THE ABSORPTIVE CAPACITIES OF BROMELIAD TRICHOMES

Tritiated glycine and leucine and microautoradiographic techniques were employed to determine whether various bromeliad species produce foliar trichomes capable of absorbing these two solutes from solutions placed on intact leaf surfaces. All trichomes borne by members of subfamily Tillandsioideae, whether on the leaf blade or sheath, accumulated both amino acids in substantial quantities. Trichomes located on the leaf sheaths of tank-forming members of Bromelioideae absorbed much smaller quantities while trichomes located on the blades of the same tank-producing bromelioids, as well as those of nontank-forming members of Pitcairnioideae and Bromelioideae, took up little or none at all. The evolution of the bromeliad trichome is considered in the light of these results. AMONG FAMILIES of flowering plants, Bromeliaceae is noteworthy for its diverse habitat preferences and for the ability of many of its species to flourish in arid, nutrient-deficient environments (Pittendrigh, 1948; Benzing, 1973). Numerous bromeliads are epiphytic, some living in dry forests while others are mesic or xeric terrestrials. Epiphytic species adapted to very xeric sites have been designated "atmospherics" because they characteristically lack the tank leaves featured by "cistern" bromeliads and have either nonabsorptive holdfast roots or none at all. As such, atmospherics must extract all required mineral nutrients directly from rainfall and other highly dilute solutions when these fluids pass over the shoot during periods of precipitation. It has been recognized for many years that the foliar trichome is the major organ of moisture absorption in species belonging to the largely epiphytic subfamily Tillandsioideae-atmospheric Tillandsia usneoides, for example (Haberlandt, 1914). The assertion has been made, but never tested by appropriate modern analyses, that this epidermal structure is involved in solute accumulation as well. The involvement of bromelioid and pitcairnioid (members of ecologically less specialized subfamilies Bromelioideae and Pitcairnioideae, respectively) trichomes in either moisture or salt absorption has received even less attention. Recent studies on the uptake of various mineral nutrients by bromeliad leaves representing all three subfamilies demonstrated that foliar permeabilities were high in some species and much ' Received for publication 25 August 1975. Much of this effort was supported by National Science Foundation grants to the first author and a National Science Foundation Undergraduate Research Participation grant to the Oberlin College Departments of Biology and Chemistry. lower in others (Benzing and Burt, 1970). These differences were associated with the type of trichome present and with its frequency of distribution over the leaf surface, with habitat preference and, in tank species, with the location of the trichomes on the leaf (blade or sheath). Foliar uptake was most pronounced in atmospheric species. In the following investigation, microautoradiographic analysis by use of tritiated leucine and glycine was employed to determine which, if any, of the foliar trichomes of selected members of all three subfamilies and the major ecological types within Bromeliaceae could be major sites of solute accumulation by these species. Although free amino acids are not abundant in foliar leachates (Tukey, 1970), they are present and therefore may be at least minor sources of nitrogen. In any case, tritiated amino acids can be employed to demonstrate absorption by bromeliad trichomes of a solute from solutions situated on the intact leaf surface. MATERIALS AND METHODS-All tissues were obtained from material grown in the Oberlin College greenhouse. Reference specimens are on deposit in the Oberlin College Herbarium. Leaves were excised in midmorning and for 0.5 hr were maintained in aerated distilled water. In some cases samples were killed by plunging them in boiling water just before the treatment solution was applied. After an appropriate pretreatment, samples designated to receive a one-half hour or longer contact with a particular treatment solution were placed in an apparatus previously utilized which permitted contact of the desired leaf surface with 10 ml of treatment solution (Benzing, 1970). All treatment solutions contained either L-leucine4,5-3H or glycine-2-3H at 10 Ac/ml, 10-5 M CaC12