Longan Fruit Tree Physiology and Its Flowering Induction

Abstract

Longan (Dimocarpus longan Lour.), a tropical tree species in the Sapindaceae family, is a perennial, flowering plant that produces delicious, drupe fruit. Longan has become an important fruit crop worldwide. Yet, longan has an irregular flowering habit as it is a biennial fruit tree. Cool temperature is the only reason known to meet its vernalization requirements to induce flowering, but it does not ensure floral initiation will occur. It is reported also that longan is the only fruit tree that can respond to potassium chlorate (KClO3), an ion-exchange oxidant, for flower bud formation. Longan has also become a model plant useful for quantifying the influence of endogenous hormones in flowering induction. Because there are important interactions between longan floral initiation and vegetative growth environmentally, the specific KClO3 effects are often not important on flowering induction. For example, in Hainan Island, southern China, where wild longan trees can still be found in its virgin tropical rainforests, commercial longan trees treated with KClO3 can only result in a small bloom, which also often fade away too soon making a profitable fruit production impossible. This chapter analyzes factors affecting longan tree reproductive growth, including environment (winter seasonal temperatures), potassium chlorine, plant hormones (auxin, gibberellins, and florigen), and plant nutrition (potassium, phosphorus, and chlorine) variations. This chapter presents some of the results from a comprehensive four-year study of longan flowering induction in Hainan Island using combined KClO3, GA3, and KH2PO4 treatments. Foliar GA inputs can accelerate longan flower budding following the floral initiation from the KClO3 reaction. The transition of longan vegetative growth into reproductive growth could occur when auxin concentrations reduce in both mature leaves and shoots and at the same time GA3 concentrations increase in shoot tips. This biennial fruit tree can produce flowers expectedly with an interesting application of potassium chlorine and gibberellins for eliminating the cold requirement for flowering induction. When longan trees can flower irrespectively of the low temperatures, more leaf K/P nutrient absorption from foliar KH2PO4 applications can inhibit tree holding of chloride (Cl–) ions released from the KClO3 inputs, which further promote fruit formation. Leaf/shoot auxin and gibberellins, leaf photosynthesis, and macronutrient K/P absorption have contributed to longan tree flowering and fruit growth.