ACTA BIOLOGICA CRACOVIENSIA Series Botanica 47/2: 111-119, 2005

Ultrastructure of the Nectary Spur of Platanthera chlorantha (Custer) Rchb. (Orchidaceae) During Successive Stages of Nectar Secretion

Malgorzata Stpiczyńska1*, Claudio Milanesi2, Claudia Faleri2, and Mauro Cresti2 

1Department of Botany, University of Agriculture, 
ul. Akademicka 15, 20–905 Lublin, Poland
2 Department of Environmental Science “G. Sarfatti”, Siena University, 
via P.A. Mattioli 4, 53 100 Siena, Italy 

*e-mail: malgorzata.stpiczynska@ar.lublin.pl

Received March 24, 2005; revision accepted: July 5, 2005

In Platanthera chlorantha, nectar is secreted into the lumen of the nectary spur by numerous unicellular hairs. These hairs arise from epidermal cells lining the spur. The nectary was studied in the presecretory (closed bud, 4 days before anthesis), secretory (day 2 of anthesis) and resorption (days  14–16 of anthesis) stages to compare the ultrastructure of the cells during various stages of nectary activity. Plant tissue was fixed for conventional TEM or prepared by freeze-substitution. Secretory cell ultrastructure was observed to change significantly during the various stages of nectary activity. During the presecretory stage, the cells have a large nucleus, dense, granular cytoplasm with numerous mitochondria and ER, and several small vacuoles. Plastids within secretory epidermal cells or subepidermal parenchyma enclose starch, but starch is absent throughout nectar secretion and resorption. During the secretory stage, plastids with a dense stroma contain tubules enclosing osmiophilic material. Abundant dictyosomes and secretory vesicles occur in the cytoplasm, particularly near the plasmalemma, indicating that granulocrine secretion operates in this species. During the resorption stage, most cells still have dense cytoplasm with numerous mitochondria and ER, but dictyosomes are uncommon. Vesicles are present adjacent to the cell wall; they are similar in size and occur in similar numbers to those seen during the secretory stage. They are probably formed by endocytosis. The ultrastructure of the cells prepared by freeze-substitution compared favorably with that seen in cells subjected to conventional chemical fixation.

Key words: Platanthera chlorantha, Orchidaceae nectary, nectar secretion, nectar resorption, ultrastructure, freeze-substitution.