Water relations

Chapters 2 and 4 describe the foliar scale of Tillandsioideae as a peltate-shaped organ comprised of a shield or plate of usually dead cells anchored to the epidermal basement by a living stalk (Fig. 2.7A,B). Briefly, four large, equal-sized central cells dominate the center of the shield and secure it to the dome cell, which constitutes the distal member of that subtending, uniserrate stalk. Extraordinarily thick tangential walls of the central disc alternately rise and fall on flexible radial walls as precipitation and evaporation alternately fill and empty the underlying lumina. Several additional rings of cells, each made up of twice as many members as the one within, surround the four central cells. An outermost, asymmetrical wing contains many more and much more elongated cells than those within (Fig. 2.7D). Absorption occurs while ion-charged fluids contact those parts of the shoot bearing trichomes.

Ultrastructure reveals that the dome cell is well equipped to mediate ion uptake. Moreover, its boundary lies just microns away from the nutrient-charged fluids that periodically to continuously engorge the central disc. Diverse organelles, especially rough endoplasmic reticulum, dictyosomes, microtubules and mitochondria, densely fill the protoplast (Dolzmann 1964, 1965). An elaborately folded plasmalemma characteristic of plant transfer cells assures intimate contact with abundant electron-dense material located just within the cell wall (Brighigna et al. 1988). Concentrations of plasmodesmata at every junction along the stalk allow extensive communication with the mesophyll. A parallel apoplastic conduit also seems likely, given the absence between cells of the cuticle that invests the outer walls of the stalk.

Several workers, including Haberlandt (1914) and Mez (1904), demonstrated that hypertonic salt solutions applied to intact leaf surfaces plasmo-lyze the mesophyll cells adjacent to the bases of the affected trichome stalks. Vital stains followed the same route, but more compelling evidence of the involvement of the foliar indumentum in nutrition would require the more sophisticated techniques that would not become available for many more decades (e.g., Benzing et al. 1976; Ehler 1977; Owen et al. 1988). Water relations would also attract continuing attention. For example, Brighigna et al. (1988) noted little difference in the ultrastructure of the stalk cells of Tillandsia usneoides whether leaves had been fixed following incubation for eight days at 80% relative humidity or desiccated up to 23% over silica gel. Foot cells of the better-hydrated samples contained larger vacuoles. These investigators also reported that the shields of mature tri-chomes sometimes retain their protoplasts, a rarely reported condition that challenges the classic explanation of how the foliar scale operates as a oneway valve.

Rather than beading up, a drop of moisture placed on the leaf of a Type Five Tillandsia spreads from one trichome shield to the next, in turn, causing the upper walls of the central discs to rise and the wings to flatten as the lumina fill with water (Fig. 2.7A,B). Mez (1904) imputed an accompanying suction mechanism (hence his term 'trichomepump') during engorgement without demonstrating that force. Moisture subsequently fluxes from the charged shield into the dome cell and on to the mesophyll until either the water potentials inside and out equilibrate or the indumentum dries. In the second case, the lumina of the central disc collapse, restoring the barrier provided by the thickened walls of the central disc. So configured, the shield prevents water from wicking out the leaf along the path of entry. Reflexed upward, the wing again scatters light, and the silvery, rough texture that highlights the shoots of Type Five Bromeliaceae returns.

In effect, trichomes of the type that invest the foliage of Type Five Tillandsioideae serve as one-way valves and energy dissipaters, alternately hydrating the plant and insulating it against water loss, photoinjury and excess heating. Controversy continued for many years over whether tri-chomes appreciably amend water deficits from adjacent moist air (Chapter 4). Plants do gain moisture by this route, but not enough to replace the need for contact with liquid moisture (Garth 1964; De Santo et al. 1976; Benzing and Pridgeon 1983; Martin and Schmitt 1989; Figs. 4.12, 4.22).

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