Improvement of Shoot-tip Grafting in vitro for Virus-free Citrus1

Abstract A 30 to 50% frequency of successful grafts was obtained by using 2-week-old dark grown seedlings as rootstocks and 0.14 to 0.18 mm long shoot tips as scions. The shoot tip was inserted into an inverted-T made at the top of the decapitated rootstock epicotyl. Most scion cultivars gave satisfactory grafts on ‘Troyer’ citrange, whereas lemon, lime and citron yielded successful grafts only on ‘Rough’ lemon. The grafted plants were allowed to develop in vitro under 16 hr daily exposure to 1000 lux Grow Lux illumination and were provided with a nutrient solution containing a high concentration (7.5%) of sucrose. The best source of shoot tips was the flush from defoliated branches of field trees or glasshouse plants. It was also possible to use shoot tips from flushes arising in excised lateral buds cultured in vitro . Grafted plants were transplantable to soil 5 to 8 weeks after grafting with over 95% survival. Preliminary data indicated recovery of cultivars freed from tristeza and psorosis viruses, stubborn spiroplasma and exocortis viroid. Pathogen-free plants showed no reversion to the juvenile phase.