Herbicides, defoliants and like plant response agents comprise a compound which in the presence of water yields the anions of 3,6-endoxo-hexahydro-orthophthatic acid or 3,6-endoxo-1,2,3,6-tetrahydro-orthophthalic acid or a mixture of such componds together with an ammonium, mono-, di- or tri-alkylammonium, mono-, di-, or tri-alkanol ammonium, alkyl alkanol-dialkylalkanol or alkyl dialkanol ammonium salt of a strong mineral acid, or a mixture of such salts, said salts when said strong mineral acid is polybasic being acid salts or normal salts in which the cations are the same or different. The compound may be the acid per se, the anhydride or an ionizable salt of, e.g. sodium, potassium, calcium, strontium, magnesium, aluminium, iron, cobalt, nickel, zinc, cadmium, mercury, copper, ammonium; mono-, di-, and tri-alkylammonium preferably having 1 to 4 carbon atoms in the alkyl group; mono-, di- and tri-alkanolammonium preferably having 2 to 3 carbon atoms in each alkanol radical, and mixed alkyl alkanol ammonium salts preferably having 1 to 4 carbon atoms in each alkyl radical and 2 to 3 carbon atoms in each alkanol radical. Many examples are given of the above organic bases including many outside the preferred range. Specified intensifiers are the sulphates, chlorides, nitrates, phosphates, acid sulphates, acid phosphates, and mixed normal sulphates and phosphates of (a) ammonia, (b) mono-, di-, tri-alkylammonium having 1 to 4 carbon atoms in each alkyl group, (c) mono-, di-, tri-alkanolammonium having 2 to 3 carbon atoms in each alkanol radical, and (d) mixed alkylanolammonium which is p N-substituted by 2 to 3 radicals of the type and carbon content indicated. The proportion of intensifier to active ingredient is from 1 : 10 to 20 : 1 and particularly from 1 : 2 to 5 : 1, but it may be 100 : 1. The compositions may be in the form of dusts or sprays. Dusts with finely divided solids, e.g. clays such as Fuller's earth, pyrophyllite, talc, bentonite, kieselguhr and diatomaceous earth; sulphur, volcanic ash, calcium carbonate, lime, lignin, wood flour and powdered wheat, soyabean, potato and cotton-seed. Sprays may be in the form of a solution, suspension, dispersion or emulsion in aqueous or non-aqueous media. Dispersions or emulsions may be prepared with, e.g. fatty alcohol sulphates, aliphatic or aromatic sulphonates and alkyl polyglycol ethers or analogous thio ethers. Solutions may contain wetting agents and humectants such as glycerin, diethylene glycol, ethylene glycol, polyethylene glycols, corn syrup and honey. Fungicides, insecticides, bactericides and other plant response agents may be present.ALSO:Herbicides, defoliants and like plant response agents comprise a compound which in the presence of water yields the anions of 3, 6-endoxo-hexahydro-orthophthalic acid or 3, 6-endoxo-1, 2, 3, 6-tetrahydroorthophthalic acid or a mixture of such compounds together with an ammonium mono-di- or tri-alkyl ammonium mono-di- or tri-alkanol ammonium alkyl alkanol-dialkylammonium salt of a strong mineral acid, or a mixture of such salts, said salts when said strong mineral acid is polybasic being acid salts or normal salts in which the cations are the same or different. The compound may be the acid per se, the anhydride or an ionizable salt of e.g. sodium, potassium, calcium, strontium, magnesium, aluminim, iron, cobalt, nickel, zinc, cadmium, mercury, copper, ammonium; mono-, di-, and tri-alkylammonium preferably having 1 to 4 carbon atoms in the alkyl group; mono-di- and tri-alkanolammonium preferably having 2 to 3 carbon atoms in each alkanol radical, and mixed alkyl alkanol ammonium salts preferably having 1 to 4 carbon atoms in each alkyl radical and 2 to 3 carbon atoms in each alkanol radical. Many examples are given of the above organic bases, including many outside the preferred range. Specified intensifiers are the sulphates, chlorides, nitrates, phosphates, acid sulphates, acid phosphates, and mixed normal sulphates and phosphates of (a) ammonia, (b) mono-, di, trialkylammonium having 1 to 4 carbon atoms in each alkyl group (c) monoditrialkanolammonium having 2 to 3 carbon atoms in each alkanol radical, and (d) mixed alkylalkanol ammonium which is N-substituted by 2 to 3 radicals of the type and carbon content indicated. The proportion of intensifier to active ingredient is from 1:10 to 20:1, and particularly from 1:2 to 5:1, but it may 100:1. The compositions may be in the form of dusts or sprays. Dusts with finely divided solids, e.g. clays such as fullers earth, pyrophyllite, talc, bentonite, kieselguhr and diatomaceous earth; sulphur, volcanic ash, calcium carbonate, lime, lignin, wood flour and powdered wheat, soyabean, potato and cotton-seed. Sprays may be in the form of a solution, suspension, dispersion or emulsion in aqueous or non-aqueous media. Dispersions or emulsions may be prepared with e.g. fatty alcohol sulphates, aliphatic or aromatic sulphonates and alkyl polyglycol ethers or analogous thio ethers. Solutions may contain wetting agents and humectants such as glycerin, diethylene glycol, ethylene glycol, polyethylene glycols, corn syrup and honey. Fungicides, insecticides, bactericides and other plant response agents may be present.