Chromium increases photosystem 2 activity in Brassica juncea

In 7-d-old seedlings of Brassica juncea chromium (VI) promoted photosystem 2 (PS 2) mediated photoreactions. The increase in PS 2 activity in the thylakoids from Cr-treated seedlings, in the presence of uncoupler (5 mM NH₄Cl), was similar to that recorded with the control thylakoids. Thus Cr enhanced PS 2 activity was not due to uncoupling of electron transport from photophosphorylation. Photon saturation kinetics revealed that the PS 2 activity of thylakoids from Cr-treated seedlings was significantly higher at almost all irradiances in comparison to that of controls. PS 2 activity of thylakoids from Cr-treated plants at 25 % of the saturating irradiance was in par with the PS 2 activity of the thylakoids from control plants at saturating irradiance. Thylakoids from both control and Cr-treated seedlings exhibited maximum PS 2 activity at pH 7.5. The PS 2 activity of thylakoids from Cr-treated plants remained high even at pH 8.0 and 8.5, demonstrating Cr enhances tolerance of PS 2 to alkaline pH.