Photosynthetic electron transfer was studied in thylakoids isolated from control and DCMU-grown wheat (Triticum aestivum L.) seedlings. When exposed to high temperature (HT) and high iradiance (HI), thylakoids showed large variations in the photosynthetic electron transport activities and thylakoid membrane proteins. A drastic reduction in the rate of whole electron transport chain (H2O → MV) was envisaged in control thylakoids when exposed to HT and HI. Such reduction was mainly due to the loss of photosystem 2, PS2 (H2O → DCBQ) activity. The thylakoids isolated from seedlings grown in the presence of DCMU showed greater resistance to HT and HI treatment. The artificial exogenous electron donors MnCl2, DPC, and NH2OH failed to restore the HI induced loss of PS2 activity in both control and DCMU thylakoids. In contrast, addition of DPC and NH2OH significantly restored the HT induced loss of PS2 activity in control thylakoids and partially in DCMU thylakoids. Similar results were obtained when Fv/Fm was evaluated by chlorophyll fluorescence measurements. The marked loss of PS2 activity in control thylakoids was evidently due to the loss of 33, 23, and 17 kDa extrinsic polypeptides and 28-25 kDa LHCP polypeptides. and K. Muthuchelian, M. Bertamini, N. Nedunchezhian.
The changes in pigment-protein composition of chloroplasts in Vigna unguiculata (L.) Walp seedlings grown under control and ultraviolet-B (UV-B) enhanced radiation at various temperatures were studied. The chlorophyll-protein (CP) complexes isolated from seedlings after 60 h of continuous UV-B enhanced radiation treatment showed temperature-dependent changes in their levels. At 10 ^C, well marked CPI, CP2a, CP47, CP2 and free pigment bands were seen in UV-B treated seedlings while control seedlings had no detectable green bands. UV-B enhanced radiation decreased the level of CPI, CP2a and CP2 complexes which was more pronounced at 20 and 30 °C than at 40 oC. The analysis of chloroplast polypeptides revealed a drastic decrease in the levels of 55, 47, 33, 25, 23, 22 and 17 kDa polypeptides in UV-B treated seedlings at 20 and 30 oC, and marginal decrease in the levels of 47, 43 and 16 kDa polypeptides at 40 ^C. In contrast to this, at 10 oC large increase in many polypeptides was noticed in UV-B treated seedlings.