GAMMA RAY STIMULATION OF GERMINATION AND DEVELOPMENT in vitro OF Laelia autumnalis

Abstract

Natural seed germination of L. autumnalis orchid is low because it requires specific conditions of the host tree and favorable environmental factors. In vitro asymbiotic germination is an efficient propagation method; however, seedling development requires one to two years. The aim of this research was to evaluate the effect of ⁶⁰Co gamma radiation to stimulate in vitro germination and seedling development of L. autumnalis. Twenty-two fruits were irradiated at different doses (3 to 30 Gy, with intervals of 3 Gy), in addition to a non-irradiated treatment used as control. Seeds were cultured in Murashige and Skoog medium without phytohormones. The experimental design was completely randomized with 16 to 32 replications; the experimental unit was a flask with 20 mg of seeds. At five days and then every 10 days the number of seeds in the stages of imbibition, photosynthetic protocorms formation, protocorms in differentiation, development of promeristems, leaves and seedlings was quantified. Analysis of variance and Tukey range test (α = 0.05) were performed. Seeds irradiated between 3 and 15 Gy formed 61 % more photosynthetic protocorms than untreated seeds. Seeds irradiated with 3, 15 and 18 Gy formed 73 % more promeristems. All protocorms treated with 3 Gy formed leaves 60 days after sowing (das); in contrast, only 12.5 % of untreated seeds formed leaves. About 66.5 % of the seeds treated with 15 Gy developed seedlings at 80 das. The radiostimulating effect of gamma radiation was confirmed at low doses; with 3 Gy promeristems, leaves and complete seedlings were formed 20, 20 and 10 days, respectively, earlier than the control treatment. Radiation at higher doses was negative by delaying germination and seedling development.