4.1.10(4)-release$ IFS=+
4.1.10(4)-release$ a=( one+two )
4.1.10(4)-release$ echo "${a[0]}"
one+two

4.1.10(4)-release$ s='one+two'
4.1.10(4)-release$ a=( $s )
4.1.10(4)-release$ echo "${a[0]}"
one

elements='hello world+sunny'

# $elements is not quoted so it is
# subject to word splitting
# based on the current value
# of IFS

array_test=( $elements )

for elem in "${array_test[@]}"; do
  printf '%s\n' "$elem"
done

printf '\nChanging IFS to %s\n\n' +

IFS=+

# we need to reassign 
# in order to perform word
# splitting based on the 
# new IFS value

array_test=( $elements )

for elem in "${array_test[@]}"; do
  printf '%s\n' "$elem"
done

4.1.10(4)-release$ ./s
hello
world+sunny

Changing IFS to +

hello world
sunny

4.1.10(4)-release$ IFS=+ read -ra arr <<<'one+two'
4.1.10(4)-release$ printf '%s\n' "${arr[0]}"
one