Influence of light on the apoplastic ph in microwounded cells of Chara corallina

Microscopic wounding of plant cell walls by pathogens or by feeding insects triggers the defense responses, including a sharp rise in pH at the cell surface (pH(o)). Using internodal cells of Chara corallina Klein ex Willd., we show here that the elevated pH(o) in the area of cell wall microincision decreases in darkness and increases on illumination. These pH(o) changes occurred specifically in cell areas affected by microincision and were lacking in intact areas with active pHotosynthesis (acid zones). Localized illumination of a remote cell region located upstream the cytoplasmic flow at a 1.5-mm distance from the analyzed area also caused a transient increase in pH(o) in the area of microwounding but had no such effect in unwounded cell regions having weakly acidic pH(o). Apparently, the increase in pH(o) after wounding is mediated by a metabolite released from illuminated chloroplasts, which is transported with the cytoplasmic flow for long distances. The transient pH(o) increase in the area of cell wall incision after illumination of a distant cell region coincided with a temporal increase in chlorophyll fluorescence F'. This implies the concurrent influence of the transported reductant (presumably NADH) on light emission of chloroplasts and on the H+ flow across the plasmalemma. We suppose that the alkalinization of cell surface in the area of microincision arises from H+ consumption in the apoplast in association with the transmembrane electron transport from cytoplasmic reducing equivalents to molecular oxygen.