Transformation of protochlorophyllide to chlorophyllide in wheat under heavy metal stress

Abstract

In angiosperms, the photoreduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is a light triggered reaction. In darkness, Pchlide is accumulated within the etioplast membranes. Only Pchlide associated with protochlorophyllide oxidoreductase can be phototransformed to Chlide, whereas the free Pchlide is photoinactive. In this study the Pchlide to Chlide photoreduction in etioplast inner membranes (EPIM) isolated from 6-day old etioladed wheat leaves was investigated. Metal ions (cadmium II, chromium VI or iron II) in the concentrations of 0.001 - 0.01 M were added to the EPIM suspension. 77 K fluorescence emission spectra were recorded after different time of metal action and fluorescence intensities at 655 (F655) and 633 (F633) nm from the photoactive and photoinactive Pchlide, respectively, were compared. The F655/F633 ratio of a control sample which was taken as 100 %, changed less than 15 % within 1 hour. Upon addition of metal ions 0.01 M) this ratio immediately decreased by 60 % (Cd) and by 10 % (Cr and Fe). During subsequent incubation F633 further increased whereas F655 decreased. Depending on metal species the F655 peak disappeared completely after several minutes (Cd), 1 h (Cr) or 6 h (Fe). When metal ions were added to EPIM suspension supplemented with 0.2 mM NADPH the F655 peak was considerably higher than in the absence of NADPH, however, its decrease with time was still observed. From these results we conclude that the toxicity of the investigated metals on the Pchlide to Chlide photoreduction can be ordered as Cd > Cr> Fe.