Diffusive Resistance, Titratable Acidity, and CO 2 Fixation in Two Tropical Epiphytic Ferns

Abstract

Crassulacean acid metabolism (CAM) is known to occur in many succulent plant species (Ranson & Thomas 1960; Ting et al., 1972; Wolf, 1960). To date, more than 184 plant species have been reported to exhibit CAM features, but none of them are ferns (Szarek, pers. comm.). Recently we reported nocturnal assimilation of C02 by Drymoglossum piloselloides (L.) Presl, an epiphytic fern (Hew & Wong, 1974). This paper presents further evidence to support our previous findings that certain epiphytic ferns do exhibit characteristics of CAM plants. Two epiphytic ferns, Pyrrosia longifolia (Burm.) Morton and Drymoglossum piloselloides, were chosen for the present investigation. These ferns are found frequently on the lower part of Acacia tree trunks. Gleichenia linearis (Burm.) Clarke, a terrestial sun fern which has been shown to be a C3 plant (Hew & Wong, 1974), also was included in the study. All three ferns grow wild around the Nanyang University campus. For comparison purposes, the flowering plant Kalanchoe pinnata, a known CAM plant, was also used as experimental material. For 14C02 fixation studies and determination of titratable acidity, detached leaves or fronds were used; the method for 14C02 fixation has been described previously (Wong & Hew, 1973). Titratable acidity of plant tissues was determined as described by Szarek and Ting (1974), except the leaf or frond extract was titrated to a pH7 end point. Diffusive resistance of intact fronds or leaves were measured at three hour intervals using an Li-60 Diffusive Resistance Meter (LiCor Limited). Titratable Acidity.-The diurnal changes in titratable acidity of Drymoglossum, Pyrrosia, and Kalanchoe were similar (Figs. 1 -3). Titratable acidity decreased in the light, and at night the acidity increased. The magnitude of dark acidification in these three species was comparable to that previously reported (Bruinsma, 1958; McWilliams, 1970; Szarek & Ting, 1974). Among the three species, titratable acidity was highest in Kalanchoe, both in light and darkness. There was no significant difference in titratable acidity between the two ferns. In contrast, Gleichenia (Fig. 4), which is a C3 plant, shows no diurnal fluctuation in titratable acidity. Diffusive Resistance.-Changes in diffusive resistance patterns of intact Pyrrosia fronds (Fig. 5) and Kalanchoe leaves (Fig. 6) in the day and at night were similar, with high diffusive resistance in the day and low at night. The values for minimum diffusive resistance (5-15 secocm-1 ) and maximum diffusive resistance (100-120 sec*cm ) also were in agreement with that of other succulent plants (Szarek & Ting, 1974; Ting et al., 1972). From the changes in patterns, one could conclude that Pyrrosia and Drymoglossum stomata were closed in the day and open at night (Nishida, 1963; Ting et al., 1972). A point worth noting is that with the onset of darkness, an increase in diffusive resistance in both Pyrrosia and