Quantitative Assessment of Symplastic Transport of Pre-Nectar Into the Trichomes of Abutilon Nectaries

Abstract

The anatomy and ultrastructure of the stalk cell region of Abutilon megapotamicum var. variegatum floral nectary trichomes has been investigated and the observations related to estimates of the rate of secretion of nectar. Evidence was obtained that an apoplastic barrier exists in the anticlinal walls of the stalk cell, and is taken to indicate that a flow of pre-nectar, equivalent in volume to the nectar that is secreted from the apex of the trichome, passes through the stalk cell protoplast. In terms of volume of water, the plasma membrane of the stalk cell might have a large enough hydraulic conductivity to cope but, in terms of sugar transport, the required flux is 3-4 orders of magnitude greater than literature values for plant cell membranes. By contrast, the plasmodesmata in the distal periclinical wall of the stalk cell provide a low resistance pathway for a bulk flow of sugar solution. The desmotubules of the plasmodesmata are clearly connected to the endoplasmic reticulum of the adjacent cells. The cytoplasmic annulus between the desmotubule and the inner face of the plasma membrane is (in the majority of cases) parallel sided and non-constricted. The hydraulic conductivity of the cytoplasmic annulus route through the plasmodesmata is 1.4 × 10^-5 m³ m^-2 (of distal wall) s^-1 bar^-1; that of the hypothetical desmotubular route is about 2.3 × 10^-6 in the same units. These conductivities are 4.5 and 45 times greater than corresponding values for root endodermal plasmodesmata reported in the literature. The results are discussed in relation to the physiology of nectar production in Abutilon.