Not displaying drop down values inside aura iteration?

I am not able to get drop down values inside aura iteration when onchange method calls.

Wrapper Object attribute:
<aura:attribute name="mapobject" type="object" access="global"/>
Aura Iteration Part:
<table class="slds-table slds-table_bordered slds-table_cell-buffer" width="100%">
    <thead>
    <tr class="slds-text-title_caps">
    <th scope="col" width="15%" class="cbcolor">
    <div class="slds-truncate" title="Brand" style="color:black"><center>Brand</center></div>
    </th>
    <th scope="col" width="20%" class="cbcolor">
    <div class="slds-truncate" title="Product Description" style="color:black"><center>Product Description</center></div>
    </th>
    </tr>
    </thead>
    <tbody> 
    <aura:iteration items="{!v.mapobject}" var="mobj" indexVar="indx">
    <tr>
    <td width="15%">
    <div class="slds-form-element slds-input-custom">
    <span>
    <lightning:select aura:id="selprod" name="{!indx+1}" value="{!mobj.selectedprod}" class="text_size" onchange="{!c.getmapmeth}">
    <option  text="--None--" label="--None--" />
    <aura:iteration items="{!v.brandlst}" var="pdesc">
    <option  text="{!pdesc}" label="{!pdesc}" />
    </aura:iteration>
    </lightning:select><br/>
    </span>
    </div>
    </td>
    <td width="20%">
    <div class="slds-form-element">
    <span>
    <lightning:select value="{!mobj.selectedproddesc}" class="text_size">
    <option text="--None--" label="--None--" />
    <aura:iteration items="{!mobj.proddesclist}" var="pdesc">
    <option text="{!pdesc}" label="{!pdesc}" />
    </aura:iteration>
    </lightning:select><br/>
    </span>
    </div>
    </td>
    </tr>
    </aura:iteration>
    </tbody>
    </table>
Helper method:
var prbwrp = component.get("v.mapobject");
        for(var n=0;n<prbwrp.length;n++)
        {
            if(index == n)
            {
                for(var k=0;k<helpervalues.length;k++)
                {
                    prbwrp[n].proddesclist.push(helpervalues[k].proddesc);
                }
                alert('Index ⌗: '+index+' = List Values: '+prbwrp[n].proddesclist);
            }
        }
Wrapper Class:
public class productdetailwrap
    {
        @AuraEnabled public String brandlist{set;get;}
        @AuraEnabled public String proddesc{set;get;}
        @AuraEnabled public list<String> proddesclist{set;get;}
        public productdetailwrap()
        {
            proddesclist=new list<String>();
        }
    }

I am seleting brand, based on selected brand i need to petch prod desc values. Values are getting js controller, but not displaying in cmp. Can anybody help me to resolve this issue.