Leaf Surface Electrostatics: Response of Detached Leaves of Beans and Maize to Humidity and Red-Infrared Radiation Under Controlled Conditions

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Abstract
The electrical fields associated with leaf surfaces of beans (P. vulgaris) and maize (Z. mays) were measured under controlled conditions to determine if detached leaves become charged and whether this relates to the environmental factors that trigger active [fungal] spore release. Electrical field intensities were measured with the measuring instrument (field mill) positioned 10 mm above the adaxial surfaces of leaves placed within a special apparatus. In saturated still air, leaf electrical field intensities were high (2700 V cm-1 maximum); however, any movement of the saturated air caused a rapid loss of surface charge. A saturated air velocity as low as 0.07 m/s caused a slight loss of charge, with much greater losses occurring at velocities of 0.21 m/s and higher. When leaves in saturated still air were briefly connected to an electrical ground, electrical field strength diminished rapidly but was partially regained when the ground was disconnected. In a moving airstream (1 m/s), highest field intensities (100-500 V cm-1) were measured whenever the leaves were subjected to low humidities (35-50% RH [relative humidity]), with lowest intensities (near 0 V) in a saturated airstream (100% RH). Exposure of leaves to red-IR radiation in a saturated airstream had a profound effect on field intensities; e.g., in one experiment, an 11-min exposure caused a 312 V cm-1 increase that rapidly diminished when the IR lamps were switched off. If IR was transmitted through water (20 cm deep), it lost much of its effectiveness in causing changes in field intensity. Repeated exposure of leaves to IR in a moving, saturated airstream indicated that the IR had caused relatively long-lasting changes in the electrical characteristics of leaf surfaces. The polarity of leaves detached from growth chamber plants was consistently positive, whereas leaves from mature field-grown plants collected during a period of unsettled weather were all negative. The reason for these polarity differences is not yet known.

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