Sugar Receding in Aril Benefits the Recalcitrant Seeds of Litchi (Litchi chinensis) and Longan (Dimocarpus longan) to Cope with Dry Spells after Maturation

Abstract

Litchi and longan are tropic/subtropic fruits harboring recalcitrant seeds that are covered with sugar-rich arils as the edible part. A rapid sugar content decline, called sugar receding, in the arils after the fruit maturation largely reduces the fruit quality, however, the mechanism is unclear. Litchi and longan fruits grow and mature in a hot and rainy season with dry spells between rainfalls. Here, we found that the seed maturation was around 2 weeks earlier than the fruit maturation, after which the sugar contents of the arils significantly decreased, while the fruits could stay on the tree for 1–2 months with high seed vigor. A continuously low-level fruit drop after the seed maturation resulted in continuous seed germination and seedling establishment in around 2–3 months. Blocking assimilate supply by storage of detached fruits or on-tree girdling-plus-defoliation for 7 days, the aril sugar contents of the treated fruits significantly decreased, while the sugar levels and vigor of the seeds increased, implying that the arils are sugar sources to maintain or even enhance the seed vigor and suggesting that sugar receding in arils benefits the recalcitrant seeds of litchi and longan to cope with dry spells after maturation.