Development and Fine Structure of the Glandular Trichomes of Artemisia annua L.

Development of capitate glands on the leaves of annual wormwood (Artemisia annua L.) was monitored with scanning and transmission electron microscopy. Differentiation of foliar cells into gland cells began in the youngest leaf primordia. After differentiation into a 10-celled biseriate structure of two stalk cells, two basal cells, and three pairs of secretory cells, the cuticle of the six secretory cells separated from the cell walls to form a bilobed sac that eventually splits to release its contents. At every developmental stage, the cells of the the gland contained relatively little vacuolar volume. The secretory cells contained extensive endoplasmic reticulum. The plastids of each cell pair were different. At maturity, the apical cells contained proplastids or leucoplasts with only occasional thylakoids. The cell pair below the apical cell pair contained amorphous chloroplasts without starch grains. The basal cell pair contained proplastids or leucoplasts and the stalk cells contained chloroplasts. The stroma to thylakoid ratio in the secretory cell chloroplasts was high. Initially, osmiophilic product was observed most frequently associated with stacked thylakoids, plastid envelopes, and smooth endoplasmic reticulum, although it was associated with all cell membranes. Near the plasma membrane adjacent to cell walls bordering the subcuticular space, the cytoplasm was enriched in smooth endoplasmic reticulum containing osmiophilic material. The apical cell wall of the apical secretory cell pair was reticulated on the inner cytoplasmic side and contained osmiophilic staining on the cuticular side. During early senescence, osmiophilic product was commonly associated with outer mitochondrial membranes.