Relationship between the morphological structure of floral nectaries and the formation, transport, and secretion of nectar in lychee

Abstract

Key message

The sugar in nectar secreted from the floral nectaries of lychee ( Litchi chinensis Sonn.) was composed of both the phloem sap and the starch synthesized by the nectariferous tissue itself. Sugar hydrolyzed from the starch and from the phloem might be transported to the endoplasmic reticulum (ER) and the Golgi cisternae, further transported via ER and Golgi vesicles, and eventually released to the spaces between the plasmalemma and cell wall by exocytosis. There was a specific sugar transport and secretory mechanism in the lychee floral nectary.

Abstract

In insect pollination of lychee (Litchi chinensis Sonn.), nectar has an important role as a reward for the floral visiting of the insects. Ultrastructural changes during development and secretion of nectariferous tissue cells were further studied to understand the pathway and features of the formation, transport, and secretion of nectar in lychee floral nectaries. Mature lychee floral nectaries consisted of an epidermis, the underlying developed nectariferous tissue, and the vascular tissues distributed in the nectariferous tissue, which contained only phloem. Externally, there were specific secretory channels on the surface of the lychee floral nectaries. Internally, developed plastids, mitochondria, endoplasmic reticulum (ER), and Golgi bodies were presented in nectariferous tissue cells of the nectaries in full bloom. One or two large starch grains were observed in the plastids and most of the starch grains were in the state of degradation. Developed phloem vascular bundles connected the base to the top of the nectary. There were developed plasmodesmata between sieve tube elements and nectariferous tissue cells, as well as among the nectariferous tissue cells. During nectary development, a series of changes occurred in the internal ultrastructures. In the early stage nectariferous tissues, there were grana structures in the plastids, and several large starch grains appeared gradually. Before nectar secretion, folds appeared on the surfaces of the starch grains, the number of mitochondria greatly increased, and ER became well developed. At nectar secretion stage, numerous vesicles were observed between the plasmalemma and cell walls of the nectariferous tissue cells. Many multivesicular bodies formed, which could release large numbers of vesicles to the outside of the cell by the fusion with the plasmalemma. ATPase located mainly in mitochondria, ER, and cytoplasm at nectar secretion stage, and had higher activity than at the stage before the nectar secretion. After nectar secretion, starch grains were nearly invisible in the plastids, and pellets composed primarily of lipids formed in the plastids. From the base to the top of the nectaries, many specific cells which have polyphenols were distributed in sheets or in rows. The epidermal cells and two to three layers of cells beneath the epidermis also contained polyphenols. Interconnected intracellular spaces developed among the specific cells. We speculated that sugar was transported and secreted to the outside of the cells by exocytosis, the process by which multivesicular bodies and/or vesicles fuse with the plasmalemma individually. The nectar outside the cell was eventually transported to the top of the nectary via the apoplast pathway, which specific cells containing polyphenols formed intercellular spaces. Nectar was eventually secreted through the multiple specific secretory channels. During the secretion stage, transport and secretion of nectar required a large supply of ATP. An important relationship was observed between the large quantities of polyphenols present in the lychee floral nectary and protection of the nectary, as well as guiding the pollinating insects to find nectar.