Photosynthetic responses of soybean (Glycine max L.) to heat-induced electrical signalling are predominantly governed by modifications of mesophyll conductance for CO₂

In recent years, the effect of heat-induced electrical signalling on plant photosynthetic activity has been demonstrated for many plant species. However, the underlying triggers of the resulting transient inhibition of photosynthesis still remain unknown. To further investigate on this phenomenon, we focused in our present study on soybean (Glycine max L.) on the direct effect of signal transmission in the leaf mesophyll on conductance for CO₂ diffusion in the mesophyll (g_m) and detected a drastic decline in g_m following the electrical signal, whereas the photosynthetic electron transport rate (ETR) was only marginally affected. In accordance with the drop in net photosynthesis (A_N), energy dispersive X-ray analysis (EDXA) revealed a shift of K, Mg, O and P on leaf chloroplasts. Control experiments under elevated CO₂ conditions proved the transient reduction of A_N, ETR, the chloroplast CO₂ concentration (C_c) and g_m to be independent of the external CO₂ regime, whereas the effect of the electrical signal on stomatal conductance for CO₂ (g_s) turned out much less distinctive. We therefore conclude that the effect of electrical signalling on photosynthesis in soybean is triggered by its immediate effects on g_m. Little is know about the interaction of electrical signals and photosynthesis, especially with regard to the physiological processes involved. In the present study we analysed the effect of heat induced electrical signalling on photosynthesis parameter in soybean plants at high temporal resolution. Our results demonstrate that mesophyll conductance for CO₂ was directly and immediately affected by the electrical signal, leading to a transient and marked drop of net photosynthesis, while impairment of photosynthetic electron transport was of minor relevance. This study shows for the first time a link between electrical signalling and mesophyll conductance, and thus provides new insights in the immediate physiological responses to stimuli like heat or wounding.