In Vitro Long-Term Cultures of Papaya (Carica papaya L. cv. Solo)

In this article, we present proliferation data from 10 years of the continuous in vitro incubation of cv. Solo papaya shoots and propose a reliable method for the long-term micropropagation of papaya, using microshoots developed from the axillary buds of papaya shoots as primary explants. Three different media were assayed. The proliferation medium (PPRM) allowed us to maintain papaya shoots under continuous proliferation for 20 years, maintaining consistent behavior. Most of the shoots developed in the PPRM rooted during the incubation and then acclimated easily, maintaining the ploidy and morphological characteristics of the parental plants, and flowering and setting fruits normally. The PPRM medium consisted of MS medium supplemented with naphthalene acetic acid (NAA) (0.1 mg L-1), benzyladenine (BA) (0.5 mg L-1), gibberellic acid (GA(3)) (0.5 mg L-1), and adenine hemisulphate (40 mg L-1). The average multiplication rate was higher than 20 shoots per explant during the long-term assay. The elongation medium (PELM) was designed to recover shoots with poor growth and allowed the development of high-quality shoots ready for rooting. It consisted of an MS basal medium supplemented with NAA (0.1 mg L-1), kinetin (KIN) (0.5 mg L-1), and GA(3) (1 mg L-1). The rooting medium (PROM) was designed to induce high-quality roots from nonrooted shoots and consisted of a half-strength MS medium and indole-3-butiryc acid (IBA) (1 mg L-1). On PROM, agar can be exchanged for expanded vermiculite. Acclimation took place inside an acclimatization tunnel under progressive hydric stress. After 4 weeks, the plant recovery rate was 90% for plants maintained under continuous proliferation for ten years. The main objective of this work was to provide a micropropagation method which would maintain healthy elite genotypes of papaya for long periods of time and produce a high number of good quality plants.