The invention relates to the use of one or more acylsulfonamide safener compounds of formula :for modifying plant development of useful plants. The optical system may include an optical isolator with a beam splitter such as a walk-off plate, a focusing element such as a GRIN lens, a combiner comprising two birefringent wedges and a non-reciprocal rotating device such as a Faraday rotator, a compensation device, and a reflector. Control is facilitated since wheel exists only at the back axis. Additionally, when the heat treatment is conducted under the condition in which a tension is applied, further improvement of the magnetic properties can be expected.