Photosynthetic responses of soybean (Glycine max L.) to heat-induced electrical signalling are predominantly governed by modifications of mesophyll conductance for CO(2).

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 CO(2) 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 CO(2) conditions proved the transient reduction of A(N) , ETR, the chloroplast CO(2) concentration (C(c) ) and g(m) to be independent of the external CO(2) regime, whereas the effect of the electrical signal on stomatal conductance for CO(2) (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) .