-sized fruit (Fig.1b) and germination problems. As it is not fit for human consumption its main use is as forage (Hechenleitner et al., 2005), but it is not cultivated as a forage crop due its various limitations, it is just gathered from the matorral and therefore has only little economic importance. As a result, there is scant information available regarding its growth conditions, phenology, and possible alternative uses of this species; i.e., search of valuable natural compounds in the latex (Kyndt et al., 2007). The germplasm potential and/ or specific traits of this species are yet to be researched. For example, the National Research Council (1989) indicated that several traits of a series of Vasconcellea species, although divergent between genera (Ocampo et al., 2006), are important due to their possible use in papaya breeding programs, especially to introduce cold adaptability and use of pathogen resistance genes present in Vasconcellea sp. highland papayas. The scant reports available for V. chilensis are mainly taxonomical studies; no information or studies on selection, morphogenic responses, grafting, nor asexual multiplication by cuttings or sexual multiplication by seeds have been reported (Hechenleitner et al., 2005, Jordan et al., 2009) up to now as far as information is available. The aim of this work was to explore the morphogenic potential existing in some V. chilensis explant-types as an alternative to achieve vegetative propagation of this endangered species.

Material and methods

In a preliminary work (results not shown here) V. chilensis showed similar developing patterns as Vasconcellea x heilbornii cv. 'babaco' and similar plant growth regulator and/or organic addenda demands for establishment, shoot formation (Cohen and Cooper, 1982; Jordan and Piwanski, 1997) as well as for somatic embryogenesis (Jordan and Piwanski, 1999). Consequently, casein enzymatic hydrolysate, adenine sulphate and L-cysteine, were included in the media, and tested at levels of 500 mg L-1; 80 mg L-1 and 10 mg L-1, respectively; besides other concentrations (Tab.1). Among plant growth regulators, thidiazuron (TDZ), 6-benzylaminopurine (BAP), dichlorophenoxyacetic acid (2,4-D) and indole -3-acetic acid (IAA), alone or in combinations were tested. As maltose was shown to be critical for shoot initiation in babaco (Jordan and Piwanski, 1997); it was also tested besides sucrose at the level of 2% added to the WPM medium (Lloyd and McCown, 1980) as well as to the MS-medium (Murashige and Skoog, 1962). After surface disinfection for 2 h in a fungicide suspension of 1.0 g L-1 triadimefon and 5.0 g L-1 captan (80% WP), under agitation, the explants were immersed for 5 min in a 0.25% (v/v) sodium hypochlorite solution and finally rinsed twice in sterile distilled water. The explants, node and petiole sections (excised 1 cm long), as well as 1 cm2 leaf sections including the midrib, were transferred to Pyrex tubes containing 13 mL solidified media (0.4% Midesa® Agar). The nodal explants were sectioned from the uppermost region of the branches (nodes from the apex to 10 cm below). Leaf sections were cultured with their abaxial side in contact with the media (Jordan and Velozo, 1996). Explants (in culture or subculture) were maintained for 2 months under an 18-h photoperiod at mol m-2s-1

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