Pursuing PhD was both painful and enjoyable experience. It's just like climbing a high peak step by step, accompanied with bitterness, hardships, frustrations, encouragements and trust with so many people's kind help. Though it will not be enough to just say thank you to all those who helped me instead I owe enormous debt of gratitude to all these people who helped me. First and foremost, I would like to express my special appreciation and thanks to my mentor and guide, Dr. Mohammad Akhtar for his guidance and supervision during entire research work. I have learnt a lot from him and without his help I could not have finished my research. I extend my special thanks to Dr. K. K. Pillai and Dr. Razia Khanam. I thank Dr. K. K. Pillai for his critical remarks and suggestions throughout my research work. I would like to thank Dr. Razia Khanam who also guided my postgraduate research project, for all her encouragement and motivations. A special thanks to my loving family. Words cannot express how grateful I am to my parents and other family members for all sacrifices that you made for me to help me reach my goal.