Pathways for Nutrient Transport in the Pitchers of the Carnivorous Plant Nepenthes alata

Abstract

Glands of the carnivorous pitcher plant Nepenthesalata are active in transport of materials into and out of the pitcher lumen, indicating dual functions in both secretion and absorption. This study examined the potential for open transport through these glands using the ultrastructural tracer lanthanum, which is restricted to the apoplast, and the fluorescent symplastic tracer, 6(5)carboxyfluorescein. Glandular uptake of lanthanum from the pitcher fluid occurred through the outer cell wall between irregularly spaced cutinized deposits, but was blocked from entering the underlying mesophyll cell walls by thick endodermal-like regions. Similarly, lanthanum localization showed an open apoplastic pathway from the petiole to the endodermal regions in the gland base. Thus, transport of materials into or out of the gland must occur through the symplast. 6(5)Carboxyfluorescein showed that these glands transport fluids directly from the pitcher fluid into vascular endings immediately beneath them via a symplastic route. When applied to the petiolar vascular system, the fluorescent tracer freely entered immature pitchers, but was blocked from entering the lumen of the mature pitcher by an endodermal zone. An ultrastructural survey showed infrequent pits with plasmodesmatal connections to adjoining subepidermal cells. These results indicate that the function of the gland is developmentally regulated. Prior to maturity, the primary function of the gland appears to be secretion. However, at maturity, secretion is blocked by an endodermal layer, which limits the function of the gland to absorption. These studies support the theory that the glands of Nepenthesalata are specialized for the bi-directional transport of materials.