Limit rotation of a plane?

I have a plane that a ball rolls around on that I’m rotating using input accelerometer values from a tablet. How can I limit the rotation in the x and z axis?

This is my current code:

private Rigidbody rb;

void Start() {
	rb = GetComponent<Rigidbody> ();
}

// Update is called once per frame
void Update () {

	float getHorizontal = Input.acceleration.x; 
	float getVetrical = Input.acceleration.y; 

	transform.Rotate (getVetrical, 0, -getHorizontal); 	

}

Use a simple if statement:

private Rigidbody rb;
    private float maxRotX = 45, maxRotZ = 45;
    float getHorizontal = 0, getVetrical = 0;

    void Start()
    {
        rb = GetComponent<Rigidbody>();

    }
    void Update()
    {
        if (transform.eulerAngles.x < maxRotX && transform.eulerAngles.x > -maxRotX)
            getHorizontal = Input.acceleration.x;

        if (transform.eulerAngles.z < maxRotZ && transform.eulerAngles.z > -maxRotZ)
            getVetrical = Input.acceleration.y;

        transform.Rotate(getVetrical, 0, -getHorizontal);
    }

If you are not going to exceed 90 degree and deal with singularity issues, following modifications might work for you:

void Update()
{
    if (transform.eulerAngles.x < maxRotX && transform.eulerAngles.x > -maxRotX)
        getHorizontal = Input.acceleration.x;
    else
        getHorizontal = 0f;

    if (transform.eulerAngles.z < maxRotZ && transform.eulerAngles.z > -maxRotZ)
        getVetrical = Input.acceleration.y;
    else
        getVetrical = 0f;

    transform.Rotate(getVetrical, 0, -getHorizontal);
}