Melatonin was found in the fresh water characeae Chara australis. The concentrations (~4 μg/g of tissue) were similar in photosynthesizing cells, independent of their position on the plant and rhizoids (roots) without chloroplasts. Exogenous melatonin, added at 10 μM to the artificial pond water, increased quantum yield of photochemistry of photosystem II by 34%. The increased efficiency appears to be due to the amount of open reaction centers of photosystem II, rather than increased efficiency of each reaction center. More open reaction centers reflect better functionality of all photosynthetic transport chain constituents. We suggest that melatonin protection against reactive oxygen species covers not only chlorophyll, but also photosynthetic proteins in general.

This paper investigates the impact of increased salinity on touch-induced receptor and action potentials of]ext was high, the background conductance increased to a lesser extent and proton pump currents were stimulated, establishing a PD narrowly negative to APthreshold. Cells did not spontaneously fire, but became hypersensitive to touch. Even slight touch stimulus induced an action potential and further repetitive firing. The duration of each excitation was extended when [Ca 2+ ]ext was low. Cell viability was prolonged in the absence of touch stimulus. Chara cells eventually depolarize and die in the saline media, but touch-stimulated and spontaneous excitation accelerates the process in a Ca 2+ -dependent manner. Our results have broad implications for understanding the interactions between mechano-perception and salinity stress in plants.

Chara australis cells exposed to media of pH 10 and above exhibit high conductance, arising from the opening of H + /OH -channels in the plasma membrane. This high conductance can be totally inhibited by 1.0 mM ZnCl2 and restored by 0.5 mM 2-mercaptoethanol (ME). Important for carbon fixation, H + /OH-channels play a key role in cell pH banding. Banding was also shown to be abolished by 1.0 mM ZnCl2 and restored in some cells by ME. The proton pump is also involved in banding, but was little affected by ZnCl2 over the periods needed for the inhibition of H + /OH-channels. Previously, we postulated that H + /OH-channels open transiently at the onset of saline stress in salt-sensitive C. australis, causing membrane potential difference (PD) noise; and remain open in latter stages of saline stress, contributing to cell deterioration. ZnCl2 totally inhibited the saline noise and the upwardly concave I/V characteristics associated with the putative H + /OH -currents. Again, ME reversed both these effects. We discuss the mode of action of zinc ions and ME with reference to animal voltage-gated H + channels and water channels.

Salt sensitive Characeae Chara australis responds to 50 mM NaCl by a prompt appearance of noise in the trans-membrane potential difference (PD). The noise diminishes with time in saline and PD depolarization, leading to altered current-voltage characteristics that could be modeled with H(+)/OH(-) channels. Beilby and Al Khazaaly (JMB 230:21-34, 2009) suggested that the noise might arise from cooperative transient opening of H(+)/OH(-) channels. Presoaking cells in 10 μM melatonin over 24 h abolished the noise in some cells, postponed its appearance in others or changed its characteristics. As melatonin is a very effective antioxidant, we postulated opening of H(+)/OH(-) channels by reactive oxygen species (ROS). Measurement of ROS using dihydrodichlorofluorescein diacetate confirmed substantial reduction in ROS production in melatonin-treated cells in saline and sorbitol media. However, ROS concentration decreased as a function of time in saline medium. Possible schemes for activation of H(+)/OH(-) channels under salinity stress are considered.

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