Development of new varietal types based on rejuvenation by somatic embryogenesis and propagation by conventional budding or microcuttings in Hevea brasiliensis

Hevea brasiliensis has been the only commercial source of natural rubber for a century. This plant species, which is planted on more than 10 million hectares, is of major importance for the economies of producing countries in Southeast Asia and West Africa. Cloning by budding on rootstocks derived from illegitimate seedlings made it possible to select the best individuals and obtain substantial genetic gain in the 1940s compared to seedling plantations. From the 1970s to date, the development of micropropagation techniques has not led to any commercial application for the multiplication of self-rooted clones. However, some serious leads have been advanced for achieving large-scale multiplication of improved material in the medium term, using biotechnologies. Although their multiplication rate is low, microcuttings and primary somatic embryogenesis techniques produce better quality planting material than budded clones. Conversely, maintained somatic embryogenesis gives a higher multiplication rate, but several developmental parameters of the in vitro plantlets are affected. A combination of embryogenlc callus cryopreservation and indirect secondary somatic embryogenesis has made it possible to reduce the length of time calli are exposed to hormones and the number of proliferation cycles. Although the quality of the in vitro plantlets has been improved, the maintained embryogenesls pathway Involving callus multiplication remains problematic. Combining primary somatic embryogenesis to rejuvenate the planting material and multiplication by budding or microcuttings seems to be an alternative for improving the planting material. Rejuvenated budded clones have been planted On 80 hectares by CIRAD and Michelin. These trials reveal better budding and growth success for four clones compared to conventional budded clones confirming previous results from small-scale trials on eleven clones. At the same time, a team at CATAS has demonstrated the merits of microcuttings from embl