الفهرس 
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Abstract
Six citrus rootstocks, Swingle Citrumello,Carrizo Citrange and
Troyer Citrange (nematode resistant rootstocks)and Cleopatra mandarin,Volkamer Lemon and Sour Orange (nematode susceptible rootstocks)were used in crosses to produce new hybrid rootstocks. Moreover, these rootstocks were subjected to molecular studies using RAPD-PCR and ISSR-PCR to estimate the genetic diversity and the efficiency of these amplicons to identify unique molecular marker for each studied rootstock. The results confirmed that pollen grains of the three citrus rootstocks showed high rates of viability. Also, Volkamer lemon which is considered as female parent seemed to be cross-compatible with Troyer citrange , Carrizo citrange and Swingle stremello as a male parents. In addition, Fertilization occurred and
pre-embryo begin to develop ١٤ days after pollination. After ٣٠ days from pollination, zygotic embryo arose at the base of the primary embryo, or from its suspensor and were found only after the primary embryo was well established. The level of random amplified polymorphic DNA (RAPD)and intersimple sequence repeats (ISSR)variations present in the six Citrus rootstocks from Egyptian germplasm sources was evaluated. In general both molecular approaches revealed a low degree of intra-rootstock polymorphism (less than ١٠% across both analyses), while high degree of inter-rootstock genetic polymorphism (over ٩٠% across both analyses)and enabled each rootstock to be uniquely fingerprinted. The genetic relationships among rootstocks were analyzed by calculating the pairwise similarity matrix between genotypes according to Nei and Li’s genetic similarity coefficient (NLc)and applying the Unweighted Pair Group Method using Arithmetic Average (UPGMA)to plot dendrograms. Both analyses were able to detect molecular genetic marker for nematode resistance. It was concluded that RAPD and ISSR could be useful and powerful tools for assessing genetic variation and genetic relationships as well as improving citrus breeding via marker-assisted selection.