Beyond 7 ppt might affect the structure and  function  of roots in mesophytes. 

It depends upon several factors viz. type of plants, even its cultivars and salt tolerance level, soil physical and chemical properties (i.e., nature of vadose zone soils), rainfall distribution pattern of the region, nature of salinity, depth of tillage operation etc. 

First , it depends of the type of plant , in some research , salt can increase the growth of plants roots ...these results are in general obtained in halophyte plants or in plant which tolerate salt..but in glycophyte plant, length and root volume decrease progressively as salinity increase... second it depends of soil .

It depends on type of plat normally halophytes show increase in root length.It also depends on the level of salinity