This invention relates to fertilizer and herbicide compositions. The present invention provides a composition for selective fertilization of a transgenic plant comprising a polypeptide which metabolizes a D-amino acid substrate; said composition comprising said D-amino acid substrate. Another aspect of the present invention provides a selective herbicide composition comprising a D-amino acid. Such a herbicide will inhibit or reduce the growth of plants which do not contain the appropriate D-amino acid metabolizing enzyme, while the growth of transgenic plants which are able to metabolize the D-amino acid will be unaffected or, more preferably, be increased or enhanced. Methods and means are also provided for selectively modulating the growth and stress tolerance such plants using D-amino acids. The methods can be used either for detoxification of phytotoxic D-amino acids such as D-alanine and D-serine, thereby allowing for selection of plants comprising said D-amino acid metabolizing enzymes, or for enhancing toxicity of lesser phytotoxic D-amino acids such as D-isoleucine, thereby allowing for selection of plants not comprising said D-amino acid metabolizing enzymes.