Effect of As on Chlorophyll and Protein Contents and enzymic Activities in Greening Maize Tissues

Arsenic is known to have several phytotoxic effects and one of them is the reduction inchlorophyll content. Determination of chlorophyll content is often accomplished to assess theimpact of most environmental stresses, as the pigment content is linked to the visual symptomsand photosynthetic plant productivity. Hence, the present investigation was undertaken to studythe effect of As on chlorophyll synthesis with a view to gain some insights into the possiblemechanism. Supply of 0.01 to 1.00 mM Na arsenate to the greening maize leaf segmentsdecreased the chlorophyll content as well as chlorophyllase activity, but decline in former wasmore pronounced than latter at high concentration of As. Supply of arsenate also reduced totalRNA, protein and acid soluble thiol content of the tissue. The inhibitory effect of As was lesspronounced in the beginning i.e. upto 4 h, but became substantial with increasing time periods.Nitrate reductase activity was inhibited more strongly than chlorophyll formation and was inducedonly slightly by the supply of inorganic nitrogen in the presence of As. Inclusion of KNO3 hadalmost no effect on inhibitory effect of As on chlorophyll formation, while NH4Cl and NH4NO3substantiated it. Percent decrease in chlorophyll content was reduced by 2-Oxoglutarate,glutamine, glycine and Na succinate, but it was increased by MgCl2 and remained unaltered byglutamate. Amongst the thio compounds tested, cysteine and 5, 5' dithio bis 2 nitrobenzoic acid could overcome the toxic effect of As partially, while mercaptoethanol anddithiothreitol enhanced it and reduced glutathione had no effect. It is suggested that toxic effect ofAs on chlorophyll biosynthesis involves limited availability of δ-aminolevulinic acid andthiol modulation of proteins.[PUBLICATION ABSTRACT]