Abstract: The invention relates to a method for protecting plants against stress or for increasing their yield, in which the plant is treated in its intense development stage or directly after flowering or before, during or maximum 5 days after the occurrence of stress with 0.001-10 kg/hectare of a compound of Formula (I), ##STR1## wherein X.sup.- is an agriculturally acceptable inorganic or organic anion. The invention also relates to a composition for protecting plants against stress or for increasing their yield, which comprises as active ingredient 0.001-95% by weight of a compound of formula (I).