For adults or saplings?

For adult trees you could try to look at the root system: avicennia has pencil roots/pneumatophores while rhizophora has prop roots.

When looking at leaf anatomy: avicennia has salt glands (can be indicated by salt accumulations on the leaves) while rhizophora has not.

I hope this is helpful...

Thanks Jeroen....

Ankush:

You may find this link useful:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.389.8129&rep=rep1&type=pdf

Best

Syed

Why you are saying Avicennia and Rhizophora are closest taxa. Morphologically they are so different and come under two widely separated groups and families.  Rhizophora is polypetalous whereas Avicennia is a gamopetalous.  Rhizophora has true viviparous germination where as Avicennia it is usually crypto-vivipary and so many other characters.

Thank you all....

There is a great diversity in mangrove species. At least about 110 species are recognized as mangrove plant species growing in saline habitats belonging to about 19 families and 17 of them to Rhizophoraceae. No single character is found common in all mangrove species. They are typically trees or large shrubs.

As Dr. Nair told Avicennia and Rhizophora are distint taxa. The former is categorised as black mangrove and the latter red mangrove. 

Thank you...

mangrove is a inter tidal vegetation.

sea - water (saline)

                 1. salt secretion glands present in Avicinnia and agicerus spp.

muddy area (least root respiration)   

            2.Pneumatophores present 

                        pencil (Avicinnia) , conical (Soneratia) , knee (kandelia), broom like                                  (ceriops and lumnitzera spp)

Shoreline plant (stand against powerful sea current.  

             3. stilt or prop root present  

                               Rhizophora spp. 

Waterlogged condition (reproduction of new plant)

              4. Viviperus (rhizophora, ceriops,  etc) or cryptoviviperus (agicerus,                                        avicinnia, etc ) germination  

this is a major morphological characters for mangrove 

Dear Mukesh sir,

Thank you for your valuable suggestion

Morphologically Rhizophora spp. can be distinguished from Avicennia by means of pneumatophore and cryptovivipary present in Avicennia spp. but in Rhizophora spp. having stilt root and true vivipary and I agree with Mohanan sir regarding floral morphology and really these are not closest taxa.

Thank you Asok sir...