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This chapter provides an in depth study on the structure, composition, and organization of viral genomes, their classification into double stranded and single stranded DNA viruses, positive and negative stranded RNA viruses with and their genome diversity. Segmentation and re-assortment of viral genomes have been discussed along with the multipartite virus genomes. Genome details of 13 different viruses have been provided as type studies for better understanding of these topics. Concepts of viral genome evolution have also been discussed.

Five popularly grown mulberry cultivars (K-2, MR-2, TR-10, BC2-59 and S-13) were subjected to drought stress by withholding irrigation, to obtain leaf water potentials (W w ) ranging from -0.75, -1.50 and -2.25 MPa. Accumulation of proline, glycine betaine and abscisic acid (ABA) were quantified in control and water stressed mulberry leaves. The activities of enzymes involved in proline accumulation including glutamate dehydrogenase (EC1.4.1.2-4), pyrroline-5-carboxylate synthetase (EC 1.2.1.41), pyrroline-5-carboxylate reductase (EC1.5.1.2), ornithine transaminase (EC 2.6.1.13) were significantly enhanced in the leaves of all the cultivars with decreasing leaf water potentials, while the activities of proline dehydrogenase (EC 1.5.1.2) were reduced with progressive increase in water stress. Accumulation of proline, glycine betaine and abscisic acid was relatively higher in S-13 and BC2-59 compared to K-2, MR-2 and TR-10 under water deficit conditions. Our results demonstrate that S-13 and BC2-59 have superior osmoprotectant mechanisms under water-limited growth regimes.