Leaf bioelectric responses to rhythmical light: Identification of the contributions from stomatal and mesophyll cells

Bioelectric responses of plant leaves to rhythmical variation of irradiance were measured using an extracellular electrophysiological technique. The 20 species studied differed in their metabolism, morphology and anatomy. They included C₃ or C₄ photosynthesis, mono- or dicotyledon, cultivated and weed species. For corn, 15 different lines and hybrids were studied. All of them responded in a frequency-dependent manner to rhythmical light. Using a resonant analysis approach, three resonant modes were identified. The resonant response with the greatest amplitude was observed for light/dark periods of about 40 min. It was associated with stomatal movements and originated from ion exchange of the guard cells. Another resonant mode was at a period about 8 min. It seems to reflect the feedback interaction involving the membrane potential and the cytosolic pH in mesophyll cells. The third resonant mode, of about 1.5 min period, may link with thylakoid-related processes. The general feedback model of these leaf responses to light is proposed and analysed, and the possible contribution of its components is discussed. It is concluded that the resonant analysis approach may be used to distinguish between the contributions of stomatal and mesophyll cells to the composite bioelectric response measured on the leaf surface. Stomatal cells are the major contributors in a frequency range of tens of minutes; the mesophyll components have smaller time constants and their contribution is substantial at higher frequencies.