Nexus framework and integration team.

When you form a Nexus, the PO moves out of the scrum team level and resides at the NIT level, so that they can collaborate with all teams.  As a Nexus is for one product, and as a product only has one product backlog, there is only one PO.  Remember, that Nexus is still scrum. 

As per the Scrum guide:  

The specific skills needed by the Developers are often broad and will vary with the domain of work. 

The appropriate member that join the NIT for Nexus sprint planning are those that can contribute to the relevant PBIs. 

As per the Nexus guide: 

The Nexus Integration Team is responsible for a Definition of Done that can be applied to the Integrated Increment developed each Sprint. All Scrum Teams within the Nexus must define and adhere to this Definition of Done. Individual Scrum Teams self-manage to achieve this state. They may choose to apply a more stringent Definition of Done within their own teams, but cannot apply less rigorous criteria than agreed for the Integrated Increment.

The NIT needs to be responsible for the DoD, as the NIT is accountable: 

that the Nexus delivers a valuable, useful Integrated Increment at least once every Sprint.

whereas, the scrum teams are accountable.

for creating a valuable, useful Increment every Sprint.

The key differentiator between the NIT & scrum teams accountabilities is integrated

Some things to consider:

1. Irrespective of the number of teams involved, one Product should have exactly one Product Backlog for which exactly one Product Owner ought to be accountable. That's the authority towards which each team should look for product ownership.
2. If so-called "senior developers" were recognized and held to be appropriate representatives for a team, what would that suggest about the quality of "bottom-up" intelligence? If a Scrum Master was in fact the most appropriate representative for that team, what concerns might he or she then have?
3. Perhaps the "bottom-up" culture you describe needs to be fostered and served within each Nexus, such that an integration team would assure transparency over Done, and the work needed for each Increment to be complete.

1)In nexus integration team there is one product owner as there is a single product backlog, wont the 3-9 smaller scrum teams consist of a product owner as a scrum team is made up of a scrum master,a product owner and dev team?

No. The Product Owner, and perhaps the Scrum Master, will be shared. Although it's best for a Developer to be on a single team in order to enhance their focus on the singular Sprint Goal, the Product Owner should be aligned with the product and support all of the teams working on the product. Likewise, the Scrum Master may be able to coach more than one team.

Another way to think about it is that each Scrum Team in the Nexus would be able to identify their Developers, their Scrum Master, and their Product Owner. Multiple teams may identify the same individual as their Product Owner or their Scrum Master, and that's fine.

2)As mentioned in nexus guide,In nexus sprint planning appropriate representatives from scrum team and product owner meet to plan the sprint, who are the appropriate ppl from the scrum?Are they the senior developers?Or the scrum masters of the indiviual scrum teams?

That's up to each team to decide. I would recommend that it is one of the Developers in order to understand what needs to be built and how to build it. Scrum doesn't recognize the concept of "senior developers" - everyone who is a Developer is a Developer. The team may want to consider knowledge and skills (which may or may not be reflected in organization-specific titles) when choosing who represents the Scrum Team at different events.

3)As mentioned in nexus guide,The nexus integration team is responsible for DOD,how then it satisfies botton up culture of scrum?Wont the people in the scrum team who are responsible for producing an increment be essential in determining DOD?

Although the Nexus Integration Team is responsible for a Definition of Done, it is common for the Nexus Integration Team to be made up of representatives of the Scrum Teams in the Nexus. In this case, the representatives of each Scrum Team would be responsible for creating the Nexus's Definition of Done.

It's also important to consider how the Nexus was formed. Was it created in the onset of the effort or did the Nexus evolve as the organization grew from one team to two teams to 3-9 teams? If the Nexus evolved from organizational growth, it's highly likely that the Definition of Done has also evolved from that first Scrum Team. Otherwise, there will be some discussion and negotiation from the different Scrum Teams to create an appropriate initial Definition of Done. Part of the role of the Nexus is to level up each team to work toward more stringent Definitions of Done, thereby allowing the Nexus's Definition of Done to become more stringent.             

Thanks everyone for the reply.Getting a clear understanding now.