TISSUE ELECTROPOTENTIALS IN KALANCHOË BLOSSFELDIANA DURING WOUND HEALING

Penetrating palladium probes were placed in the stem and leaf of Kalanchoe blossfeldiana. The electropotentials arising in the probes at the wound site were continuously monitored during the subsequent healing process. The potential change was always in the positive going direction, beginning at an average of 320 millivolts and stabilizing at an average of 601 millivolts relative to a standard hydrogen electrode. The healing process can be divided into two distinct phases: an initial phase of approximately 6 hr duration in which the potential rises rapidly and a second phase extending to approximately dusk on the third day in which the potential rises at a substantially lower rate. There is also a definite change of the transition potential waveshape at the beginning and end of the 16-hr light period during the healing process. Rewounding results in a potential recovery waveshape similar to the original healing response but of shorter duration. The data are interpreted in terms of the redox status of the wound site. THIS STUDY iS concerned with the time change in the tissue electropotentials after the insertion of a palladium probe into the stem and leaf of Kalanchoe blossfeldiana. While there is no previous literature which considers this physical situation, there is abundant work on the time course of the anatomy and biochemistry of the repair mechanism after various portions of the plant have been cut or abraded (Wylie, 1930a, b, 1931; Bloch, 1941, 1952; Lipetz, 1970). The time course of wound healing is anatomically divided by Tucker (1975), following a previous delineation by Wylie, into three phases: the formation of the pseudocicatrice, the formation of the cicatrice proper, and the formation of the periderm. Wylie (1930b) observed in deciduous leaves the formation of a cicatrice 3-4 days after wounding, with completion in about 10 days. Lipetz (1967) observed the first mitosis 36 h after wounding in Kalanchoe diagremontiana. Jones and Villiers (1972) observed the first evidence of new cell wall formation 4 days after wounding. Sussex, Clutter, and Goldsmith (1972) observed the first division of pith cells of tobacco stems 3 days after wounding. Welch (1945) observed this same timing with Bryophyllum calycinum. The time course of wound healing has been examined at a genetic level as well. Borchert and McChesney (1973) traced the level of DNA synthesis in potato tubers and observed an 8-h latency, a rise and peak in 14-18 h, then a fall to normal in 24 h, at which time the earliest cell divisions occurred. These latter workers drew the same spatial distinctions as Wylie concerning the wound healing ' Received for publication 4 April 1977; revision accepted 10 October 1977. site. It is interesting to note that Lipetz (1970), in a recent review on wound healing in plants, states that it is impossible to draw any convincing overall schemes explaining wound healing. The specific objective of the present study is to examine the timing of the d.c. electropotential value and the evolution of the transition potential waveshape during the healing period. The electropotential is a measure of the redox state of the wound site (Glasstone, 1942; Cater and Silver, 1961; Lehninger, 1970). A determination of the redox state as a function of time throughout the major portion of the healing process could provide a coherent experimental measure of the underlying biochemical changes. MATERIALS AND METHODS-Two architecturally similar cultivars of Kalanchoe blossfeldiana were used: Gelbe Melodie and Type D, Sunnyside Nursery, Hayward, CA. Plant height was approximately 15 cm. Probes were placed in three locations: in the stem between the second and third node, in the basipetal section of the second or third apical leaf contiguous to the midrib, and next to the midrib at its outermost extension in the leaf. Approximate leaf size was 4 cm long by 3 cm wide. The plants themselves were placed in 10-cm diam pots filled with vermiculite which were in turn immersed in a modified Hoagland's hydroponic nutrient solution (O'Leary and Prisco, 1970). Air was continuously bubbled through the solution. The plants were propagated from cuttings in a growth chamber and then transferred to a second growth chamber held at 26 C ? I C. Light level in the second growth chamber was 17,000 lux