Abstract
Stimulation of the stigma of Hibiscus flowers by pollen, wounding (heat), or cold shock (4[deg]C) evokes electrical potential changes in the style, which propagate toward the ovary with a speed of 1.3 to 3.5 cm s-1. Potential changes were measured intracellularly by microelectrodes inserted in the style. The resting potential ranged from -90 to -112 mV (n = 20) in cells of the vascular tissue and from -184 to -220 mV (n = 22) in cells of the pollen-transmitting tissue. The amplitude of the potential changes was between 40 and 150 mV, depending on the kind of stimulus. Self- as well as cross-pollination hyperpolarized the resting potential after 50 to 100 s, followed by a series of 10 to 15 action potentials. In contrast, cooling of the stigma caused a single action potential with a different shape and duration, whereas wounding generated a strong depolarization of the membrane potential with an irregular form and a lower transmission rate. To determine the physiological function of the different signals measured in the style, the gas exchange and metabolite concentrations were measured in the ovary before and 10 min after stimulation of the stigma. Self- and cross-pollination caused a transient increase of the ovarian respiration rate by 12%, which was measured 3 to 5 min after the stigma was stimulated. Simultaneously, the levels of ATP, ADP, and starch increased significantly. In contrast, both cold shock and wounding of the stigma caused a spontaneous decrease of the CO2 content in the measuring chamber, as well as reduced metabolite concentrations in the ovary. Since the transport of labeled auxin from the top to the base of the style lasts at least 45 min, the influence of a chemical substance transmitted within 10 min is unlikely. Thus, our results strongly support the view that different, stimulus-dependent electrical signals cause specific responses of the ovarian metabolism.
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