Effects of acid treatment and mechanical scarification on seed germination of Encephalartos altensteinii

Abstract

Cycads are one of the most threatened plant groups in the world, mainly due to their continued illegal removal from their natural habitat. While South Africa remains a diversity hotspot for cycads with 29 species out of 38 species found worldwide, seventy-eight percent of South African species remain threatened with extinction compared to the worldwide average of 62%. The increasing demand for aesthetic horticultural and medicinal use of the species have led to serious over-harvesting of wild populations. While this shortage of specimen plants presents an opportunity for potential profitable large-scale production of cycads, most species are slow growing and remain difficult to propagate with complex reproductive cycles. This study focused on the sexual propagation of the vulnerable Encephalartos altensteinii also known as eastern Cape giant cycad which continue to show low germination rates with prolonged germination periods and high seed viability losses at Pretoria National Botanical Garden. It was hypothesised that the low seed viability results from inadequate environmental storage conditions, desiccation intolerance of seed and germination periods which are influenced by morphophysiological seed dormancies. To investigate desiccation tolerance, seeds were subjected to hydration methods during storage to measure embryo maturity, moisture content and seed viability which were measured at 8 weeks’ intervals. To break seed dormancy, seeds were pre-treated with mechanical and chemical scarification using different concentrations of sulphuric acid (H2SO4) and, or gibberellic acid (GA3). The study reported that the growth of embryos of E. altensteinii became visible after 6 months of storage while seed hydration did not have significant effects on the embryo growth of this species. Seed moisture content was significantly different (P < 0.01) when compared to untreated seeds. The results also showed significantly (P < 0.05) high seed viability on hydrated seeds at 86.67% which decreased to 73.33% and compared to 33.33% in the control. Mechanical scarification had a significantly (P < 0.05) high germination of 60% on sanded seed beds with a reduced germination period when compared to other treatments. Cracking of seed coats resulted in negative effects on germination. While mechanical scarification was successful to improve germination percentage of E. altensteinii, the combination of GA3 with mechanical scarification did not show a difference on seed germination. Seed pre-treatment with varying concentrations of H2SO4 did not influence germination either, however, H2SO4 in combination with GA3 presented a significant difference (P < 0.05) on the final seed germination of E. altensteinii. The results indicated that the highest final germination of 73.33% was achieved with the H2SO42a+GA31 treatment (25% of H2SO4 for 0.5hr followed by 1000ppm GA3 for 24hrs). These results suggest that while E. altensteinii seeds are sensitive to desiccation, seed hydration treatment can slow down moisture content, improve viability loss and extend seed longevity. The seed pre-treatment with both mechanical and chemical scarifications can improve final germination of E. altensteinii. The outcome of the study holds promising possibilities for propagation and cultivation of E. altensteinii as well as to reintroduce plants back into the wild.