You don't need to reboot system to re-read partition table after applying changes. I suppose that you already know this and came here to get the strict answer so I will shortly describe basic tool for this task.

To re-read partition table you need to use partprobe utility which is a part of parted package. To install it execute command:

$ sudo apt-get install parted

To just check changes without kernel update execute command:

$ sudo partprobe -ds
/dev/sda: msdos partitions 1 2 3 4 <5 6>

To re-read partition table and update kernel execute command:

$ sudo partprobe

You can specify the device in any case:

$ sudo partprobe /dev/sda