Localization of Thallium in Stomata is Independent of Transpiration

Abstract

The specific locations of the evaporating surfaces in the leaf are not yet known. However, Maercker (1965) repeated successfully the early experiments of Stahl (1894) and concluded from her evidence that the cuticular component oftranspiration could be further divided into "cuticular" and "peristomatal" transpiration, where the latter term refers to transpiration from the surfaces of the guard cells and their subsidiary cells. Peristomatal transpiration has been the subject of considerable recent interest (e.g. Lange et al. 1971).Using the method of Stahl (1894), Maercker (1965) placed cut leaves of Zea mays into 3% thallium sulphate solution and allowed the leaves to transpire in light or darkness, and in either normal air or CO2-depleted air. Under conditions which led to stomatal opening (Le. in light or in darkness in CO2-free air), TI+ was found exclusively in the guard cells, whereas it was located exclusively in the subsidiary cells of leaves with closed stomata. This evidence, in conjunction with experiments using tritiated water, led her to suggest that the terminal site of transpiration was in the guard cells when stomata are open and in the subsidiary cells when stomata are closed. Raschke (personal communication) suggested, however, that these observations merely indicate that Tl+ ions follow the shuttle of K + between the guard and subsidiary cells which occurs during stomatal movement in Z. mays (Pallaghy 1971;Raschke and Fellows 1971). The results briefly reported in this communication prove that TI+ cannot be employed as an indicator to locate the terminal sites of transpiration. Results and DiscussionThe leaf material (Z. mays) used was that described by Pallaghy (1971). Experiments, lasting 2-5 hr, were carried out using 0'5% thallium sulphate solution. Leaves were kept in distilled water for about 15 min prior to each experiment. Localization of Tl+ in the leaf epidermis was observed as a black precipitate of thallium chloride when the leaf was submerged for about 5 min into a solution of 3% NaCI in 50% methanol. Control experiments were carried out in the manner according to Maercker (1965). Similar experiments, but employing either normal or CO2-free aerated solutions, were carried out with leaves entirely submerged in the solution to prevent transpiration.The results obtained with both transpiring and submerged leaves (Fig. 1) were similar to those described by Maercker (1965), proving that Tl+ localization is independent of transpiration. The method of micro-autoradiography using tritiated water to localize transpiration sites near the stomatal pore is also subject to doubt. The topography of the stomatal apparatus is considerably different between the open and closed states, as has been well demonstrated in an extreme situation for Z. mays * Manuscript

Effects of Cadmium, Thallium, and Vanadium on Photosynthetic Parameters of Three Chili Pepper (Capsicum annuum L.) Varieties

“…Furthermore, Tl produced more drastic effects than did Cd, Ni, and Pb. Being isomorphic to K, Tl tends to mimic its movement in guard and companion cells, responsible for stomata opening and closing [45]. This characteristic places Tl as the metal that most inhibits stomatal opening with respect to other metals studied.…”